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+*** START OF THE PROJECT GUTENBERG EBOOK 77516 ***
+
+
+
+
+                                                               _THE
+                                                               CONCISE
+                                                               KNOWLEDGE
+                                                               LIBRARY_
+
+
+                               ASTRONOMY
+
+[Illustration:
+
+  Photograph of the Moon taken at Paris by MM. Loewy and Puiseux with
+    the great Coudé Equatorial, February 14, 1894.
+]
+
+
+THE CONCISE KNOWLEDGE LIBRARY
+
+
+
+
+                               ASTRONOMY
+
+                                   ❧
+
+                                   BY
+
+                            AGNES M. CLERKE
+
+ AUTHOR OF A POPULAR HISTORY OF ASTRONOMY DURING THE NINETEENTH CENTURY
+
+                     A. FOWLER, A.R.C.S., F.R.A.S.
+
+              DEMONSTRATOR TO THE ROYAL COLLEGE OF SCIENCE
+
+                   J. ELLARD GORE, F.R.A.S., M.R.I.A.
+
+    AUTHOR OF THE SCENERY OF THE HEAVENS, THE WORLDS OF SPACE, ETC.
+
+[Illustration: Black-and-white publisher’s emblem with a tree above a
+shield featuring stylized letters, flanked by scrolls reading ‘Inter
+Folia Fructus.’]
+
+                                NEW YORK
+                        D. APPLETON AND COMPANY
+                                  1898
+
+
+                         _Authorized Edition._
+
+
+
+
+                                PREFACE
+
+
+This work aims to present in concise form a popular synopsis of
+Astronomical Knowledge to date.
+
+In Section I., Miss Agnes M. Clerke, author of “A Popular History of
+Astronomy during the Nineteenth Century,” gives a brief historical
+sketch of the science from Hipparchus to the present time. In Section
+II., an attempt is made by Mr. A. Fowler, A.R.C.S., F.R.A.S.,
+Demonstrator of Astronomical Physics to the Royal College of Science, to
+briefly outline the general principles of spherical and gravitational
+Astronomy, and to describe the instrumental means now at the command of
+observers in the various branches of Astronomical research. The author
+begs to record his indebtedness to Loomis’ “Treatise on Astronomy,” and
+Young’s “General Astronomy,” which have been frequently consulted,
+especially for memorial data; to Mr. W. Shackleton, for assistance in
+proof-reading; and to Mr. C. P. Butter, for valuable help in preparing
+the diagrams. Section III., contributed by Miss Agnes M. Clerke, deals
+with the Solar System; and Section IV., written by Mr. J. Ellard Gore,
+F.R.A.S., M.R.I.A., treats of the Sidereal Heavens.
+
+The work is illustrated by a large number of diagrams and other
+illustrations, prepared expressly for its pages, as well as by a number
+of photographic and other reproductions of photographs and drawings made
+by distinguished astronomers in Europe and America. In this connexion
+numerous acknowledgments are due.
+
+The Editor begs to express his sense of indebtedness to the following
+astronomers and publishers, for kind permission to reproduce original
+photographs and drawings from their works:
+
+To M. Loewy, Director de l’Observatoire, Paris, for permission to
+reproduce the photograph of the Moon, which forms the frontispiece of
+this volume; to Professor Edward S. Holden, Director of the Lick
+Observatory, for permission to reproduce drawings and photographs of the
+Observatory at Nice, p. 2; the Thirty-six Inch Reflector of Lick
+Observatory, p. 40; the Meridian Circle of the Paris Observatory, p.
+203; the Spectroscope adapted to the eye end of the Lick Telescope, p.
+221; and Jupiter showing the Red Spot, p. 322; to Dr. Isaac Roberts, for
+permission to reproduce his photograph of the photographic telescope
+used by him, p. 196; to Messrs. Trichnor & Co., of Berlin, for
+permission to reproduce two illustrations of Donati’s Comet, pp. 228 and
+363; and one of Sun-spots and Magnetic Variations, p. 246—all from
+Langley’s “New Astronomy”; to Messrs. Witherby & Co., for permission to
+reproduce the photograph of a Sun-spot by Janssen, p. 243; the
+photograph of Jupiter, p. 328; the photographs of Swift’s Comet, pp. 374
+and 375, Brooks’ Comet, p. 381, and the Milky Way, p. 557, from
+_Knowledge_; to Messrs. Taylor & Francis, for the diagram of curves
+showing the development of Sun-spots, p. 257; to Professor George E.
+Hale, of Kenwood Observatory, Chicago, for his illustrations of Eruptive
+Prominences photographed at Kenwood, March 24th, 1896, pp. 264 and 265,
+reproduced from the _Astrophysical Journal_; to the Council of the Royal
+Society, for the illustration of the Eclipsed Sun, p. 267, reproduced
+from “Philosophical Transactions”; to Professor Barnard, for the
+photographs of the Corona, reproduced on p. 269; and the drawings of the
+Transit of Jupiter’s Satellite, on p. 330, reproduced from the _Monthly
+Notices_ of the Royal Astronomical Society; the Eye of Mars, p. 302; and
+of Saturn and his Rings, p. 335; to the editor of the _Astronomische
+Nachrichten_, for the map of Mercury, by Schiaparelli, reproduced on p.
+276; to the editor of _Nature_, for the drawing of Venus by Mascari, on
+p. 280; to Messrs. George Philip & Son, for the map of the Moon, given
+on p. 291, from Fowler’s “Telescopic Astronomy”; to Messrs. Longman,
+Green & Co., for the Chart of Mars, p. 300, and the diagram of the
+Apparent Orbit of the Companion of Sirius, p. 439, from Proctor’s “Old
+and New Astronomy”; to Professor W. W. Payne of Goodsell Observatory,
+for the use of the drawing of the Oases of Mars, p. 304, from “Popular
+Astronomy”; and the photograph of Holmes’ Comet, p. 378, from the same
+work; to Messrs. A. & C. Black, for the illustrations of the Great Comet
+of September, 1882, p. 361, reproduced from Miss Clerke’s “History of
+Astronomy”; to Messrs. Crosby, Lockwood & Co., for permission to
+reproduce the illustration of the Nebula in Andromeda 31 Messier, p.
+398, from the frontispiece of Mr. J. E. Gore’s “Visible Universe”; and
+also for the same authority, regarding the diagrams, showing the Stars
+visible in the Northern Hemisphere, p. 401; the Stars visible in the
+Southern Hemisphere, p. 403; the drawing showing the position of the
+Solar Apex, according to different computers, p. 429; and the
+photographs of the Spiral Nebula 51 Messier, p. 533; and the Milky Way
+in Sagittarius, p. 555, all from the same work; to Messrs. A. D. Innes &
+Co., for the use of the diagram, Apparent Orbit of Zeta Herculis, p.
+436; Triple Stars, p. 451; and for permission to reproduce the
+photographs, 37 Messier, p. 505; the star cluster, Omega Centauri, p.
+512; the Nebulæ of Orion, p. 521; and the Magellanic Clouds, p. 537,
+from “The Worlds of Space”; and to Messrs. Sutton & Co., for the use of
+the diagram of the Apparent Orbit of 70 Ophiuchi, p. 443; the
+photographs of the Double Cluster of Perseus, p. 503; the Star Cluster
+in Gemini, p. 504; the Star Cluster in Hercules, p. 507; and the drawing
+of the Trifid Nebula, Sagittarius, p. 525, from “The Scenery of the
+Heavens”; and the drawing of the Temporary Star of 1572, p. 481, from
+“Planetary and Stellar Studies,” both published by them.
+
+                                                                A. H. M.
+
+
+
+
+                                CONTENTS
+
+
+               SECTION I.—HISTORY      By Agnes M. Clerke.
+ Chap.                                                              Page
+    I. FROM HIPPARCHUS TO LAPLACE                                      3
+   II. A CENTURY OF PROGRESS                                          21
+
+
+     SECTION II.—GEOMETRICAL ASTRONOMY AND ASTRONOMICAL INSTRUMENTS.
+                                        By A. Fowler, A.R.C.S., F.R.A.S.
+
+    I. THE EARTH AND ITS ROTATION                                     41
+   II. THE EARTH’S REVOLUTION ROUND THE SUN                           55
+  III. HOW THE POSITIONS OF THE HEAVENLY BODIES ARE DEFINED           65
+   IV. THE EARTH’S ORBIT                                              72
+    V. MEAN SOLAR TIME                                                78
+   VI. THE MOVEMENTS OF THE MOON                                      87
+  VII. MOVEMENTS OF PLANETS, SATELLITES, AND COMETS                   98
+ VIII. ECLIPSES AND OCCULTATIONS                                     110
+   IX. HOW TO FIND OUR SITUATION ON THE EARTH                        122
+    X. THE EXACT SIZE AND SHAPE OF THE EARTH                         129
+   XI. THE DISTANCES AND DIMENSIONS OF THE HEAVENLY BODIES           139
+  XII. THE MASSES OF CELESTIAL BODIES                                151
+ XIII. GRAVITATIONAL EFFECTS OF SUN AND MOON UPON THE EARTH          162
+  XIV. INSTRUMENTAL MEASUREMENT OF ANGLES AND TIME                   171
+   XV. TELESCOPES                                                    176
+  XVI. INSTRUMENTS OF PRECISION                                      198
+ XVII. ASTROPHYSICAL INSTRUMENTS                                     211
+
+
+                     SECTION III.—THE SOLAR SYSTEM.
+                                                     By Agnes M. Clerke.
+
+    I. THE SOLAR SYSTEM AS A WHOLE                                   229
+   II. THE SUN                                                       237
+  III. THE SUN’S SURROUNDINGS                                        253
+   IV. THE INTERIOR PLANETS                                          273
+    V. THE EARTH AND MOON                                            283
+   VI. THE PLANET MARS                                               297
+  VII. THE ASTEROIDS                                                 310
+ VIII. THE PLANET JUPITER                                            318
+   IX. THE SATURNIAN SYSTEM                                          333
+    X. URANUS AND NEPTUNE                                            343
+   XI. FAMOUS COMETS                                                 352
+  XII. NATURE AND ORIGIN OF COMETS                                   368
+ XIII. METEORITES AND SHOOTING STARS                                 385
+
+
+                    SECTION IV.—THE SIDEREAL HEAVENS.
+                                                 By J. E. Gore, F.R.A.S.
+
+    I. THE STARS AND CONSTELLATIONS                                  399
+   II. DOUBLE, MULTIPLE, AND COLOURED STARS                          410
+  III. THE DISTANCES AND MOTIONS OF THE STARS                        417
+   IV. BINARY STARS                                                  431
+    V. VARIABLE AND TEMPORARY STARS                                  458
+   VI. CLUSTERS AND NEBULÆ                                           497
+  VII. THE CONSTRUCTION OF THE HEAVENS                               538
+
+
+
+
+                         LIST OF ILLUSTRATIONS
+
+
+        _Frontispiece to Volume_—PHOTOGRAPH OF THE MOON.
+
+
+                           SECTION I.—HISTORY.
+                                                                  _Page_
+        _Frontispiece_—THE OBSERVATORY AT NICE                         2
+
+
+                   SECTION II.—GEOMETRICAL ASTRONOMY.
+
+        _Frontispiece_—THE LICK REFRACTOR OF THIRTY-SIX INCHES
+          APERTURE                                                    40
+ _Fig._
+    1.— ROUGH MEASUREMENT OF EARTH’S DIAMETER                         43
+    2.— HORIZONS AT TWO PLACES ON THE EARTH                           44
+    3.— FOUCAULT’S PENDULUM EXPERIMENT                                49
+    4.— SUN’S RAYS—DAY AND NIGHT                                      52
+    5.— ATMOSPHERIC REFRACTION                                        54
+    6.— APPARENT PATHS OF SUN AT EQUINOXES AND SOLSTICES              56
+    7.— ABERRATIONAL ORBIT OF A STAR                                  58
+    8.— THE SUN’S ALTITUDE IN SUMMER AND WINTER                       62
+    9.— THE SUN’S ALTITUDE AT THE EQUINOXES                           63
+   10.— THE MIDNIGHT SUN                                              64
+   11.— ALTITUDE AND AZIMUTH                                          66
+   12.— RIGHT ASCENSION, DECLINATION, ETC.                            69
+   13.— ELLIPTIC FORM OF EARTH’S ORBIT                                72
+   14.— THE ELLIPSE                                                   73
+   15.— HOW TO DRAW AN ELLIPSE                                        74
+   16.— ILLUSTRATING KEPLER’S SECOND LAW                              76
+   17.— EFFECT OF OBLIQUITY OF ECLIPTIC UPON THE EQUATION OF TIME     81
+   18.— THE MOON’S PHASES                                             90
+   19.— THE LUNAR MONTH                                               91
+   20.— THE MOON’S ROTATION                                           93
+   21.— THE MOON’S NODES                                              94
+   22.— POSITION OF ECLIPTIC AT SUNSET AT VERNAL EQUINOX              96
+   23.— MOVEMENT OF AN INTERIOR PLANET                                99
+   24.— MORNING AND EVENING STARS                                    100
+   25.— MOVEMENT OF AN EXTERIOR PLANET                               103
+   26.— APPARENT PATHS OF CERES, PALLAS, JUNO, AND VESTA, IN 1896    104
+   27.— OPPOSITION OF MARS                                           105
+   28.— ELEMENTS OF AN ELLIPTIC ORBIT                                107
+   29.— THE EARTH’S SHADOW                                           110
+   30.— THE LUNAR ECLIPTIC LIMIT                                     112
+   31.— ECLIPSES OF THE SUN                                          114
+   32.— DURATION OF A SOLAR ECLIPSE                                  115
+   33.— TRACK OF ECLIPSE OF APRIL 16, 1893                           117
+   34.— DETERMINATION OF LATITUDE                                    124
+   35.— ANCIENT MODE OF MEASURING LATITUDE                           125
+   36.— TRIANGULATION                                                132
+   37.— MOVEMENTS OF THE EARTH’S POLE, 1890–95                       138
+   38.— PARALLAX OF A HEAVENLY BODY                                  140
+   39.— DIAMETER OF A HEAVENLY BODY                                  142
+   40.— MEASUREMENT OF THE MOON’S DISTANCE                           143
+   41.— RELATIVE DISTANCE OF VENUS                                   145
+   42.— THE PARALLAX OF MARS                                         147
+   43.— THE TRANSIT OF VENUS                                         148
+   44.— THE MOON’S CURVILINEAR PATH                                  155
+   45.— THE TIDES                                                    163
+   46.— NUTATION                                                     169
+   47.— SECTION OF READING MICROSCOPE                                172
+   48.— THE READING MICROSCOPE                                       173
+   49.— THE ACHROMATIC OBJECT-GLASS                                  177
+   50.— THE NEWTONIAN REFLECTOR                                      179
+   51.— THE CASSEGRAIN REFLECTOR                                     181
+   52.— THE PHOTOGRAPHIC TELESCOPE                                   196
+   53.— THE MERIDIAN CIRCLE OF THE PARIS OBSERVATORY                 203
+   54.— THE MICROMETER APPLIED TO A BINARY STAR                      208
+   55.— THE SPECTROSCOPE ADAPTED TO THE EYE END OF THE LICK
+          TELESCOPE                                                  221
+
+
+                     SECTION III.—THE SOLAR SYSTEM.
+
+        _Frontispiece_—DONATI’S COMET                                228
+
+    1.— PHOTOGRAPH OF A SUN-SPOT                                     243
+    2.— SUN-SPOTS AND MAGNETIC VARIATIONS                            246
+    3.— CURVES SHOWING THE DEVELOPMENT OF SUN-SPOTS                  257
+    4.— ERUPTIVE PROMINENCE                                          264
+    5.— THE SAME, 18 MINUTES LATER                                   265
+    6.— THE ECLIPSED SUN                                             267
+    7.— THE CORONA OF JANUARY 1, 1889                                269
+    8.— MAP OF MERCURY                                               276
+    9.— VENUS, FROM A DRAWING BY MASCARI                             280
+   10.— MAP OF THE MOON                                              291
+   11.— PHOTOGRAPH OF THE TOTALLY ECLIPSED MOON                      296
+   12.— CHART OF MARS                                                300
+   13.— THE “EYE OF MARS”                                            302
+   14.— THE OASES OF MARS                                            304
+   15.— JUPITER, SHOWING THE RED SPOT                                322
+   16.— PHOTOGRAPH OF JUPITER                                        328
+   17.— TRANSIT OF JUPITER’S FIRST SATELLITE                         330
+   18.— SATURN AND HIS RINGS                                         335
+   19.— GREAT COMET OF SEPTEMBER, 1882                               361
+   20.— DONATI’S COMET                                               363
+   21.— PHOTOGRAPH OF SWIFT’S COMET                                  374
+   22.— THE SAME, 24 HOURS LATER                                     375
+   23.— PHOTOGRAPH OF HOLMES’ COMET                                  378
+   24.— PHOTOGRAPH OF BROOKS’ COMET                                  381
+
+
+                    SECTION IV.—THE SIDEREAL HEAVENS.
+
+        _Frontispiece_—NEBULA IN ANDROMEDA 31 MESSIER                398
+
+    1.— STARS VISIBLE IN THE NORTHERN HEMISPHERE                     401
+    2.— STARS VISIBLE IN THE SOUTHERN HEMISPHERE                     403
+    3.— DIAGRAM SHOWING “SOLAR APEX” AS ROUNDLY COMPUTED             429
+    4.— APPARENT ORBIT OF ZETA HERCULIS                              436
+    5.— APPARENT ORBIT OF THE COMPANION OF SIRIUS                    439
+    6.— APPARENT ORBIT OF 70 OPHIUCHI                                443
+    7.— TRIPLE STARS                                                 451
+    8.— THE TEMPORARY STAR OF 1572                                   481
+    9.— THE DOUBLE STAR CLUSTER IN PERSEUS                           503
+   10.— STAR CLUSTER IN GEMINI                                       504
+   11.— 37 MESSIER                                                   505
+   12.— STAR CLUSTER IN HERCULES                                     507
+   13.— THE STAR CLUSTER, OMEGA CENTAURI                             512
+   14.— THE ORION NEBULÆ                                             521
+   15.— THE NEBULA ROUND ETA ARGUS                                   523
+   16.— THE TRIFID NEBULA, SAGITTARIUS                               525
+   17.— SPIRAL NEBULA, 51 MESSIER                                    533
+   18.— MAGELLANIC CLOUDS                                            537
+   19.— PHOTOGRAPH OF MILKY WAY, SAGITTARIUS                         555
+   20.— THE MILKY WAY                                                557
+
+
+
+
+                               ASTRONOMY
+
+
+[Illustration:
+
+  THE OBSERVATORY AT NICE.
+
+  [_See page 192_
+]
+
+
+
+
+                          SECTION I.—HISTORY.
+
+                          BY AGNES M. CLERKE.
+
+
+
+
+                               CHAPTER I.
+                      FROM HIPPARCHUS TO LAPLACE.
+
+
+In the year 134 B.C., a temporary star blazed out in the constellation
+Scorpio. It was observed by a man of extraordinary genius, and furnished
+the incentive to one of his most memorable works. This was the
+construction, on essentially modern principles, of a catalogue of 1,080
+stars. Hipparchus thus, with deliberation and singular prescience,
+furnished a standard by which future changes in the heavens might be
+detected. He was a native of Rhodes, but belonged to the school of
+Alexandria; and at Alexandria, after three centuries, he found an able
+and ambitious successor.
+
+Claudius Ptolemæus was one of the many “inheritors of unfulfilled
+renown.” He combined, completed, and preserved what his predecessors,
+eminent or obscure, had done. Gathering materials from all quarters, and
+adding much of his own, he reared an astronomical edifice so imposing,
+coherent, and substantial, that the lapse of fourteen centuries left it
+virtually unassailed, and, to a superficial judgment, unassailable.
+Fitly, then, this monument of industry and ingenuity kept the title
+bestowed upon it by the Arabs of “Almagest,” signifying “the Greatest.”
+It bears, nevertheless, perennial witness to the possibility of
+satisfying the human mind with the truth of appearances, apart from the
+truth of things. For although the Almagest embodies a large amount of
+real knowledge, that knowledge is throughout falsely interpreted. The
+Ptolemaic system was constructed on the principle of “saving the
+phenomena”—that is, of providing expedients geometrically valid, even if
+physically inadmissible, by which to represent the apparent movements of
+the heavenly bodies. That they might, to a great extent, be apparent
+only, was obvious to the cultivated Greek mind. The rotation of the
+earth on an axis was a familiar Pythagorean doctrine; it was adopted by
+Plato, and Aristarchus of Samos went to the length of ranking our green
+world as a planet revolving yearly round the sun. The idea, however, was
+too recondite for vulgar apprehension; it was tainted with a suspicion
+of impiety, and its development would, besides, have proved extremely
+embarrassing to the nascent science of that age. So Hipparchus chose the
+prudent alternative of treating astronomy from the purely mathematical
+standpoint; he submitted to the restrictions imposed by the hypothesis
+of equable circular motion; and, with wonderful skill, fitted the
+Apollonian eccentrics and epicycles to expound celestial wanderings.
+Ptolemy inevitably followed suit. He set some five dozen spheres in
+motion, while leaving the earth at rest; and at rest it remained until,
+in long meditations by the foggy shores of the Baltic, a grave-browed
+ecclesiastic elaborated certain cogent arguments in favour of its
+motion.
+
+During the interval between Ptolemy and Copernicus, astronomy kept in
+the Alexandrian groove. Early in the eighth century, the seat of
+learning having been transferred to Baghdad, the charge of its
+crystalline machinery devolved upon Arabs and Jews, men of fine
+technical acquirements, but small originative power, men of the kind
+described in the “Vicar of Wakefield,” who, “had they been bred
+cobblers, would all their lives have only mended shoes, but never made
+them.” Not but that they executed the necessary repairs with uncommon
+ingenuity, modifying the cumbrous structure given into their keeping to
+suit the fresh inequalities brought to light by their patient
+watchfulness. But their improvements consisted in adding to already
+intolerable complications—in piling orb on orb, in devising
+“trepidations” and oscillations, of which nature took small heed; so
+that the better they observed, the worse their system became.
+
+The science was diligently cultivated. Al-Mamûm, son and successor of
+“good Harûn-al-Raschid,” founded at Baghdad a school of astronomy, of
+which Albategnius, called “the Ptolemy of the Arabs,” was the brightest
+ornament. He discovered, early in the tenth century, the movement of the
+“sun’s apogee”—that slow revolution of the longer axis of the earth’s
+orbit, regarded by astronomical glacialists as a factor in the
+production of recurring Ice Ages. The Persian grandee, Al-Sûfi (903–986)
+belonged to the same group. His “Description of the Stars” was a revised
+edition, not a simple reprint, of the Alexandrian list, and has the
+value derived from personal consultation with the skies. Thus, Algol,
+now purely white, is recorded in it as a decidedly red star. About a
+century later, Aboul Wefa detected the moon’s “variation,” independently
+noted, after five centuries, by Tycho Brahé. Then the Tartars had their
+turn. Nasir Eddin (1201–1274) was a native of Khorassan; but his love of
+learning drew him to the city of the Khalifs, where he assembled a band
+of experts for the construction of new planetary tables, the old ones
+having lapsed into hopeless disaccord with the heavens. Last came Ulugh
+Beigh, grandson of the furious Tamerlane. He founded at Samarcand a kind
+of astronomical Solomon’s House, built a grand observatory, and worked
+in it assiduously. His vigorous and ennobling reign of forty years was
+terminated by one of those domestic catastrophes which ordinarily fix
+the chronology of Eastern dynasties. He was murdered by his son in 1447,
+and the sands of the desert thereupon closed, so to speak, over his
+civilising efforts. His star catalogue, edited by Francis Baily in 1843,
+is the outcome of fresh observations made in the old way. A Tartar
+prince, he ranks as an Arab astronomer.
+
+Mohammedan science had already fulfilled its appointed task. A torch,
+still alight, had been handed on from East to West. Its extinction would
+have been a calamity. A total break in the cultivation of astronomy, for
+instance, would have cost ages to repair. The Ptolemaic system, it is
+true, disguised rather than revealed nature; yet it constituted a
+regulated body of knowledge, only looked at from a wrong point of view.
+An unbiassed spectator had merely to shift his position and open his
+eyes, in order to perceive the simplicity of the real celestial
+mechanism. No better illustration could be adduced of Bacon’s aphorism
+that “truth emerges more easily from error than from confusion.”
+
+It was from the Moors in Spain that Christian Europe took its first
+lessons in antique science. The Alphonsine Tables were due to Oriental
+industry. They were compiled at Toledo about 1270 by an assemblage of
+Arab experts directed by Hassan, the Jew delegate of Alfonso X. of
+Castile. But they caught Western attention, and drew Western
+intelligence towards the abstruse art they exemplified. Thus a little
+treatise on the Sphere composed about 1230, by John Holywood, a
+Yorkshireman, known to cosmopolitan fame as Johannes de Sacrobosco,
+obtained astonishing popularity; at least sixty-five Latin editions of
+it appearing between 1472 and 1647, besides French, Italian, German, and
+Spanish versions, and endless commentaries. With the revival of
+classical learning, the Almagest, previously known in blundering Latin
+translations from the Arabic, came to be read in the original Greek, and
+thus re-emergent, roused fresh enthusiasm. Inspired by the afflatus,
+George Purbach (1423–1461) and his brilliant pupil, Johannes Müller of
+Königsberg in Franconia (Regiomontanus), successively professors of
+mathematics at Vienna, applied themselves to burnishing up the ancient
+epicyclical apparatus; while in Italy, the seductive opinions of the
+Pythagorean school gained ground, as evidence came to light, that there
+had been astronomers before Ptolemy no less than kings before Agamemnon.
+The orthodox doctrine naturally continued to be taught at the
+universities; but some of the professors held esoteric opinions of a
+different cast, which they freely imparted to privileged disciples. The
+earth’s rotation was spoken of as a matter of common knowledge by
+Lionardo da Vinci; it was inculcated in rhyme, before the close of the
+fifteenth century, by Girolamo Tagliavia, a Calabrese poet; it was
+debated by scholars and pedants; on all sides influences wrought to
+shatter the integrity of Ptolemaic convictions.
+
+True progress, however, consists less in destruction than in
+re-organisation. And this demands powers of a high order. They were
+brought into play just at the right moment. Nicholas Copernicus was born
+at Thorn on the Vistula, February 19, 1473. At the age of twenty-three,
+having exhausted the teaching resources of the university of Cracow, he
+crossed the Alps in quest of instruction in Greek and mathematics.
+Towards the close of 1496, then, he was enrolled as a student at
+Bologna, and shortly afterwards became the pupil, assistant, and friend
+of the Ferrarese astronomer, Domenico Maria Novara. Here, beyond
+reasonable doubt, Copernicus adopted Copernican opinions. The question,
+_An terra moveatur?_ was incessantly mooted at Bologna; advanced
+thinkers replied in the affirmative; Novara himself most likely took his
+intellectual beliefs from Plato and Aristarchus, while looking to
+Ptolemy for his daily bread. The transalpine scholar, at any rate,
+brought back with him to Poland in 1505, an unalterable persuasion that
+the heliocentric system belonged to the reality of things. He devoted
+eighteen years of his abode within the cathedral precincts of
+Frauenburg—from 1512 to 1530—to demonstrating its detailed conformity
+with the phenomena of the heavens; but allowed only a sketch of his
+results to be published. It was only at the earnest request of the
+Bishop of Culm that he finally delivered up to him the manuscript of “De
+Revolutionibus Orbium Coelestium,” the first printed copy of which was
+laid on his deathbed, May 24, 1543.
+
+The immediate effect was small. The new system of astronomy was admired,
+but not adopted. It indeed contradicted the evidence of the senses, and
+failed to compel assent from the understanding. For its author had not
+completely broken with tradition. He unfortunately retained the false
+supposition of equable circular motion, and thus greatly marred the
+simplicity of his scheme of the heavens. Orbs still kept rolling upon
+orbs, Mercury alone demanding a combination of seven to bear him over
+his course. But if seven, it might have been asked, why not seven times
+seven? The principle of representing appearances by transcendental means
+remained the same as before. Ignorance of the laws of motion raised
+other formidable objections. A whirling earth, it was thought, should
+leave behind all detached objects; absolute repose was taken to be the
+condition _sine quâ non_ of stability. Then the seeming immobility of
+the stars implied for them a remoteness so extravagant, according to
+prevalent ideas, that even Kepler admitted it to be “a big pill to
+swallow.” Copernicus was fully aware that the earth’s orbital revolution
+must occasion stellar perspective displacements; indeed, he staked the
+truth of his theory upon future measurements of annual parallax.
+Nevertheless, four centuries passed before they were successfully
+executed.
+
+Tycho Brahé was the last great mediæval observer. Like Hipparchus, he
+was summoned by a star—the marvellous “new star” of 1572; and, having
+obtained from Frederick II. of Denmark the grant of an islet in the
+Sound, he built upon it a mansion “royal, rich and wide,” erected
+magnificent instruments, and used them, not only with consummate skill,
+but also with a certain princely pomp, donning robes of state before
+admitting the bright “populace of heaven” to audience. His stormy
+temper, however, led to disputes with the young King Christian IV.; he
+forsook Uraniborg, and died at Prague in 1601. Curiously enough, the
+very accuracy of his observations led him astray from speculative truth.
+For it enabled him to perceive the incompatibility of many facts with
+Copernican expedients for harmonising them, and intensified the
+difficulty raised to Copernican views by the absence of stellar
+parallax. So he devised a system of his own, in which the planets
+revolved round the sun, but the sun round the earth. It scarcely
+survived its contriver.
+
+The invention of the telescope created descriptive astronomy. Without
+it, the mechanism of the solar system could have been laid bare, and the
+law of force regulating its action discovered; and in point of fact,
+Kepler’s achievements owed nothing, and Newton’s very little, to the
+optician’s art. Inquiries, on the other hand, into the nature of the
+heavenly bodies were wholly inspired by it; it disclosed the amazing
+multitude of the stars, and opened endless vistas of research. No one
+could at first have divined the momentous character of the accident by
+which Hans Lippershey, a spectacle-maker at Middleburg in Holland, hit
+upon an arrangement of lenses serving virtually to abridge distance. It
+happened in 1608; and Galileo Galilei (1564–1642), hearing of it shortly
+afterwards at Venice, prepared on the hint a “glazed optic tube,” and
+viewed with it, early in 1610, the satellites of Jupiter, the mountains
+of the moon, the star streams of the Milky Way, and in 1611, the phases
+of Venus, the spots on the sun, and the strange appendages of Saturn.
+Thus, amid a tumult of applause, the telescopic revelation of the
+heavens began. It was brilliantly illustrative, although not
+demonstrative, of Copernican theory; and Galileo drove his own vivid
+conviction on the subject home to general apprehension by the literary
+skill with which he treated it in his famous “Dialogues” (1632). He most
+substantially promoted the new views, however, by his recognition of the
+laws of motion, and of force as the cause of motion. The problem of the
+heavens, stript thereby of metaphysical obscurities, was laid bare to
+the reason as one of pure mechanics; the planets came to be treated as
+ordinary projectiles, and distinct reasoning about the nature of their
+paths was rendered possible. Newton’s great task was thus prepared and
+defined by Galileo.
+
+Kepler’s (1573–1630) three generalisations formed a still more
+indispensable prelude to its accomplishment. Their immediate effect was
+to sweep away the Copernican remnants of Ptolemaic lumber, and to
+disclose the harmonious plan upon which our system is ordered. But it
+was a geometrical plan only. Kepler indeed divined the influence of a
+central power, which he surmised to be of a magnetic nature; and he
+aspired towards the establishment of a truly physical astronomy. Yet he
+was far from perceiving the full implications of the laws he had
+himself, after half a lifetime of trial and failure, at last
+triumphantly discovered. These laws are:
+
+(I.) The planets travel in ellipses of which the sun occupies one focus.
+
+(II.) They travel at rates varying in such a manner that the “radius
+vector”—or imaginary line joining each to the sun—describes equal areas
+in equal times.
+
+(III.) The cubes of their mean distances from the sun are proportional
+to the squares of their periods of revolution.
+
+Now these are precisely the conditions under which planetary circulation
+should proceed if governed by a force emanating from the sun, and
+decreasing as the square of the distance from him increased. Moreover,
+Hooke, Halley, and Wren separately got so far as to perceive that it
+could be explained on this principle. But Isaac Newton alone could
+demonstrate what they divined, and even his supreme faculties were
+dangerously strained by the laborious process. This was not all. He
+showed that the earth exerts on the moon just the same kind of pull that
+the sun exerts on the planets; a pull identical with the familiar
+“attraction of gravitation,” by which the globe we inhabit holds
+integrally together, retains its oceans in their beds, and bears with it
+through space its “cloud of all-sustaining air.” Its domestic affairs
+are thus guided by the same unchanging rule that dominates its foreign
+relations.
+
+The publication in 1687 of Newton’s “Principia” marked an unprecedented
+advance in knowledge. The advance consisted in unification. A science of
+celestial physics, capable of indefinite future expansion, was founded
+on the sure basis of terrestrial experience. Canons of interpretation,
+derived from immediate perception, were proved applicable to the
+phenomena of the heavens. The line drawn in antique philosophy between
+the “corruptible” things under our feet and the “incorruptible” over our
+heads was forever rubbed out. Sublunary and empyreal regions were thrown
+together into one vast domain.
+
+Although Newton’s law is, in itself, of extreme simplicity, its actual
+workings are highly intricate. Because dependent upon a universal and
+unintermittent influence, they are self-modifying, so that each
+consequence becomes a cause, and to each cause is attached an endless
+train of effects. They can be dealt with only with the aid of the
+infinitesimal calculus, and then, not directly, but by successive and
+tedious approximations, or by arts and devices of almost superhuman
+ingenuity. Hence Newton’s laurels would have remained comparatively
+barren had he not found successors in a group of men of extraordinary
+ability. What he had begun, Clairaut, D’Alembert, Euler, Lagrange, and
+Laplace carried on by showing the adequacy of a single law to account
+for every traceable deviation from undisturbed elliptical motion. In the
+course of a long and arduous campaign, they carried every position that
+they attacked. Over and over again, the principle of gravitation seemed
+to be compromised; over and over again, it was vindicated by these
+intrepid champions.
+
+This process of gradual verification began in 1747, when Clairaut and
+D’Alembert sent to the Paris Academy of Sciences, on the same day, the
+first satisfactory solutions of the “Problem of three Bodies.” The
+motions of the moon, nevertheless, did not at once fall in with the
+general theory; they were rendered amenable only after years of anxious
+toil. Barely the initial difficulties had been overcome when Euler, in
+1753, published his “Theory of the Moon,” from which Tobias Mayer of
+Göttingen constructed lunar tables. Now tables are the test of theories.
+Every row of figures they contain is a prediction, by the fulfilment, or
+non-fulfilment of which the underlying scheme must stand or fall.
+Through such comparisons, mathematical astronomers find out the
+shortcomings of their methods, or the insufficiency of their hypotheses,
+and are incited to refine the first, and correct the second. Demands for
+the application of the nicer criteria thus afforded suggest
+observational improvements, which seldom fail to bring to light minor
+discrepancies with theory, impelling to fresh efforts for their
+abolition. Such alternations of advance along the abstract and the
+practical lines result in a continual diminution in the _scale_ of
+error, although not in its annihilation; absolute exactitude being, as
+it were, an asymptote, continually approached, but touched only at
+infinity—that is, never, under subsisting conditions. Even now the
+length of the moons tether is four or five miles. To that extent, she
+may go astray from her computed path, not without occasioning
+disquietude to the responsible authorities.
+
+So far as could be ascertained in the eighteenth century, her subjection
+to known law was completed by the dispersal of the mystery surrounding a
+slight, continuous acceleration of her orbital velocity detected by
+Halley in 1693. It had been in progress since the earliest recorded
+eclipse in 721 B.C., if not longer; there was no sign of its cessation
+or reversal, and the grave question arose, Was the principle of
+universal attraction, elsewhere unreservedly obeyed, here fatally
+complicated by the action of a resisting medium involving the eventual
+collapse of the earth-moon system? Laplace gave the answer, November 19,
+1787, by proving the observed quickening of pace to be a necessary and
+simple consequence of a secular diminution in the ellipticity of the
+earths orbit. This, however, will not go on for ever in the same
+direction; after many ages the tide of change will turn, and a complete
+restoration to the _status quo ante_ will ensue.
+
+Another master-stroke of Laplace’s genius was his explanation, also in
+1787, of the “long inequality” of Jupiter and Saturn. He demonstrated
+its strictly gravitational origin in the mutual disturbance of the two
+giant planets, rendered up to a certain point cumulative by the
+approximate commensurability of their periods. While Jupiter performs
+five circuits Saturn accomplishes nearly two, and the perturbation set
+up at their conjunction is hence both intensified and balked of
+compensation for 918 years.
+
+The epoch of trial and confirmation immediately following the
+publication of the “Principia” lasted then a full century. During its
+course, difficulties had arisen only to be overcome; suggested
+qualifications of the single and simple law of gravity had proved
+unnecessary; at its close, recalcitrance had everywhere been overcome,
+and there was victory all along the line. And not only were the workings
+of the planetary system exhibited as depending upon an elementary
+principle, but they were further shown to be perfectly equilibrated. It
+contained within itself, so far as could be ascertained, no seeds of
+decay; its destruction could only come from without. This remarkable
+conclusion was established in a series of splendid treatises by Lagrange
+and Laplace. The special adaptation to permanence of the solar mechanism
+was demonstrated in them. Ruinous disturbances were shown to be excluded
+by the overwhelming disparity of mass between the central body and its
+attendants, no less than by the regularity and harmony of their
+movements and distribution. Thus only slight oscillatory changes can
+occur. Millions of years will elapse without producing any fundamental
+alteration. The machine is so beautifully adjusted as to right itself
+automatically through the mutual action of its various parts. And it is
+the force which perturbs that eventually restores.
+
+The astronomical acquisitions of the century were embodied in Laplace’s
+“Mécanique Céleste,” published 1799–1805. This “Almagest of the
+eighteenth century,” as it has been termed, is in a rare degree
+comprehensive and complete. It leaves nothing enigmatical. Every
+question propounded in it receives an answer, if not definitive, at
+least highly authoritative; and the range of these questions is very
+wide. All the phenomena which the Greeks and Arabs had rightly observed,
+but wrongly interpreted, are not merely “saved” by geometrical
+artifices, but derived as a connected whole from one physical cause,
+absolutely prescribing that they should be thus, and no otherwise. The
+work is a record of unmixed triumphs. It seems as if the author, for
+want of more worlds to conquer, had laid down the sword of the calculus
+to take up the pen of the chronicler. With grave exultation, he proceeds
+from point to point, recounting the events of the campaign,
+commemorating the battles won by the brilliant staff of mathematical
+heroes to which he himself belonged, and expatiating in the broad
+subjugated plain. He scarcely looked beyond. There was indeed at that
+time no “beyond” where his methods of investigation were applicable. The
+“Mécanique Céleste” hints at no unsatisfied ambitions; it is a book of
+the _teres atque rotundus_ sort—a world in itself well arranged and
+compact, to which outlying perplexities are allowed no access. Nor
+should this be counted a defect. As a monument to one of the greatest
+periods in the history of science, its fitting character was that of an
+ordered collection of acquired certainties.
+
+The countrymen of Newton took no part in the striking series of
+operations by which the intricate consequences of the law of gravity
+were deduced and shown to correspond with reality. During the whole of
+the eighteenth century, they stood aside from the race towards
+verification. Their effacement was due to no lack of ability, but to a
+mistaken choice of means. Newton’s synthetic method was a veritable Bow
+of Ulysses. It was too tough to be bent by other hands than his own.
+Thus, no sequel could be given to the “Principia.” There was no
+possibility of following up the line of demonstration pursued in it.
+Newton himself would have vainly attempted to carry it much further. In
+order to advance, it was necessary, as Dr. Whewell remarked, to begin
+afresh. This, British mathematicians were unwilling to do. The easy and
+flexible analytical method brought to perfection on the continent
+remained strange to them. With inadequate strength, they persisted in
+wielding the cumbrous weapon of a giant—in using main force, so to
+speak, where skill and agility were required. Our insularity in this
+respect lasted until about 1816, when, by the joint efforts of the
+younger Herschel, Charles Babbage, and George Peacock (afterwards Dean
+of Ely), mathematical studies were revolutionised at the University of
+Cambridge.
+
+The neglect in England of theoretical research was, however, partly
+compensated by the steady progress of practical astronomy. For a century
+and a half after its foundation in 1675, the Royal Observatory at
+Greenwich continued to be the main—almost the only source of information
+regarding the places of the heavenly bodies. Thence were obtained the
+data necessary for the correction of theory, since there alone the
+visible positions of the sun, moon, and planets were systematically
+determined. _Actual_, compared with _predicted_, movements gave
+so-called “tabular errors”; and tabular errors indicated theoretical
+shortcomings, the rectification of which led gradually, but surely,
+towards a higher plane of knowledge.
+
+John Flamsteed (1646–1719), the first astronomer-royal, was, in
+Professor De Morgan’s phrase, “Tycho Brahé with a telescope.” By his
+diligence and insight he set on foot modern astronomy of precision. The
+“British Catalogue” of nearly 3,000 stars, was, in its day, an unique
+and most valuable work. His lunar observations were indispensable to
+Newton’s calculations, which, indeed, through the insufficient supply of
+them, now and again came to a halt; he constructed new solar tables, and
+kept watch over the careers of planets and comets. His completion, in
+1689, of a seven-foot mural quadrant, constituted a marked advance in
+the art of instrument-making. It was firmly fixed in the meridian, so
+that the distances from the zenith of the heavenly bodies at the moment
+of culmination could be read off on the limb, the time being
+simultaneously noted by a clock. Their positions in the sky relative to
+a set of forty otherwise known stars were thus completely determined,
+and they were determined essentially after the manner still in use.
+
+On Flamsteed’s death in 1719, Edmund Halley (1656–1742) succeeded to his
+place. An expedition to St. Helena in 1677, for the purpose of observing
+stars invisible in these latitudes, got him the name of the “southern
+Tycho.” They were the very first so situated to be located on the sphere
+(except those few that came within Ptolemy’s range), and a list of them,
+to the number of 341, was appended to the “British Catalogue.” The
+purpose to which Halley devoted most sustained attention was, unluckily,
+that in which he was least successful. Early in life he formed the
+design of observing the moon through an entire revolution of its nodes,
+so as to bring lunar tables to the perfection required for solving the
+prize-problem of longitudes. But the _contumax sidus_—his opprobrious
+term for our satellite—proved more than a match for him. The eighteen
+years’ watch was kept, notwithstanding that the watcher had reached the
+age of sixty-five before he was able to set about it; but in vain;
+nothing came of it. Halley’s varied performances were, nevertheless, so
+considerable as to warrant Lalande in describing him as “the greatest of
+English astronomers”; and he ranked next to Newton among contemporary
+English men of science.
+
+His cometary labours alone sufficed to perpetuate his name. He initiated
+the computation, on Newtonian principles, of the orbits traversed by
+such bodies—then a most toilsome process; and, among twenty-four, found
+three so much alike as to suggest the identity of the great comets of
+1531, 1607, and 1682. A renewed apparition might then be expected in
+1758, and he appealed to “candid posterity to acknowledge that this was
+first discovered by an Englishman.” The prediction roused widespread
+interest, and as the epoch for its fulfilment drew near, Clairaut
+undertook the formidable task of determining to what extent it might be
+postponed by the retarding influence of Jupiter and Saturn. Many times
+he despaired of its execution, even with the efficient aid of Lalande
+and Madame Lepaute, the wife of a Paris clock-maker; and at last, after
+months of wearisome calculation, having succeeded in forming the
+differential equations representing the comet’s disturbed motion, he
+threw down the paper on which they were written, with the exclamation,
+“Now, integrate them who can!” Eventually this, too, was done; and the
+comet, caught sight of on Christmas Day, 1758, by Palitzsch, a rustic
+star-gazer in Saxony, passed the sun within the month’s “law” permitted
+to it by the French geometer. This signal triumph laid the sure
+foundation of cometary astronomy.
+
+In 1679, Halley drew attention to the importance of transits of Venus
+for measuring the sun’s distance; and developed later a method
+extensively used in observing the eighteenth century pair of transits in
+1761 and 1769. But the accuracy actually attained in determining the
+instants of contact between the limbs of the sun and planet fell far
+short of what he had anticipated as attainable. The “black drop”
+interposed its pernicious effects, and occasioned wide discrepancies.
+The margin of uncertainty regarding the value of the great unit was,
+none the less, diminished, although it still remained uncomfortably
+wide; while the public interest excited by such rare events, the
+adventurous character of the expeditions sent to the uttermost parts of
+the earth for their utilisation, and the combined efforts of various
+nations towards the same end, served to popularise astronomy, and to
+give it something of that cosmopolitan stamp now borne by it.
+
+Besides the discovery of the secular acceleration of the moon’s motion,
+that of the long inequality of Jupiter and Saturn was due to Halley; he
+ascertained, in 1718, the proper movements of Sirius, Aldebaran, and
+Arcturus, thereby virtually demonstrating the non-existence of “fixed”
+stars; he associated auroræ with terrestrial magnetism; noted the
+globular star clusters in Hercules and Centaur; and divined nebulæ to be
+composed of “a lucid medium shining with its own proper lustre,” and
+filling “spaces immensely great.” Yet, in spite of the comprehensiveness
+of his genius, his administration at Greenwich was a failure. He was a
+better astronomer than astronomer-royal.
+
+James Bradley (1693–1762), who came after him, gave a narrower scope to
+his abilities, yet was of unsurpassed sagacity in connecting effects
+with their causes. Robert Hooke (1635–1703) had observed, in 1669,
+annual displacements of γ Draconis, a star nearly crossing the zenith of
+London, which he took for results of parallax; and Flamsteed, in 1694,
+had similarly interpreted a similar affection of the pole-star. They had
+both been misled by an “aberration,” due to the progressive transmission
+of light combined with the advance of the earth in its orbit. Bradley
+determined to sift the matter thoroughly, and observed Hooke’s star
+continuously from 1725 until 1728, first at Kew with Molyneux, then at
+Wanstead in Essex. It evidently described a small ellipse in the sky
+with a period of one year; yet its place in the ellipse was not what it
+should have been on the parallactic hypothesis; so he remained for some
+time in the dark about it. During a water-party on the Thames, however,
+in September 1728, he noticed that the slant of the pennant varied with
+changes in the boat’s course, the wind remaining steady throughout. This
+gave him the clue he wanted; and his discovery of the “aberration of
+light” was communicated to the Royal Society in the month of January
+following. That of the nutation of the earth’s axis followed in 1748.
+Both, setting aside their importance in themselves, were indispensable
+as preliminaries to accuracy in fixing the places of the heavenly
+bodies. For they are vital elements in the process of “reduction,” by
+which the ore of truth contained in observations is extricated from the
+dross of casual circumstances. The raw material, collected by timing
+transits and reading circles, must be so refined and purified that the
+facts contained in it become mutually comparable. Before Bradley’s time
+allowance was indeed roughly made for refraction in our atmosphere, and
+for the precession of the equinoxes; and, in the case of the moon, for
+parallax; but the effects of aberration and nutation had remained mixed
+up with a mass of disguising errors. Their elimination constituted an
+inestimable improvement.
+
+In the immediate art of observation Bradley was a master. He did not
+live to possess an achromatic telescope; neither astronomical circles
+nor equatorial mountings were at his disposal. His leading instrument
+was an eight-foot quadrant, by John Bird, certainly of admirable
+workmanship; although of a type long since, and for good reasons,
+superseded. He amassed with it, nevertheless, a treasure of high-class
+observations. The bulk of them remained in manuscript until 1798, so
+that it was reserved for this century to turn them to account; but their
+value has only developed with the efflux of years. Those relating to the
+moon and planets, reduced by Sir George Airy, lent efficient aid towards
+perfecting the theories of those bodies. Those of 3,222 stars formed
+into a catalogue by Bessel were published in 1818 with the proud, but
+not unmerited title of “Fundamenta Astronomiæ.” The same original data,
+again in 1886 reduced with the utmost nicety of care by Dr. Auwers of
+Berlin, afforded a splendid accession to knowledge of stellar proper
+motions. Acquaintance with Bradley’s stars now extends over 144 years;
+and the amount and direction of their progress across the sphere during
+that long interval have, for the most part, become defined with
+tolerable certainty.
+
+Nathaniel Bliss (1700–1764), the fourth astronomer-royal, filled the
+post only two years. Yet the observations made under his care form a
+sequel to Bradley’s well worth having. The reign of his successor, Nevil
+Maskelyne (1732–1811), extended over forty-six years. His determinations
+of the sun, moon, and planets, were in great demand abroad for the
+correction of tables, and as criteria of theories; while, of the stars,
+he paid attention only to thirty-six, catalogued as reference-points in
+1790. Their proper motions served Herschel for his second investigation,
+in 1805, of the sun’s translation through space. By the close of the
+century, Maskelyne’s instruments had lapsed into decrepitude; and only
+the stimulus supplied by Pond’s strictures roused him to order one of
+Troughton’s improved circles. But he died before it was mounted, and its
+employment fell to the share of his critic, John Pond (1767–1836), the
+sixth astronomer-royal. Maskelyne’s most enduring title to fame is his
+foundation, in 1767, of the “Nautical Almanac.”
+
+English observers were ably seconded by English artists. Graham, Sisson,
+Cary, Bird, Ramsden, had, from the beginning to the end of the
+eighteenth century, no foreign competitors of note. Their quadrants and
+sectors were distinguished both for stability and for refinement of
+execution. The mechanical skill displayed in their construction was no
+less necessary for the promotion of practical astronomy than the
+subtlety of eye and hand needed to employ them to the best advantage.
+Bradley’s work was conditioned by the performances of Graham and Bird.
+Without Graham’s sector he could not have discovered the aberration of
+light; without Bird’s quadrant the perennial worth of his Greenwich
+observations would have been impaired, if not destroyed. Observatories
+all over the continent were furnished in the latter half of the
+eighteenth century with instruments of English make; the art of
+accurately dividing circular limbs was invented in England, and nowhere
+else successfully practised. The innovation of substituting entire
+circles for quadrants was effectively introduced by Ramsden; and Piazzi
+came from Palermo in 1788 for the purpose of securing from him a
+five-foot altazimuth, at that date the finest sky-measuring machine in
+the world. Edward Troughton (1753–1835) ably carried on the tradition of
+his predecessors, and brought the altazimuth, transit circle, and
+equatorial up to the modern standard of efficiency. But they were no
+longer in exclusive demand. The foundation, in 1804, of Reichenbach’s
+Institute at Munich finally abolished the British monopoly in supplying
+astronomers with their exquisite and ingenious tools.
+
+The improvement of refracting telescopes ran a somewhat similar course.
+The essential step of combining flint and crown glass, so as to bring
+differently-coloured rays to one focus, was taken in 1733 by Chester
+More Hall, a gentleman of fortune in Essex; but he published nothing,
+and the re-invention of the “achromatic” lens was left to John Dollond
+(1706–1761) a Spitalfields weaver. “I obtained,” he wrote in 1758, “a
+perfect theory for making object-glasses, to the apertures of which I
+could scarcely conceive any limits.” The excise duty on glass, however,
+which was repealed only in 1845, drew these limits very narrowly in this
+country; and it was through the extraordinary perseverance of a Swiss
+artisan named Guinand, in overcoming the difficulties connected with
+glass-making, and the genius of Joseph Fraunhofer (1787–1826) in
+moulding the material thus placed at his disposal, that refractors began
+at Munich to rise towards their present power and perfection.
+
+The history of the reflecting telescope is British throughout. It was
+invented by Newton, made practically effective by John Hadley
+(1682–1744), and brought very near to theoretical perfection by James
+Short of Edinburgh (1710–1768); yet it is remarkable that not a single
+observation of lasting interest was made with any of his instruments, a
+few of which have survived, and are regarded with admiration to this
+day. The career of reflectors as engines of discovery began, but did not
+end, with William Herschel.
+
+
+
+
+                              CHAPTER II.
+                         A CENTURY OF PROGRESS.
+
+
+On March 13, 1781, an event occurred without precedent in the history of
+astronomy. A new member of the sun’s immediate retinue was disclosed. A
+hard-worked music-teacher at Bath performed this startling—indeed,
+according to antique notions—impossible feat; and the name of Herschel
+became known _urbi et orbi_. It was far from being by chance that the
+“new planet swam into his ken.” The Octagon Chapel organist was no
+ordinary lucky amateur. He had, some time previously, made two notable
+resolutions. The first was to push the improvement of telescopes to the
+furthest verge of what was possible; the second, to leave no corner of
+the starry heavens unexplored. And he applied himself with marvellous
+energy, in despite of accumulated professional engagements, to carry
+them into execution. He thus rapidly grew to be an adept in the art of
+constructing specula, and a master in the art of using them.
+
+Two lines of effort, accordingly, converged, in his case, towards
+celestial discoveries. With all his diligence in “reviewing” the
+heavens, he could not have distinguished at sight Uranus from a fixed
+star, but for the uncommon excellence of his seven-foot reflector; nor
+would the reflector, had it been used in the ordinary erratic fashion of
+casual stargazers, been at all likely to have encountered the little
+bluish disc of the remote orb then slowly wending its way through the
+constellation of the Twins. The direct, and a momentous result of the
+discovery was to secure for astronomy the undivided powers of the
+extraordinary man who had made it. George III. attached him to his
+Court, delivered him from the drudgery of teaching, and gave him the
+means of carrying out his grand designs.
+
+Their fulfilment involved the construction of great light-gathering
+machines. Herschel ardently desired to see as far and as much as the
+conditions of mortality permitted; he was the first to connect depth of
+penetration into space with extent of reflective surface; and he
+accordingly strained every nerve to secure the means by which to compass
+the end he had mainly in view. Nor was he content with mere size. His
+mirrors were as remarkable for beauty of figure as for breadth of
+aperture. They bore, on proper occasions, enormously high magnifying
+powers, and the precise roundness of the star-images formed by them
+excited the incredulous wonder of contemporaries. The quality of some of
+his largest instruments was guaranteed by the heavens themselves. Their
+approval was signified to the seven-foot reflector through the detection
+with it of Uranus; the “large twenty-foot,” with a speculum of eighteen
+inches, revealed in January 1787, two Uranian moons, Oberon and Titania;
+and the monster forty-foot, through the tube of which George III.
+promenaded with the Archbishop of Canterbury, brought into view, within
+three weeks of its completion, Enceladus and Mimas, the innermost and
+hardest to observe of Saturn’s numerous family of satellites.
+
+The forty-foot was “Herschel’s furthest”; he fully recognised that with
+it he had touched the line which divides failure from success. If,
+indeed, he had not overpassed it; for the subsequent career of the great
+telescope hardly bore out the promise of its start. It was an unwieldy
+engine, demanding vastly more time and labour to bring into play than
+the twenty-foot; and Herschel took such account of minutes as few men do
+of hours or days. His fiftieth birthday had in fact gone by before his
+optical ambition was satisfied; while his appetite for exploration was
+only whetted by what he had already accomplished. He estimated, however,
+that a “review of the heavens” with the forty-foot would have occupied
+800 years; hence it was used only on special occasions. The Orion nebula
+was the last celestial object upon which, January 19, 1811, “its broad,
+bright eye” rested; and it was then, with due honour, placed on the
+retired list.
+
+Two years before his death, which occurred August 25, 1822, the elder
+Herschel initiated his son into the secrets of speculum-building. The
+pupil was worthy of the master. John Herschel (1792–1871) aimed only at
+producing generally available instruments, and his success was easy and
+unqualified. His eighteen-inch mirrors seem to have been all but
+faultless. They certainly afforded him better views of the nebulæ than
+had been obtained by his father. Thus he first saw the “Dumb-bell” in
+its true oval shape; and his remarks upon annular lines of structure in
+elliptical nebulæ prove that features unmistakably imprinted upon Dr.
+Roberts’ photographs had been antecedently visible to him, and probably
+to him alone.
+
+The next stride in the enlargement of reflectors was made by an Irish
+nobleman, the third Earl of Rosse (1800–1867). His leviathan telescope,
+six feet in aperture, and fifty-four in length, has, in point of actual
+size, never been surpassed. Distinguished rather for light-grasp than
+for precise definition, it found its appropriate field in the nebular
+realms of the sphere; and the discovery of spiral nebulæ, with which it
+made its début, was one of high and wide significance.
+
+William Lassell (1800–1881) of Starfield, near Liverpool, set the
+example, in 1840, of mounting reflectors equatorially, so as to enable
+them, by the application of clock-work, to follow automatically the
+diurnal movement of the heavens. His specula were of almost unrivalled
+perfection in form and finish. One twenty-four inches in diameter, now
+at Greenwich, left a splendid record. With it Lassell detected, October
+10, 1846, the satellite of Neptune; September 18, 1848, simultaneously
+with W. C. Bond of Cambridge, U.S., Hyperion, the seventh in order of
+distance and last in order of discovery of Saturn’s eight moons; and
+October 24, 1851, Ariel and Umbriel, the inner pair of Uranian
+satellites, of which Sir William Herschel had possibly, although not
+very probably, caught transient glimpses. He erected a similar
+instrument of fourfold capacity at Malta in 1861, registered with its
+aid 600 new nebulæ, and delineated the complex structure of many others,
+previously less well seen.
+
+The four-foot reflector built in 1870 by Thomas Grubb of Dublin for the
+Melbourne Observatory disappointed expectation. An apparatus so delicate
+that the abrasion of 1/20,000th of an inch makes all the difference
+between good and bad definition, is ill-fitted to endure the
+rough-and-tumble experiences of an ocean-voyage; and that it in some way
+“suffered a sea-change” is scarcely doubtful. It was the last great
+telescope of its kind, metallic specula, having, in the seventies, been
+superseded by mirrors made of glass upon which a thin layer of silver
+has been chemically deposited. These have many advantages over their
+predecessors. They are considerably more reflective; they are more
+easily constructed; their shape is less liable to injury; their
+brilliancy, although more evanescent, can be readily restored. They have
+the drawback, however, of being extremely sensitive to changes of
+temperature. A three-foot mirror of this description by Calver, was
+employed by Dr. Common at Ealing with surprising success, early in 1883,
+for the purpose of photographing the Orion nebula. It was mounted at the
+Lick Observatory, California, in 1896. Dr. Common has since himself
+constructed a similar instrument of five feet aperture, which is the
+most potent light-collector ever yet turned to the skies. It is curious
+to learn that the silver spread over its surface weighs less than one of
+the “fourpenny bits” some time ago withdrawn from circulation; the
+reflecting film is in fact only 1/280,000 inch thick.
+
+Reflectors are perfectly, and _naturally_, achromatic, rays of all
+colours being thrown back at the same angle, and consequently meeting at
+the same focus. This gives additional brilliancy to the images formed by
+them, compared with those given by object-glasses, the colour-correction
+of which has hitherto been so imperfect that much light has to be
+“thrown away” as worse than useless. New kinds and combinations of
+optical glass have, however, of late been invented, by which this grave
+defect may be cured. Reflecting telescopes, on the other hand, are less
+manageable, and suffer more from distortion through change of position.
+Their cheapness recommends them to amateurs; but they should, on
+principle, be reserved for special departments of work, such as nebular
+photography and the chemical delineation of stellar and nebular spectra.
+
+The growth of refractors, like that of reflectors, has obtained from
+time to time the sanction of unexpected disclosures. Thus a superb
+fifteen-inch, turned out at Munich in 1847, for Harvard College,
+Cambridge, U.S., showed Hyperion to Bond, September 16, 1848, and on
+November 15, 1850, surprised him with a view of Saturn’s dusky ring.
+This telescope was surpassed, after fifteen years, through the energy
+and genius of Alvan Clark, the famous self-taught American optician,
+originally a portrait-painter at Cambridgeport, Massachusetts. Before it
+had left the workshop, an eighteen-inch achromatic, now the leading
+instrument at the Dearborn Observatory, Evanston, Illinois, won maiden
+honours by disclosing to Alvan G. Clark, one of the maker’s sons,
+January 31, 1862, the dim companion of Sirius, which, before being seen,
+had made itself _felt_ by gravitational disturbances of its radiant
+primary. The Washington twenty-six-inch, by the same firm, was rendered
+illustrious by Professor Hall’s discovery, in 1877, of a pair of Martian
+moons; the Lick thirty-six-inch, by bringing within the range of
+Professor Barnard’s keen eyesight, September 9, 1892, Jupiter’s tiny
+“fifth satellite.” The diploma performance of the Yerkes forty-inch,
+mounted in 1896 at the Chicago University Observatory, is yet to come.
+Meanwhile, several very perfect refractors, up to thirty-two inches of
+aperture, have been built on this side of the Atlantic by Sir Howard
+Grubb of Dublin, and the MM. Henry of Paris; and a twenty-five-inch,
+finished so long ago as 1868, and at the cost of his life through the
+labours which it entailed, by Thomas Cooke of York, after having lain
+for upwards of a score of years choked by the fog and smoke of
+Gateshead, has recently begun a promising career at Cambridge, under the
+care of Mr. Frank Newall, son of the original owner.
+
+And now we cannot but ask ourselves, has the _ne plus ultra_ in
+telescopic magnitude been attained? There is no reason to suppose that
+it has, provided that due allowance be made for inexorable conditions.
+Climate is one of these. The largest instruments are those most readily
+crippled by atmospheric hindrances. The greater their powers, the fewer
+are the nights on which they are likely to be available. If they are to
+“shine in use,” and not “rust unburnished,” they must then be erected in
+exceptionally favourable localities, such as the summit of Mount
+Hamilton (the site of the Lick Observatory), or the Harvard College
+southern station at Arequipa in Peru. In South Africa, too, but “up
+country”—not in the Cape peninsula—splendid facilities for astronomical
+observation are to be found.
+
+From Professor Keeler’s report it can readily be gathered, and he indeed
+explicitly states, that the Yerkes forty-inch marks the limit of useful
+size in equatorials. For the character of the star-images formed by it
+slightly change their character when it is directed to different parts
+of the sky; and this implies that its lenses become, as it moves,
+infinitesimally deformed through the effects of their own weight. No
+larger instrument, accordingly, can safely be permitted to swing in
+mid-air. The huge light-concentrating machines of the future will lie in
+wait for the objects to be observed, instead of pursuing them. They will
+either be supported horizontally, or mounted in the “Coudé” fashion
+invented by M. Loewy. In either case, the necessary movement will be
+performed vicariously by a plane mirror.
+
+Thus, the optical and mechanical outlook is decidedly better than the
+atmospheric. The question, How to build giant telescopes? is more easily
+answered than the question, Where to place them when built? The ultimate
+barrier to seeing indefinitely far into space is the rigid circumstance
+that we live on an air-girt globe. The prospects of astronomy are deeply
+involved in the forecast of its hampering effects. The dependence of
+those prospects upon telescopic improvements became obvious when
+Herschel took the whole contents of the sphere “for his province.” These
+are indefinitely numerous, indefinitely far-off, indefinitely faint. The
+task of their correlation undertaken by Herschel, and inherited from him
+by modern astronomers, can at no time be more than approximately
+fulfilled; but for each successive approximation more light is needed.
+Those who would investigate the universe can never get enough of that
+too scarce commodity.
+
+Until Herschel conceived the novel idea of a comprehensive science of
+the stars, they had been chiefly regarded as convenient sky-marks, by
+which to track the wanderings of our nearer neighbours in space. When it
+was perceived that the sky-marks were not fixed, it became necessary to
+determine their movements; and this was very roughly done for
+fifty-seven stars by Tobias Mayer of Göttingen, in 1757; and more
+accurately for thirty-six by Maskelyne, a third of a century later. But
+if the stars were travelling, the sun could not be supposed to stand
+still; and the possibility of laying down his line of march through
+space, by extricating a common element from the confused network of
+mutually-crossing stellar paths, occurred to Mayer, and was actually
+realised by Herschel in 1783. His inquiry, with the scanty materials
+then at command, was a wonderful stroke of audacity, which very nearly
+hit the mark; yet few believed in his result until it was confirmed by
+Argelander in 1837.
+
+The various attempts made, prior to 1782, to measure the parallaxes of
+some of the brighter stars were instigated by the wish to find a
+demonstrative argument in favour of the Copernican theory of our system.
+They had no reference to sidereal structure. Herschel, however, took up
+the subject simply for the purpose of fixing the scale of that vast
+edifice. Before sounding the skies, he sought to ascertain the length of
+his fathom-line. He never ascertained it. To the end of his life, he
+could only make plausible assumptions as to the distances of the stars.
+Their real parallaxes were insensible with his instrumental means. But
+he fortunately chose for his experiments Galileo’s “double-star method.”
+This consisted in determining the relative positions of two close stars,
+one of which, taken to be indefinitely remote, was designed to serve as
+a standard of reference for the perspective shiftings of the other. It
+was thus that Herschel’s attention was directed to double stars. He
+found them to be astonishingly numerous—far more numerous than could
+have been anticipated by the doctrine of probabilities. In January,
+1782, he presented to the Royal Society a catalogue of 269 star-pairs,
+and he had collected 434 more by December, 1784. From their abundance
+alone, the Rev. John Michell inferred their character of binary systems;
+and Herschel, after twenty years of observation, was able, in 1802, to
+announce the fact of their mutual revolutions. Thus was taken the second
+great step towards the unification of the Cosmos. Newton proved that
+terrestrial gravity dominates the solar system; Herschel showed that a
+law of attraction, presumably (and assuredly) identical in its mode of
+operation, extends through sidereal space.
+
+One cannot reflect without amazement that the special life-task set
+himself by this struggling musician—originally a penniless deserter from
+the Hanoverian Guard—was nothing less than to search out the
+“construction of the heavens.” He did not accomplish it, for that was
+impossible; but he never relinquished, and, in grappling with it, laid
+deep and sure the foundations of sidereal science. No one before him had
+thought of approaching the subject otherwise than by way of speculation;
+he alone had the boldness to attack it experimentally. Having invented
+for the purpose an ingenious method of “star-gauging,” based upon the
+hypothesis that the stars are, on an average, scattered evenly through
+space, he concluded in 1784, from its application, that the Milky Way is
+the visual projection of a disc-shaped stellar aggregation, within which
+our sun is somewhat excentrically placed. The progress, however, of his
+telescopic studies convinced him that the continued action of a
+“clustering power” had long ago drawn the stars into many separate
+allotments, and annulled the original uniformity of their distribution.
+So the disc theory was given up, and the Milky Way came to be regarded
+as a collection of genuine clusters, arranged into an irregular ring
+encircling the solar system. This view, implicitly held by the elder
+Herschel from 1802, was explicitly stated by his son in 1847. The
+results that Herschel expected from star-gauging may, in the future, be
+derived from the more elaborate process of star-gauging by magnitudes,
+photographically executed; and the sky-charting work, rapidly
+progressing in all parts of the world, will at least supply ample
+materials for sounding the star-depths.
+
+These are stored besides with the curious objects called “nebulæ.” They
+were little noticed until Herschel, on March 4, 1774, made
+
+                     “That marvellous round of milky light
+             Below Orion,”
+
+the subject of his earliest recorded observation. Except, indeed, as
+impediments to comet-hunting. Thus, Messier, one of the keenest
+sportsmen in that line who have ever scanned the sphere, tried to
+eliminate by enumerating them, and drew up in 1771 a list of 45 such
+misleading objects, enlarged in 1781 to 103. And Lacaille, during an
+expedition to the Cape in 1752–1755, picked up 42 more. So far this
+department of knowledge had been cultivated when Herschel began to
+“sweep the heavens.” To _sweep_ them, be it remembered. Not merely to
+gaze at hap-hazard, or to look out for show specimens, but to gather in
+the celestial harvest methodically, zone by zone, so as to “leave no
+spot of the heavens unvisited.” The fruits were proportioned to his
+diligence. The nebulæ discovered by him amounted, in 1802, to 2,500. And
+he did not merely discover; he investigated them as well. He separated
+them into classes, noted the mode of their distribution, and searched
+out their relationships. To begin with, he believed them to be of a
+purely stellar nature—to be, in fact, independent galaxies. Miss Burney
+was informed by him in 1786 that he had “discovered fifteen hundred
+universes.” A few years later, however, he reasoned out for himself the
+gaseous nature of a great many nebulæ, such as that in Orion, and those
+of the “planetary” sort; and published in 1811 a complete theory,
+strikingly illustrated with examples taken from his telescopic
+experiences, of stellar development out of nebulous stuff. The
+supposition that they included the revelation of “exterior universes”
+was thus rendered, to say the least, superfluous; yet it was not
+perhaps, even by him, wholly abandoned. It was, moreover, revived in
+consequence of the performances of the great Rosse reflector, from 1845
+onwards, in resolving apparent nebulæ into “bee-like swarms” of stars.
+Meanwhile Sir John Herschel’s examination of those wonders of the
+southern heavens, the Magellanic Clouds, had virtually decided nebular
+standing. For they contain within a limited compass, as Dr. Whewell
+argued in 1853, “stars, clusters of stars, nebulæ, regular and
+irregular, and nebulous streaks and patches. These, then, are different
+kinds of things in themselves, not merely different to us.” That stars
+and nebulæ co-exist in every part of the heavens, has since been fully
+established; while the laws respectively governing their distribution
+over the sphere are related in such a manner as to leave no doubt that
+these two classes of sidereal objects unite to form the grand galactic
+whole. Hence, to all reasonable apprehension, “island universes” have
+vanished into the inane.
+
+Sir John Herschel accomplished the unparalleled feat of sweeping the
+heavens from pole to pole. Having, within eight years from 1825, revised
+his father’s work at Slough, he conceived the noble idea of rounding it
+off in the southern hemisphere; and, in 1833–4, transported his
+instruments from Slough to Feldhausen near Cape Town. During the four
+years of his residence there, he not only executed his proposed survey,
+registering 1,790 nebulæ—300 of them for the first time—and discovering
+and measuring 2,100 double stars, but carried out a number of special
+researches. He catalogued the miscellaneous contents of the Magellanic
+Clouds—systems _sui generis_, as he justly termed them—made a detailed
+and laborious study of the Argo nebula, applied pretty extensively the
+paternal method of star-gauging, observed Halley’s comet at its second
+predicted return, measured the sun’s heat-emissions, carefully watched
+the spot-maximum of 1837, and finally, struck with a sudden rise in
+magnitude of η Argûs, brought to general knowledge that star’s
+extraordinary character. These varied results were embodied in a
+monumental volume, published in 1847.
+
+One of the greatest triumphs of modern science has been the
+establishment of an “Astronomy of the Invisible.” It was primarily due
+to Bessel’s inquiries into the disturbed proper motions of the
+“Dog-stars,” Sirius and Procyon. They convinced him that each of these
+brilliant orbs is attended by a massive satellite, round which it
+revolves as it advances, its path in the sky being thus not straight but
+wavy. Telescopic verification of his forecast was, nevertheless, delayed
+until 1862 in the case of Sirius, until 1896 as regards Procyon. The
+earliest, and still the most memorable result in this line is the
+discovery of Neptune. Bessel knew that the thing was to be done, and in
+1840 planned the doing of it. But his powers began, soon afterwards, to
+be crippled by deadly illness, to which he succumbed, March 17, 1846.
+_Uno avulso, non deficit alter._ Adams and Leverrier separately
+undertook the enterprise he had relinquished, and each with perfect
+success. It was a formidable one. The _direct_ problem of perturbations
+taxes the highest mathematical resources; the _inverse_ problem is not
+only more arduous, but was then untried. Laplace and Lagrange had shown
+how to determine the perturbations produced by a known disturbing body;
+it was left for Adams and Leverrier to find an unknown body through its
+disturbing effects. Irregularities in the movements of Uranus betrayed
+the presence of Neptune, and by the powerful analysis brought to bear
+upon them, were made to serve as an index to his actual place in the
+heavens at a given epoch. This was done by Adams in September, 1845; but
+his calculations, deposited at the Royal Observatory in the hope that
+they would incite to a telescopic search for the new planet, remained
+there buried in a drawer. Sir George Airy had no faith in them, and he
+unaccountably received no reply to a test-question addressed to their
+author. In the following June, however, he was roused by the
+intelligence of Leverrier’s advance towards the goal already attained by
+Adams, to arrange an exploratory campaign with the Cambridge
+“Northumberland equatorial.” But here again, disbelief—reinforced by the
+absence of a detailed star-map—stepped in to retard proceedings
+conducted by Professor Challis in so leisurely a fashion that the object
+“wanted” was found before he had sifted his observations, September 23,
+1846, by Galle of Berlin, acting under Leverrier’s precise directions.
+It proved on inquiry to have been twice observed at Cambridge during the
+previous couple of months.
+
+Gravitational astronomy won its crowning distinction by the discovery of
+Neptune. It afforded the first instance of a body made known as an
+unseen power previously to being visually detected. Many stellar
+systems, however, have since then been ascertained to include members
+which can only be _felt_, owing to their partial, if not total
+obscurity. Again, the spectroscope tells of the existence of others
+entirely beyond the range of direct vision with the most powerful
+optical appliances; not because they do not shine (although this is
+sometimes also the case), but because they revolve so close to their
+primaries as to form with them single and indissoluble telescopic
+objects.
+
+The spectroscope and the photographic camera have been mentioned as aids
+to astronomy. Their adoption has profoundly modified the science,
+widening its borders, inviting it to undertake novel tasks, endowing it
+with previously undreamt-of powers. Realms of knowledge deemed
+inaccessible to human faculties have, as if at the touch of a magician’s
+wand, been thrown open; and of the many paths leading into the interior,
+only a few have yet been pursued, and that for a short distance. The
+prospects of exploration are hence unlimited, and of bewildering
+variety.
+
+Spectrum analysis is essentially a chemical method. It depends upon the
+principle firmly established in 1859 by Kirchhoff and Bunsen, two
+professors at the university of Heidelberg, that different kinds of
+glowing vapour give out distinctive rays of variously coloured light,
+commonly called “lines,” simply because, for the purpose of getting rid
+of overlapping images, and for convenience of measurement, they are
+transmitted through a narrow slit. Thus, the presence of a familiar, and
+almost ubiquitous deep-yellow line, named by Fraunhofer “D,” and shown
+by a moderately powerful apparatus to be double, _infallibly_ testifies
+to the presence of sodium; iron, rendered gaseous by heat, gives out
+several thousand lines ranging from end to end of the spectrum, not one
+of which is common to any other substance; hydrogen shows a radiant
+sequence exclusively its own; and so of all the remaining elements. To
+apply this mode of detection, the light from the source to be studied
+must be analysed, or dispersed into its various component colours
+through the unequal action upon them of a prism, or train of prisms.
+Dispersion can also be effected by “diffraction”; and since the spectrum
+thus produced is “normal,” or dependent wholly upon wave-length, it is
+always employed where a high degree of exactitude is aimed at. The
+coloured fringes of shadows originate in this way, through the
+interference of ethereal undulations; while the rainbow is a prismatic
+phenomenon, drops of water performing the refractive office of actual
+prisms.
+
+The rainbow exemplifies too—although less perfectly than the electric
+light—what is called a “continuous spectrum.” Its tints merge one into
+the other insensibly, without any sensible dark interruption. Now,
+incandescent liquids and solids of every kind and quality give
+rainbow-like spectra; they emit light which _rolls out_ into an unbroken
+band of colour. Hence there is nothing characteristic about them. They
+are to the chemical enquirer absolutely uncommunicative. Vapours and
+gases alone can be induced to show the _badge_ of their particular
+nature.
+
+Celestial spectrum analysis began with the sun. The solar spectrum is
+furrowed transversely by a multitude of fine dark lines, known as
+“Fraunhofer lines,” because Fraunhofer brought them within scientific
+cognisance by carefully mapping and measuring them. Their significance
+remained a standing puzzle until Kirchhoff, in 1859, furnished the key
+to it, by demonstrating the correlation of radiation and absorption. In
+other words, vapours and gases have the faculty of arresting those
+precise rays of light which they are in a condition to emit. Hence, the
+ignited, although relatively cool vaporous envelope of a white-hot body
+like the sun, or the carbons of the electric arc, acts predominantly as
+an intercepting medium, stopping more than it sends out of its peculiar
+rays. There results a continuous spectrum crossed by dark lines of the
+same chemical significance as if they were bright. They would, in fact,
+show as bright if the brilliant background, upon which they are seen
+projected, could be withdrawn. The interpretation, upon this principle,
+of the Fraunhofer lines, proved the sun to be surrounded by hydrogen in
+vast quantities, by incandescent sodium, magnesium, iron, calcium, and a
+number of other metals. Spectrum analysis in this way assumed a double
+aspect. The hieroglyphics of coloured light were rendered legible,
+whether positively or negatively written. And the spectra of the
+heavenly bodies are actually found to be inscribed, some in one way,
+some in the other; not unfrequently, in both combined.
+
+The new and marvellous power of investigation thus acquired was in 1864
+applied to the stars by Dr. Huggins and his coadjutor, Professor W. A.
+Miller. They ascertained the presence in the atmospheres of Aldebaran
+and Betelgeuse, of nine or ten terrestrial elements, thereby setting on
+foot the science of stellar chemistry. Moreover, on August 29, in the
+same year, Dr. Huggins made the signal discovery of gaseous nebulæ.
+Admitting the dim rays of a “planetary” in Draco through the slit of his
+spectroscope, he perceived it to be composed of three bright green
+lines, one of them Fraunhofer’s “F”—an emanation of hydrogen. This one
+observation verified after seventy-three years Herschel’s inference of
+the existence in the heavens of a “fiery haze,” destined, according to
+his long forecast of creative processes, eventually to “subside into
+stars.”
+
+By the discovery of celestial spectrum analysis, a third stadium of
+progress towards the unification of the sciences was reached. The first
+step was taken with the demonstration that the force retaining the
+planets in their orbits is no other than that which causes rivers to
+flow, and apples to fall upon the earth. The extension of the same law
+to the stellar universe through the discovery of binary stars, showing
+that matter, wherever existing, possesses at least one unchanging
+quality, constituted the second. It was now learned that the sun and
+stars were composed of the identical _species_ of matter scattered in
+the dust of the earth, dug up from its bowels, condensed to make its
+oceans, entering into the very framework of our own bodies. An universal
+chemistry was established, based upon the relations of light to material
+molecules, and of material molecules to the ether filling space; and, as
+an inevitable consequence, the new branch of knowledge, termed
+“astrophysics,” made its ardently welcomed advent. By it astronomy has
+entered into close alliance with the rest of the sciences. No laboratory
+experiment is any longer indifferent to her; and laboratory experiments,
+on the other hand, derive from the connexion vastly augmented
+importance. The youth of learning seems renewed. Secrets of nature,
+formerly believed to lie beyond the scope of investigation, have been
+penetrated; _nil desperandum_ is the motto which astro-physicists have
+earned the title to adopt as their own.
+
+The old art of direct observation has, during the latter half of the
+present century, developed in sundry novel directions. By the use of
+auxiliary appliances, the telescope has gained a wonderful increase of
+subtlety and power. Modern astronomical work may be divided into four
+classes:—telescopic, spectroscopic, photographic, and spectrographic or
+spectrophotographic. Daguerre’s invention was almost immediately tried
+with the sun and moon; J. W. Draper and the two Bonds in America,
+Foucault and Fizeau in France, and Warren de la Rue in this country,
+being among the pioneers of celestial photography. But it was not until
+after the introduction of the collodion process that really useful
+results were obtained. With the regular employment at Kew, from 1858
+onwards, of De la Rue’s “photoheliograph,” began the daily
+selfregistration of sun-spots, suggested by Sir John Herschel in 1847;
+and pictures of the eclipsed sun, obtained with the same instrument at
+Rivabellosa in Spain, July 18, 1860, terminated a prolonged dispute as
+to the nature of the red prominences by exhibiting them as undeniably
+solar appendages. Lunar photography was meanwhile successfully
+prosecuted, and Henry Draper’s picture, of September 3, 1863, remained
+unsurpassed for a quarter of a century. Star-prints were first secured
+at Harvard College, under the direction of W. C. Bond in 1850; and his
+son, G. P. Bond, made, in 1857, a most promising start with double-star
+measurements on sensitive plates, his subject being the well-known pair
+in the Tail of the Great Bear. The competence of the new method to meet
+the stringent requirements of exact astronomy was still more decisively
+shown in 1866 by Dr. Gould’s determination from his plates of nearly
+fifty stars in the Pleiades. Their comparison with Bessel’s places for
+the same objects proved that the lapse of a score of years had made no
+sensible difference in the configuration of that immemorial cluster; and
+Professor Jacoby’s recent measures of Rutherfurd’s photographs, taken in
+1872 and 1874, enforced the same conclusion. To the “collodion period”
+also belongs the earliest spectrograph, taken by Dr. Huggins in 1863;
+but the analysed light of Sirius left an uncharacteristic, although a
+strong impression. No lines were visible in it; a “virgin page” was
+presented. Before prosecuting the subject, fresh developments had to be
+awaited.
+
+The invention of gelatine dry plates was the decisive event in the
+history of celestial photography. Dr. Huggins turned it to account with
+marked success for depicting the spectrum of Vega, December 21, 1876,
+and was able, three years later, to exhibit to the Royal Society
+photographs of the spectra of six white, or Sirian stars, stamped with
+the ultra-violet series of hydrogen lines, then for the first time
+recognised, whether on the earth, or in the sky. The uses of the camera
+have since then multiplied at a prodigious rate. Its versatility appears
+unbounded. There are very few departments of astronomy left in which the
+eye has the advantage over it. A volume might be written on its
+successes; its comparative failures would scarcely fill a page. Its
+extraordinary power of penetrating space would have amazed and delighted
+William Herschel. This is due to the indefinitely prolonged exposures
+rendered practicable by the employment of dry plates; and these
+exposures can be interrupted and resumed at pleasure. Three-night
+photographs are now quite commonly taken, following the example given by
+Dr. Roberts in 1889. Now every additional minute of exposure brings
+intelligence from further and further sky-depths, owing to the happy
+faculty of sensitive plates for accumulating impressions. The eye sees
+at once, or not at all; the chemical retina sees by degrees, storing up
+insensible effects until they become sensible, and this without
+definable limit. This is its most essential prerogative. For the
+portrayal of nebulæ and comets, it is inestimable; and by its means the
+boundaries of the sidereal system may be laid down before the twentieth
+century is far on its way. A picture of the great comet of 1882,
+standing out from a richly spangled background, taken at the Cape
+Observatory under Dr. Gill’s direction, was the object-lesson by which
+the advantages of photographic star-charting were effectually learnt.
+They have been practically illustrated in the _Cape Durchmusterung_, a
+southern continuation, by photographic means, of Argelander’s
+corresponding telescopic work at Bonn; and are being turned to account
+on a magnified scale, in the International Survey of the heavens, now in
+progress at seventeen observatories scattered over the face of the
+globe. Special problems have, meanwhile, been investigated with striking
+success, by the chemical method, and its fresh applications are
+innumerable. Hitherto, performance has usually outrun promise; but
+promise has now so quickened its pace as to make the issue of the race
+dubious. We can only be sure that the future will be full of surprises.
+
+
+
+
+                               ASTRONOMY
+
+
+[Illustration:
+
+  THE LICK REFRACTOR OF THIRTY-SIX INCHES APERTURE.
+]
+
+
+
+
+    SECTION II.—GEOMETRICAL ASTRONOMY AND ASTRONOMICAL INSTRUMENTS.
+
+                    BY A. FOWLER, A.R.C.S., F.R.A.S.
+
+
+
+
+                               CHAPTER I.
+                      THE EARTH AND ITS ROTATION.
+
+
+It is a common remark that we are creatures of circumstances, and in no
+sense is this truer than in its application to the conditions under
+which we view the heavenly bodies. At the commencement of a study of
+astronomy it is accordingly important to first ascertain as far as
+possible the nature of the earth on which we are situated, and to
+determine in what way our observations are affected by our local
+conditions.
+
+THE HORIZON.—When we look at the sky we see a vast hemispherical vault
+of which we seem to occupy the centre. If we are at sea, the water and
+sky appear to meet at a certain distance, in whatever direction we look.
+Where these meet we have what is called the visible horizon. On land,
+the horizon is usually broken up by terrestrial objects, such as hills,
+buildings, or vegetation, but otherwise the appearances are the same as
+at sea.
+
+SHAPE OF THE EARTH.—When we observe the horizon, whether from land or
+sea, our eyes are at a certain elevation above the level of the ground
+or water, as the case may be, and the higher we are situated, the
+greater is the distance of the visible horizon, although the circular
+outline is retained. No matter where we may be, the same appearances are
+noted, and we are thus led to infer that the earth is a globe, as no
+other shape could appear circular from all points of view.
+
+There are other considerations which lead to the same conclusion with
+regard to the shape of the earth. One of the most familiar proofs that
+the earth cannot be flat is found in the aspects of a ship putting out
+to sea or coming into port, when observed from a somewhat elevated
+position on shore. A ship does not become visible in its entirety, as it
+would if diminishing distance were the only cause affecting its
+visibility; the masts are seen first, and then the lower parts of the
+vessel gradually make their appearance. This finds a simple explanation
+in the curvature of the surface of the sea, and as similar appearances
+can be seen in all parts of the world, a globular form is indicated.
+
+The fact that one may continue to travel westward and yet return to the
+point of starting, is quite in harmony with the supposition that the
+earth is globular, but it does not furnish a proof. This facility would
+evidently be equally afforded by a cylindrical earth, or even by a flat
+earth of which the Pole occupied the centre.
+
+Still another indication of the rotundity of the earth is given by the
+phenomena of an eclipse of the moon. On these occasions, as will appear
+later, the moon passes through the shadow of the earth, and as this
+shadow is always circular, nothing but a spherical, or nearly spherical,
+body can be in question.
+
+SIZE OF THE EARTH ROUGHLY MEASURED.—Granting then that the earth is
+spherical, a measurement of its curvature will enable us to determine
+its size. To do this it is necessary to measure the distance of the
+visible horizon from the eye at a known elevation. Then it can be shown
+that if the height of the eye is only a small fraction of the diameter
+of the earth, the diameter is as many times larger than the distance of
+the horizon as that distance is greater than the height of the eye.
+Thus, to an observer whose eye is 5 feet above sea level, the horizon is
+2¾ miles distant, while from the top of a lighthouse 66 feet high the
+sky would appear to meet the sea at a distance of 10 miles. One way in
+which an approximate measurement may be made is illustrated in Fig. 1.
+Three posts are placed in line, with their tops at the same height above
+the surface of some calm stretch of water such as is afforded by a
+canal. A telescope fixed to the first post, so that its centre is at the
+top, is directed to the upper end of the third post, and it is seen to
+sight the middle one at some distance from the top. When the posts are a
+mile apart, the line joining the two extremes turns out to be 8 inches
+below the top of the middle one.
+
+[Illustration:
+
+  FIG. 1.—_Rough Measurement of Earth’s Diameter._
+]
+
+In our diagram this 8 inches is represented by the distance _b d_, and
+if we imagine an arc of a circle _d e_ concentric with the surface of
+the water, the part which it intersects on the end post, namely _a e_,
+will also be 8 inches. This means that to an eye at _a_, 8 inches above
+the surface represented by _d e_, the visible horizon at _d_ would be a
+mile distant. Applying the proportion named above, it results that the
+earth is 7,920 miles in diameter.
+
+Owing to various causes, this method only furnishes a rough indication
+of the dimensions of our globe; but, if we had no other evidence, the
+result would suffice to explain that the irregularities of the earth’s
+surface, though seeming so large to us who dwell upon it, are not
+inconsistent with the idea that the surface forms part of a sphere. The
+highest mountains with which we are acquainted do not exceed 5½ miles in
+height, and this is only ¹⁄₁₄₀₀th part of the earth’s diameter. On a
+globe 14 inches in diameter, representing the earth, the highest
+mountains would be less than a hundredth of an inch on the same scale;
+so that, taking the earth generally, it is practically a smooth globe.
+
+DIFFERENT HORIZONS AT DIFFERENT PLACES.—So far then we have learned that
+the earth is a globe about 8,000 miles in diameter. This enables us to
+understand that persons in different parts of the earth will see the sky
+in different ways. At any given place we can see only what is above our
+horizon, and it results from the spherical form of the earth that no two
+observers have precisely the same horizon. If we consider a section of
+the earth, such as is shown in Fig. 2, an observer at the point _a_ will
+have a horizon represented in section by the line _b c_, while the
+horizon of an observer at _d_ will be represented be _e f_. It is clear
+then that an external distant object, such as the sun or a star, which
+may appear on the horizon in the direction _a b_, as seen from the point
+_a_, will be at a considerable angle above the horizon when seen from
+the point _d_.
+
+[Illustration:
+
+  FIG. 2.—_Horizons at Two Places on the Earth._
+]
+
+SENSIBLE AND RATIONAL HORIZON.—Having this conception of the horizon as
+a thing terrestrial, we may consider its astronomical relationships a
+little further. If we imagine the plane of the horizon prolonged until
+it cuts the distant sphere on which the stars and other celestial bodies
+seem to lie, it will meet that sphere in what is called the _sensible
+horizon_. A parallel plane passing through the centre of the earth is
+called the _rational horizon_, but as the starry sphere is at an almost
+infinite distance, the rational and sensible horizons coalesce into one
+celestial horizon.
+
+Closely associated with the horizon is the point vertically overhead
+which is called the _zenith_, and the point vertically below which is
+called the _nadir_. As the plane of the horizon is tangential to the
+earth’s surface at the point of observation, the zenith is simply the
+prolongation into space of the line joining the centre of the earth with
+the place of observation; at the point _a_ in Fig. 2, for example, the
+zenith is in the direction _o a z_.
+
+The zenith as thus defined, however, is not the astronomical zenith, but
+what is called the geocentric zenith. As will appear later, the earth is
+not truly spherical, so that the direction of gravity does not pass
+exactly through the earth’s centre, and the astronomical zenith is
+overhead in the direction of gravity.
+
+DIURNAL MOTION OF THE HEAVENS.—In the day-time, when the sky is clear,
+we see the sun; at night, we sometimes see the moon, always some stars,
+and occasionally a comet. If we continue our observations, even for a
+few hours, we begin to recognise that the heavenly bodies have an
+apparent movement towards the west, very similar to the daily motion of
+the sun, with which everyone must have been familiar from childhood.
+
+Continuing such observations, it is found that the great majority of the
+stars do not appear to change their positions relatively to each other,
+although their apparent places in the sky are different at different
+times. These have consequently been called the “fixed stars,” but in the
+light of our present knowledge, the name is not to be taken too
+strictly. On account of this seeming fixity, the stars have been divided
+from very remote times into _constellations_, or groups, which enable us
+to name and identify individual members of the starry host. Other bright
+objects having the appearance of stars, when they are viewed merely by
+the naked eye, may be seen to change their positions with regard to the
+stars in that part of the sky in which they appear. These are the
+_planets_—the “wandering stars” of the ancients, to whom five were
+known, namely, Mercury, Venus, Mars, Jupiter, and Saturn.
+
+Comets also are seen to share in the general westward movement of the
+heavenly bodies, but, in addition, they have another movement relative
+to the stars situated in the same part of the sky.
+
+If we closely observe the stars in Europe, we shall find some of them
+rising due east, and setting due west; others, again, will be found to
+rise in the north-east, and to travel nearly overhead; still others will
+be seen to rise south of east, attain only a small elevation above the
+horizon, and pass from our view as far south of west as they rise south
+of east. One point in the heavens appears stationary, and all the stars
+seem to traverse their daily courses round this as a centre. This
+stationary point is the north _celestial pole_. It is marked by no star,
+but a fairly conspicuous star is at present only about a degree and a
+half removed from it The name given to this star is the Pole Star, or
+Polaris. As seen from London, stars within 51½° from the celestial pole
+never set, and such stars are said to be _circumpolar_.
+
+When our place of observation is changed from one of middle latitude to
+one very near the Equator, these appearances are modified. We still see
+the stars rising and setting daily, but there will be _two_ points which
+do not seem to move, one on the northern and the other on the southern
+horizon. One of these stationary points is identical with that seen from
+higher latitudes, and the other, which is called the _south celestial
+pole_, is diametrically opposite to it What is more, stars which were
+not visible at all at our first place of observation will be seen in the
+south. All the stars will rise and set, and will alike be above the
+horizon for twelve hours.
+
+If we could see the stars from the North Pole, the Pole Star, which is
+on the horizon of places at the Equator, would be found overhead, and
+all the stars visible to us would be ever above the horizon. Not only
+this, not one of the glittering stars which adorns the southern heavens
+would ever be seen at all.
+
+In place of the rising and setting of stars, which lends such a great
+interest to their observation in other parts of the world, as seen from
+the poles the stars will simply travel round and round in circles
+parallel to the horizon.
+
+To produce the apparent daily revolution of the heavens, and the changes
+in the appearances observed at different places, one of two causes must
+be at work; either the celestial bodies themselves must be performing a
+daily majestic movement from east to west round a motionless earth, or
+the earth itself must be whirling round from west to east, and so
+changing the situation of the observer’s horizon with regard to external
+bodies. In the early days of astronomical observations this observed
+revolution of the heavens was thought to be real, but, with our present
+knowledge, we are no longer justified in regarding the earth as
+occupying a place of any such importance as that of the centre of the
+universe. By the earth’s rotation, an observer, unless situated exactly
+at the North or South Pole, is carried round in a circle, and his
+horizon is gradually swept round so that on one side stars are setting
+and on the other side rising. The appearances at different places find a
+simple and sufficient explanation in the varying inclination of the
+observer’s horizon to the earth’s axis of rotation as the place of
+observation is changed.
+
+A very simple experiment will assist one to comprehend the varying
+position of the horizon in different latitudes, and its effect upon the
+apparent diurnal movement of the heavens. Through the middle of an
+orange pass a knitting-needle, so that the two together may be taken to
+represent the earth and its axis. A circular piece of thin card pushed
+on to the needle at one end will represent the polar horizon, and, if
+the orange be rotated, it will be at once realised that such movement
+produces no change in the plane of this horizon, although different
+points on the visible horizon will be successively brought in line with
+different groups of stars or other external bodies.
+
+Another piece of card should next be fixed on the orange by means of a
+pin at a point corresponding to the Equator. Again spinning the model
+earth on its axis, this horizon will be seen to constantly change its
+plane with regard to outside objects, and in a manner which perfectly
+accounts for the apparent movement of the heavens as observed from a
+point on the Equator.
+
+A third piece of card touching the surface of the orange at an
+intermediate place will have an oblique movement, and as referred to
+this plane, the stars appear to traverse their daily rounds in oblique
+circles.
+
+EXPERIMENTAL PROOFS OF ROTATION.—Not only does a supposed rotation of
+the earth accord perfectly with all that we can glean from observations
+of the heavens, but actual demonstrations of the reality of this
+movement are forthcoming. Sir Isaac Newton suggested one experimental
+method of setting the matter at rest. The further a thing is removed
+from the centre of the earth, the greater is the circle which it
+describes in a day, and the greater, consequently, the speed with which
+it must travel. Thus the top of a high tower moves more quickly than its
+base, and the surface of a mine than the bottom of the shaft. A stone
+let fall from the top of a tower thus starts with a greater forward
+velocity than that of objects at the base, and when it reaches the
+earth’s surface, it will be a little east of the point where a
+plumb-line let down from its starting-point reaches the surface. This
+experiment has been tried, but there are so many disturbing causes
+affecting the movement of the falling stone that the results are not
+very satisfactory, although generally confirming the earths rotation
+from west to east. Evidently this method would fail at the Pole, and
+would be most effective at the Equator.
+
+A much more beautiful and perfect proof is furnished by the celebrated
+Foucault’s pendulum experiment. Again fancying ourselves at the North
+Pole, let us imagine a long and heavy pendulum, suspended in such a
+manner that the plane in which it swings is not affected by the earth’s
+rotation. The trace of such a pendulum on a bed of sand placed beneath
+it would remain in a constant position if the earth were at rest. As the
+earth rotates, the bed of sand is twisted round, and the path of the
+pendulum apparently changes. The experiment was first actually carried
+out by Foucault in 1851, at the Pantheon in Paris, and it created a
+widespread interest. Since then, pendulums have been erected in various
+parts of the world, and all agree in essential results. The experiment
+can be seen in actual operation in the science section of the South
+Kensington Museum. The pendulum bob is a very heavy one, and before
+commencing the experiment, it is held out of the vertical by a loose
+band, which is fixed to the wall by a piece of string. On burning the
+string, the band falls off, and the pendulum starts its swing with
+little or no movement out of a plane. The pendulum bob is suspended by a
+long piano wire which is attached to a bracket carrying a conical pivot.
+The pivot rests on an agate plate at the end of a beam, and the weight
+of the bracket is compensated by an adjustable weight (Fig. 3). When
+swinging, the pendulum has a constant tendency to remain in one plane,
+and the turning of the beam beneath the pivot has no effect on the
+absolute direction of the plane of swing. Beneath the pendulum is a
+table divided into degrees, and the hourly apparent movement of the
+plane of swing at Kensington is observed to be nearly 12°.
+
+[Illustration:
+
+  FIG. 3.—_Foucault’s Pendulum Experiment._
+]
+
+If the experiment could be performed at the North Pole, the pendulum
+plane would apparently rotate from east to west, making a complete
+rotation once a day. At the South Pole the direction of movement would
+be reversed, but the rate would be the same as at the North Pole. The
+experiment, however, fails altogether at the Equator, while at places
+between the Poles and Equator the rate of movement varies with the
+latitude.
+
+A more compact piece of apparatus for demonstrating the earth’s rotation
+is the gyroscope, which we also owe to Foucault’s ingenuity. The
+principle is exactly the same as in the case of the pendulum. A heavy
+disc is set in very rapid rotation, and is suspended in such a way that
+its points of support may be turned round without disturbing its plane
+of rotation. The results obtained with this instrument substantiate
+those derived from pendulums.
+
+These experimental proofs of the rotation of the earth further teach us
+the same fact that we learn from observations of the stars, namely, that
+the earth makes a complete turn on its axis once a day.
+
+LATITUDE AND LONGITUDE.—Having thus arrived at the conclusion that the
+earth is a globe turning on an axis once in twenty-four hours, the
+_North and South Poles_ may be defined as the points where the axis of
+rotation meets the surface, while the _Equator_ is the circle passing
+through places midway between the Poles. Imaginary circles passing round
+the earth through the Poles are called _meridians_, while circles
+parallel to the Equator are called _parallels_. These conceptions enable
+us to define very precisely the situation of any particular place upon
+the terrestrial sphere. We measure its angular distance from the
+Equator, as seen from the centre of the earth, and call this its
+_latitude_; London, for instance, is 51½° north of the Equator, and this
+is abbreviated to lat. 51½°N. All places on the same parallel have the
+same latitude, so that another measurement is required to designate the
+exact location of any one place. For this purpose the meridian passing
+through some place is agreed upon as a start-point, and we can then say
+that the place in question is so many degrees east or west; such a
+measurement represents the _longitude_ of the place. At present there is
+no universal agreement as to the initial meridian, but in all British
+maps the meridian passing through the centre of the transit instrument
+at the Royal Observatory, Greenwich, is taken as the start-point.
+Longitudes are reckoned up to 180° E. and 180° W. New York, for example,
+is in long. 73° 58′ W., and Berlin in long. 13° 24′ E.
+
+THE CARDINAL POINTS.—For general convenience in expressing the situation
+of an object, it is usual to say that it is towards the north, south,
+south-west, etc., as the case may be. A north or south line at any
+place, or a _meridian line_, as it is called, is in the direction of the
+terrestrial meridian passing through the place. The north point of the
+horizon is thus the point in which the meridian line meets the horizon
+towards the North Pole. The opposite point is south; while the east and
+west points lie in the directions at right angles. There are various
+ways in which a meridian line may be drawn. One of the simplest is to
+erect a vertical rod and to observe when its shadow thrown by the sun is
+shortest; at that moment the shadow marks the direction of north and
+south. This method is not very exact, as it is so difficult to tell when
+the shadow is shortest. A more accurate result may be obtained by
+drawing a circle round the stick as centre, and noting the points on
+this circle reached by the end of the shadow before and after noon; the
+point midway between these, marks the position of the shadow when
+shortest. By taking the average result of observations made with more
+than one circle, a good approximation can be obtained.
+
+For a somewhat rough determination of the direction of the cardinal
+points, a watch showing the correct time may be utilised. Directing the
+hour hand to the sun, the south point will lie midway between that and
+XII. In the case of a watch having a dial marked up to XXIV., and
+reading XII. at mid-day, the latter figure would always point to the
+south when the hand indicating the hour was directed towards the sun.
+This will be easily understood if it be remembered that the sun is in
+the south at intervals of (approximately) twenty-four hours.
+
+[Illustration:
+
+  FIG. 4.—_Day and Night._
+]
+
+DAY AND NIGHT.—The succession of days and nights by which our daily
+arrangements are regulated is at once explained by the fact that the
+earth is round, and turns on its axis once a day. At any particular
+instant of time the sun can only shine on that half of the earth which
+is turned towards it. At all places included in the illuminated part the
+sun will be above the horizon, and it will be day. One half of the earth
+will be turned away from the sun, and to all places in that part it will
+be night. Under the conditions represented in Fig. 4, to a person
+situated at the point P it will be midnight; he will, however, be
+carried by the earth’s rotation along the circle P Q R; when he arrives
+at a point on _a b_, the sun will be rising to him, and his day will
+commence. On reaching the point R the sun will be on the spectator’s
+meridian, and it will be noon. After another interval he will arrive at
+the boundary of light and shade, and his night will commence.
+
+ATMOSPHERIC REFRACTION.—In common with other substances through which
+light can pass, the atmosphere by which the earth is surrounded has the
+effect of bending rays of light out of their courses, and on account of
+this we do not see the heavenly bodies in their true positions. If the
+air were of uniform density the effect of this refraction would be as
+illustrated to the left in Fig. 5. The light from a star S will reach
+the observer at O after striking the atmospheric shell at _a_ and being
+refracted along the line _a_ O; consequently the observer will see it in
+the direction O S′, and not in the direction O S, which it would have if
+the air were absent. As a matter of fact, the atmosphere becomes less
+dense in passing upwards, so that the rays of light are subjected to a
+succession of small deviations; two such refractions are illustrated at
+the right of Fig. 5. When a star is overhead there is no refraction, and
+the greatest displacements of a star’s positions are produced on the
+horizon, where the light has to pass through a great thickness of
+atmosphere.
+
+Refraction always makes the heavenly bodies appear higher in the sky
+than they otherwise would be, and some very curious effects can be
+traced to it. Thus the sun becomes visible on account of refraction some
+time before it has actually risen, and remains visible for a little
+while after it has really descended below the horizon. The amount of
+refraction varies with the temperature and pressure of the air, but the
+average amounts for different elevations above the horizon are as
+follows:
+
+                             TABLE OF MEAN
+                             REFRACTIONS.
+                       ┌───────────┬───────────┐
+                       │ Altitude. │Refraction.│
+                       ├───────────┼───────────┤
+                       │         0°│    34′ 54″│
+                       │         2°│    18′  9″│
+                       │         4°│    11′ 39″│
+                       │         6°│     8′ 23″│
+                       │         8°│     6′ 29″│
+                       │        10°│     5′ 15″│
+                       │        12°│     4′ 23″│
+                       │        14°│     3′ 45″│
+                       │        16°│     3′ 17″│
+                       │        18°│     2′ 54″│
+                       │        20°│     2′ 35″│
+                       │        25°│     2′  2″│
+                       │        30°│     1′ 38″│
+                       │        40°│     1′  8″│
+                       │        50°│     0′ 48″│
+                       │        60°│     0′ 33″│
+                       │        70°│     0′ 21″│
+                       │        90°│     0′  0″│
+                       └───────────┴───────────┘
+
+Refraction is responsible, among other things, for the curiously
+distorted appearances of the sun and moon, when they are very near the
+horizon.
+
+TWILIGHT.—The atmosphere, or rather the solid and liquid particles which
+it always contains, has the property of reflecting light, and hence it
+does not suddenly become dark when the sun has set. Even until the sun
+has descended 18° below the horizon, the upper parts of the air continue
+to reflect his beams, and this is the origin of _twilight_. In the
+tropics the sun sets almost vertically, so that it gets below the
+twilight limit comparatively quickly, and this explains the short
+twilight which is remarked by all who have visited a tropical country.
+In our own country the sun has an apparent oblique motion, and a
+relatively long period elapses before twilight ends. The increase in the
+duration of twilight is, indeed, very noticeable in merely travelling
+from London to the north of Scotland in summer-time.
+
+[Illustration:
+
+  FIG. 5.—_Atmospheric Refraction._
+]
+
+Within the Arctic Circle, at places where the sun itself is never
+visible for months together, its reflected beams in the form of twilight
+may be seen for months.
+
+
+
+
+                              CHAPTER II.
+                 THE EARTH’S REVOLUTION ROUND THE SUN.
+
+
+APPARENT MOVEMENTS OF THE SUN.—During any day on which we may observe
+the sun, it will be seen to rise at a certain place on the horizon,
+gradually ascend into the heavens to a certain point, then as steadily
+sink towards the west until it disappears at some point on the western
+horizon. If we watch the sun about the 20th of March, we shall find it
+to rise due east, and set due west; it will be above the horizon for
+exactly twelve hours, and below for the same length of time. When this
+happens, we have the _vernal_ or _spring equinox_, as the nights are
+then equal in all parts of the world. From this time to the third week
+in June, we shall find the sun to rise more and more to the north of
+east, and to set gradually further north of west. This is accompanied by
+a daily increase in the apparent height of the sun at noon, and by
+increasing length of day and reduction of night. For some days before
+the 21st of June the change of the sun’s place of rising and setting is
+very slow, and after this day the places of rising and setting begin to
+recede to the south. We then have the _summer solstice_, so-called
+because the sun seems to stand still, in so far as its northward travel
+is concerned. The point of rising or setting of the sun goes on moving
+nearer to the south point of the horizon, until about September 22, we
+again have the sun above the horizon for twelve hours, and below the
+horizon for an equal period; this is the _autumnal equinox_. The
+southward movement is continued until December 21, after which the
+rising begins to take place further towards the north. When furthest
+south, we have the _winter solstice_ in the Northern Hemisphere, the sun
+being above the horizon for only a short time, and reaching only a small
+altitude at noon. From December 21 to March 20, the sun rises further to
+the north, at first very gradually, and afterwards more rapidly. These
+varying amounts of sunshine correspond to the short days of winter, and
+the long days of summer. A diagrammatic representation of the apparent
+path of the sun at the solstices and equinoxes for some place, such as
+London, is given in Fig. 6.
+
+[Illustration:
+
+  FIG. 6.—_Apparent Paths of Sun at Equinoxes and Solstices._
+]
+
+It is clear, then, that our relations to the sun are very different from
+our relation to the stars, inasmuch as the apparent position of the sun,
+as projected upon the sky, is constantly changing, but returns to
+similar conditions at the end of a year. If our place of observation is
+changed, the apparent diurnal movement of the sun is affected in the
+same way as that of the stars.
+
+To explain these annual changes of the sun, with regard to an observer’s
+horizon, it is only necessary to suppose that the sun marches northwards
+towards the celestial pole from the winter to the summer of the Northern
+Hemisphere, and southwards from summer to winter. It is not to be
+imagined, however, that this apparent movement towards or from the north
+celestial pole is necessarily a real movement of the sun; we shall, in
+fact, very shortly see that it is only an apparent movement due to the
+changing situation of the earth with respect to the sun.
+
+THE ECLIPTIC.—A very small amount of actual observation, without the aid
+of instruments, suffices to show that the changes in the sun’s relation
+to any observers horizon at different parts of the year are associated
+with a change in its situation among the stars. If we direct our gaze
+towards the south at midnight, we are looking towards that part of space
+which is directly opposite to the sun, as will be evident from Fig. 4,
+and if the sun’s apparent movement were only in a polar direction, we
+should always see the same stars in the same part of the sky at the same
+hour. Such, however, is not the case. The stars are found more and more
+towards the west at the same hour as the year advances. Sirius, for
+instance, is due south about midnight on December 31; but at the end of
+January it will pass through the south point shortly before ten P.M.
+Similar changes are noted in the case of all the stars, and they
+indicate either an easterly movement of the sun among the stars, or a
+westerly motion of the stars with regard to the sun. If it were possible
+to see the stars in the immediate neighbourhood of the sun, this
+relative motion could be directly observed; but under the actual
+circumstances, the apparent track of the sun amongst the stars must be
+determined indirectly. When we make observations at midnight, we know
+that the sun is opposite to stars which are due south at that moment;
+and the height which it reaches above the horizon at noon indicates its
+angular distance from the celestial pole. It is thus possible to trace
+the sun’s apparent path on a map of the stars, or upon a celestial
+globe; this is called the _ecliptic_, and it is found to be a great
+circle of the celestial sphere—that is, it is a circle contained in a
+plane which passes through the centre of the sphere.
+
+The observed movement of the sun among the stars might be produced
+either by a revolution of the sun round the earth in a year, or by a
+revolution of the earth round the sun in the same period, the stars
+being supposed at rest at a greater distance than the sun. There are
+many phenomena which indicate that it is the earth which moves round the
+sun, but the most direct proof is found in what is known to astronomers
+as the aberration of light.
+
+ABERRATION AS A PROOF OF THE EARTH’S REVOLUTION.—While engaged on an
+observation having for its object the determination of the distance of a
+star, Dr. Bradley made a discovery of very great interest and importance
+to astronomers. What he found practically amounts to this, that in order
+to see a star exactly at the centre of the field of view of a telescope
+we must direct the optical axis of the instrument at a small angle to
+the line joining the earth and star, irrespective of other deviations,
+such as that produced by refraction. The direction of this displacement
+is constantly changing throughout the year, but it is common to all the
+stars, and the fact that the original apparent position is regained at
+the end of a year at once associates aberration with a revolution of the
+earth round the sun.
+
+[Illustration:
+
+  FIG. 7.—_Aberrational Orbit of a Star._
+]
+
+In Fig. 7 we have a perspective view of the earth’s orbit with the sun
+at S. A star _s_ would appear in the direction A _s_ when seen from the
+earth, supposed at rest at the point A; actually it is seen at _a_,
+ahead of its place, and in the course of a year it describes the
+_aberrational orbit_, _a b c d_, these points corresponding to positions
+A B C D of the earth in its annual path.
+
+As a result of aberration, then, each star appears to revolve once a
+year in a small elliptic path about its average position. The breadths
+of these ellipses vary according to their angular distances from the
+ecliptic, but all have precisely the same length of about 41″. Half the
+length of the ellipses, which amounts to 20″.5, is accordingly called
+the _constant_ of _aberration_.
+
+The fact that the earth’s velocity in its orbit forms a sensible
+fraction of the velocity of light is the cause of aberration. If we let
+an object fall down the middle of a tube which is at rest, it will fall
+to the bottom without touching the side if the tube be held vertically.
+When the tube has a forward movement, however, it must be inclined at an
+angle in order that the falling body may pass clear to the bottom, and
+the greater the speed of the tube the more it must be inclined. So it is
+with light which comes from a star and traverses the tube of a telescope
+situated on a moving earth; the tube must be inclined to the actual path
+of the light rays.
+
+Other proofs that it is the earth which moves round the sun are
+furnished by the parallaxes of the stars, and by spectroscopic measures
+of the earth’s velocity.
+
+APPROXIMATE SCALE OF EARTH’S ORBIT.—A very beautiful application of the
+constant of aberration is in the measurement of the distance of the
+earth from the sun. We have only to bear in mind that the apparent size
+of the sun does not change very much, in order to realise that the path
+of the earth must be very nearly a circle; if the distance changed very
+much there would be a correspondingly great change in the sun’s apparent
+diameter. Now the constant of aberration is a measure of the relative
+velocity of the earth in its orbit and the velocity of light. There are
+several ways of determining the velocity of light, and it is known to be
+very nearly 186,300 miles per second. In a right-angled triangle having
+one angle equal to the constant of aberration, the side opposite to this
+angle would represent the velocity of the earth, if the longer side
+represented that of light. In such a triangle the proportion between
+these sides would be nearly as 1 to 10,000. That is, the velocity of
+light is about 10,000 times that of the earth in its orbit. The earth’s
+velocity is thus found to be about 18½ miles per second, so that the
+distance which it traverses in a year is found by a simple
+multiplication. In this way the circumference of the earth’s orbit is
+obtained, and it is easily deduced that the radius of the orbit, which
+is nothing more than the sun’s distance, is not far from 93,000,000
+miles.
+
+THE ZODIAC.—The space about 8° above and below the ecliptic constitutes
+what is called the _zodiac_. The zodiac is of very great antiquity, and
+marks out the region traversed by the sun and all the planets known to
+the ancients. It is divided into twelve parts of 30° each, called signs
+of the zodiac, from the supposed outlines of animals, etc., marked out
+by the stars. The names of these signs are probably familiar to everyone
+from the well-known rhyme:
+
+                “_The Ram, the Bull, the Heavenly Twins,
+                And next the Crab the Lion shines,
+                  The Virgin, and the Scales,
+                The Scorpion, Archer, and the Goat,
+                The man that bears the Watering-Pot,
+                  And Fish with glittering tails._”
+
+The astronomical names and symbols corresponding to these are as
+follows:—
+
+                    ♈︎ Aries,       The Ram.
+                    ♉︎ Taurus,      The Bull.
+                    ♊︎ Gemini,      The Twins.
+                    ♋︎ Cancer,      The Crab.
+                    ♌︎ Leo,         The Lion.
+                    ♍︎ Virgo,       The Virgin.
+                    ♎︎ Libra,       The Balance.
+                    ♏︎ Scorpio,     The Scorpion.
+                    ♐︎ Sagittarius, The Archer.
+                    ♑︎ Capricornus, The Goat.
+                    ♒︎ Aquarius,    The Water-Bearer.
+                    ♓︎ Pisces,      The Fishes.
+
+The sun enters the sign Aries at the vernal equinox in March, and the
+others in successive months. On account of the precession of the
+equinoxes (see p. 69), however, the sun no longer enters the
+_constellation_ Aries at the vernal equinox, but it is still said to
+enter the _sign_ Aries.
+
+INCLINATION OF THE EARTH’S AXIS.—The revolution of the earth round the
+sun provides us with a very satisfactory explanation of the apparent
+easterly movement of the sun among the stars. There is, however, another
+very important point. We have seen that during a year the sun has a
+movement towards and from the Pole, as well as an easterly movement. The
+plane of the earth’s orbit, therefore, cannot be coincident with the
+plane of the Equator; if it were, the sun would have the same apparent
+movement every day—it would always rise due east, and set due west, in
+all parts of the earth. The ecliptic, moreover, would be coincident with
+the celestial equator. When the ecliptic is determined by observations
+in the way already explained (p. 57), it is found to intersect the
+celestial equator in two points, and the plane containing it is inclined
+at an angle of very nearly 23½° to the equatorial plane. This
+inclination of the Equator to the ecliptic, or “obliquity of the
+ecliptic,” indicates that the earth’s axis of rotation is inclined to
+the plane in which the revolution round the sun is performed, the actual
+inclination being about r66½°.
+
+Further, the axis of rotation must remain parallel to itself during the
+revolution of the earth. Otherwise, the situation of the celestial pole
+would be seen to change, and the Pole Star would no longer serve to show
+us which way lies north.
+
+It is precisely this inclination of the earth’s axis which brings about
+the varying lengths of days and nights which we associate with different
+seasons.
+
+THE SEASONS.—Let us in the first place contrast the conditions in summer
+with those which obtain in winter. Imagine that we can view the sun and
+earth from a very distant point lying in the plane of the ecliptic, and
+situated so that a line joining it with the sun is perpendicular to the
+line joining the sun and earth in summer or winter.
+
+[Illustration:
+
+  FIG. 8.—_The Sun’s Altitude in Summer and Winter._
+]
+
+The sun will thus appear in some position represented by O in Fig. 8; in
+the summer of the Northern Hemisphere the earth will be in the position
+S, and in winter in the position W, since it travels half way round its
+orbit in six months’ time. An observer situated at London will be 38½°
+from the North Pole, and he is represented by the point A in our
+diagram. The horizon at noon of such an observer is represented by the
+line H R, tangential to the surface of the sphere at the point A. At
+noon, then, the altitude of the sun is equal to the angle O A H. When it
+is winter in the Northern Hemisphere, the earth’s axis is inclined away
+from the sun, and our observer at London is so situated that at noon his
+horizon is the line H′ R′, while the sun’s altitude is the angle O A′
+R′, which is no less than 47° smaller than in summer. People who dwell
+in the Southern Hemisphere enjoy the long days of summer at the time
+when our own days are shortest, and _vice versâ_, and the reason for
+this is clearly that when the position of the earth’s axis presents the
+greatest part of the Northern Hemisphere towards the sun, the greater
+part of the southern half of our globe is turned away from the sun.
+
+At the equinoxes, which occur very nearly midway between the solstices,
+the earth’s axis is directed neither towards nor away from the great
+source of light and heat, so that both hemispheres are presented to the
+sun under exactly the same conditions. This state of affairs is shown
+diagrammatically in Fig. 9. The sun’s altitude at noon at the
+commencement of spring is equal to that at the beginning of autumn, and
+depends only upon the observer’s latitude. The half of our globe which
+is then flooded with the sun’s rays comprises both the North and South
+Poles, and it is evident that as the earth turns round, every place upon
+it, whether in Arctic or equatorial regions, receives the benefit of
+twelve hours sunshine, and at the same time has a night of twelve hours
+duration.
+
+[Illustration:
+
+  FIG. 9.—_The Sun’s Altitude at the Equinoxes._
+]
+
+THE MIDNIGHT SUN.—The facilities which are now offered for foreign
+travel have induced many people to pay a visit to the north of Norway,
+one of the objects in view frequently being to witness the so-called
+“midnight sun.” It seems somewhat paradoxical to speak of night when the
+sun is above the horizon, but it simply means that in high latitudes the
+sun may be seen over the northern horizon when it is midnight at places
+further south which have the same longitude. We have seen that in our
+own country there are certain stars which never set, and when we get to
+the Pole itself, all the stars which are there visible will present this
+peculiarity.
+
+In order to see the sun at midnight, then, what we have to do is to
+travel towards the Pole until we reach a latitude where the sun itself
+becomes circumpolar. At the Pole this would be the state of things
+during the whole of the northern summer, when the sun is north of the
+Equator, and since the sun never travels northward more than 23½°, it
+can only be circumpolar at places within that angular distance from the
+Pole, that is, within the Arctic Circle.
+
+[Illustration:
+
+  FIG. 10.—_The Midnight Sun._
+]
+
+Let A in Fig. 10 be such a place, the sun being to the left. At noon the
+horizon of A is represented by H R, and the sun will appear in the south
+at a certain altitude, S A H. At midnight the earth’s rotation will
+change the observers position to A′ and his horizon to H′ R′, but it
+will not have taken him out of sunshine. The sun will then appear due
+north, but, except at the Pole, its altitude, S A′ H′, will be lower
+than at noon. At a place situated on the Arctic circle, latitude 66½°,
+the midnight sun would only be visible for one night at the summer
+solstice, were it not that refraction causes it to appear above the
+horizon when it is geometrically more than its own apparent diameter
+below.
+
+At Tromsö the midnight sun is visible from May 19 to July 22, and at the
+North Cape from May 12 to July 29.
+
+Nature, however, exacts compensation for this lavish share of summer
+sunshine in high latitudes, and there is a correspondingly number of
+dreary days in winter when the sun does not rise at all.
+
+
+
+
+                              CHAPTER III.
+         HOW THE POSITIONS OF THE HEAVENLY BODIES ARE DEFINED.
+
+
+TWO MEASUREMENTS REQUISITE.—In order to make a more precise study of the
+movements of the heavenly bodies, it is essential that we should have
+some very definite means of specifying their positions upon the
+celestial sphere. To define the position of any object, at least two
+measurements are required. If, for example, one wishes to draw attention
+to a particular letter on the page of a book, it is only necessary to
+say that it is so many lines from the top, and a certain number of
+letters from the end of the particular line on which it lies. In the
+same way, latitude and longitude sufficiently indicate the situation of
+a place on the surface of the earth, and similar measures can be
+employed to indicate the places of the heavenly bodies.
+
+ALTITUDE AND AZIMUTH.—The horizon and zenith at any place—being in a
+constant position with reference to the earth—may be utilised for
+indicating the positions of external bodies. We may say, for instance,
+that at noon on June 24, the sun, as seen from London, is 62° above the
+horizon, or 28° from the zenith. Technically, the former is called the
+_altitude_ of the sun, being the angular distance above the horizon,
+while the latter measure is called the _zenith distance_.
+
+[Illustration:
+
+  FIG. 11.—_Altitude and Azimuth._
+]
+
+We may next note that an object, besides having a certain altitude, is a
+certain number of degrees from the north, south, east, or west points,
+measured horizontally; if we reckon from the north point through E, S,
+and W, from 0° to 360°, such a horizontal measurement is called
+_azimuth_; if reckoned north or south of the east or west points it is
+called the _amplitude_ of the body. Fig. 11 illustrates these terms. In
+this diagram the observer is placed at O, N S and E W respectively
+representing a north and south, and an east and west line in the
+horizon; the point Z is the zenith, and S a heavenly body. A vertical
+circle drawn from Z through S will meet the horizon at a point A. The
+azimuth of S is thus the angle N O A, and its amplitude is the angle E O
+A, while the altitude of S is simply the angle A O S. Measurements of
+altitude and azimuth are made by means of an instrument called the
+altazimuth, an account of which will be found on page 202.
+
+DECLINATION.—Altitude and azimuth only specify the position of a star
+for a particular place at a particular time. A better system is
+evidently one which is independent of the observer’s situation on the
+earth. Of the two measurements required, one is readily decided upon; we
+can say that the sun, or star, or other heavenly body is a certain
+number of degrees from the north celestial pole; or, what is just as
+good, we can state the number of degrees north or south of the celestial
+equator, which lies midway between the poles. The former measurement
+gives what is called the _north polar distance_ of the star, and the
+latter its _declination_.
+
+RIGHT ASCENSION.—Just as the latitude of a place on the earth does not
+tell whether it is in Europe or North America, so declination alone
+fails to locate a heavenly body. We must have some measurement
+equivalent to terrestrial longitude, and it is therefore necessary in
+the first instance to select a start-point, which shall do for stars
+what Greenwich does for our geographical maps. By universal consent the
+fundamental point for the stars is a point situated on the celestial
+equator where it is crossed by that part of the ecliptic occupied by the
+sun at the vernal equinox. This zero mark is called the _First Point of
+Aries_, and is frequently denoted by the symbol ♈︎ identical with that
+employed for the corresponding sign of the zodiac.
+
+The location of this reference point being thus determined, the _right
+ascension_ of a celestial body may be defined as its angular distance
+from the First Point of Aries, as measured along the celestial equator.
+Like terrestrial longitude, it may be stated in degrees, but it is more
+usually expressed in hours, minutes, and seconds of time, for the reason
+that in general the measurement of a right ascension consists of an
+observation of the time at which the body in question comes to a certain
+position.
+
+The right ascensions and declinations of stars are best determined when
+they are on the meridian of the place of observation, and such
+measurements are made by means of a transit instrument. When a star is
+on the meridian, its declination is estimated by the angle at which the
+instrument is inclined to the celestial equator when directed to the
+star. The fact that the earth is turning on its axis furnishes us with a
+simple method of finding the right ascensions of the heavenly bodies.
+Imagine a plane passing through the observers position on the earth and
+through the earth’s axis. This, prolonged indefinitely, cuts the
+celestial sphere in his meridian, and it is evident that on account of
+the earth’s rotation it will turn completely round every twenty-four
+hours. It may therefore be regarded as the hour-hand of a clock, which
+is provided with figures ranging from I. to XXIV. When this gigantic
+clock hand sweeps past the First Point of Aries, all stars then seen in
+the plane—that is, all stars which are on the meridian—will have zero
+right ascension. After a complete rotation it will again sweep through
+the First Point of Aries.
+
+USE OF STAR TIME.—Meanwhile, suppose we have a clock regulated so that
+it marks twenty-four hours between these two meridian passages of the
+First Point of Aries. Evidently, then, the time by this clock at which
+any object in the sky is seen on the meridian will depend upon its
+angular distance from the celestial meridian passing through the First
+Point of Aries. As the earth is rotating through 360° in twenty-four
+hours, reckoned by our clock, the meridian plane will travel at the rate
+of 15° per hour, so that, for example, a star 60° from the celestial
+meridian passing through the First Point of Aries, will appear to cross
+the observer’s meridian at IV. hours by the clock. A clock so regulated
+to keep time with the stars is called a sidereal clock, and the sidereal
+time at which a celestial body crosses the meridian, or “souths,” is the
+right ascension of that object. Such a time measurement can be converted
+into angular measure by allowing 15° per hour, 15′ per minute, and 15″
+per second of time.
+
+CELESTIAL LATITUDE AND LONGITUDE.—In some astronomical questions it is
+often convenient to adopt a different system of co-ordinates to indicate
+the situation of a celestial body. Just as the earth’s equatorial plane
+serves as a basis for the measurement of declination, the earth’s plane
+of revolution—that is, the plane of the ecliptic—is used as the term of
+reference for _celestial latitude_, which may be defined as the angular
+distance of an object above or below the plane of the ecliptic.
+_Celestial longitude_ is the angular distance from the First Point of
+Aries measured along the ecliptic.
+
+A diagram such as that in Fig. 12 may assist the comprehension of these
+co-ordinates. Here the observer is supposed to be situated at the point
+O, at the centre of the celestial sphere. To him the north and south
+celestial poles will appear in some such positions as N and S, and the
+celestial equator will be represented by a great circle at right angles
+to the line joining these two points. The apparent path of the sun—the
+ecliptic—will be indicated by another great circle, which is inclined to
+the Equator; and the poles of the ecliptic will be represented by P and
+P′.
+
+The Equator crosses the ecliptic at the First Point of Aries, marked ♈︎.
+Considering now a star which the observer sees in the direction of the
+line O S, its position would be reckoned as follows in the two systems:—
+
+                 Right       Ascension = Angle ♈︎ O R }
+                 Declination           =   „   S O R }
+
+                 Celestial   Longitude = Angle ♈︎ O L }
+                 „           Latitude  =   „   S O L }
+
+Either pair of co-ordinates can, by a mathematical process, be expressed
+in terms of the other.
+
+[Illustration:
+
+  FIG. 12.—_Right Ascension, Declination, Celestial Latitude, and
+    Celestial Longitude._
+]
+
+PRECESSION OF THE EQUINOXES.—It is not too early to remark that the
+First Point of Aries is not absolutely a fixed point on the celestial
+equator. This is on account of the precession of the equinoxes, which
+consists of a backward movement of the First Point, due to a change in
+the position of the earth’s equator. As a point common to the ecliptic
+and equator, it is conveniently retained as the starting-point of right
+ascensions and celestial longitudes, but in consequence of precession,
+these co-ordinates are subject to a constant change. The amount of
+precession for a point on the Equator is 50″·2 per annum, and this
+movement requires 25,800 years for a complete revolution.
+
+GEOCENTRIC AND HELIOCENTRIC POSITIONS.—When observing objects at a very
+great distance, they will appear in the same direction to a spectator on
+the earth as they would if he could by some means be transferred so as
+to be able to see them from the sun. If, for instance, one sees the Peak
+of Teneriffe from a distant ship, its apparent direction will be very
+slightly affected by a change of a mile in the ship’s position. But a
+similar change of place would produce a greater difference of direction
+when a nearer body was under observation. If an object is relatively
+near to the sun and earth, its direction, and, therefore, its apparent
+position on the celestial sphere, will be different, as seen from the
+earth and sun. Such will be the case with planets and other bodies which
+lie in our immediate neighbourhood, speaking astronomically. Hence, it
+is often convenient to distinguish between the _geocentric_ position of
+a celestial body—referring it to the position it would occupy if it
+could be seen from the centre of the earth—and the _heliocentric_
+position, representing it as it would appear to an observer occupying
+the centre of the sun. We thus have geocentric and heliocentric
+latitudes and longitudes of the nearer heavenly bodies.
+
+STAR CATALOGUES.—The problem of constructing catalogues showing the
+positions of the stars is one of considerable practical value, as well
+as one of great scientific importance. In the first instance, such
+catalogues were of necessity compiled from data acquired by naked eye
+observations, so that the ancient catalogues comprise only a small
+number of stars.
+
+As far back as 295 B.C., the positions of stars were determined by
+Timocharis with sufficient accuracy to lead Hipparchus to his great
+discovery of the precession of the equinoxes about 170 years later. From
+observations at Rhodes, Hipparchus drew up a catalogue of 1,022 stars,
+giving their latitudes and longitudes; this is preserved for us in
+Ptolemy’s “Almagest,” where the positions are corrected for precession,
+and reduced to the epoch 150 A.D. The next catalogue of importance was
+due to the industry of Tycho Brahé (1546–1601), who gave the positions
+of 1,005 stars with greater accuracy than had been previously obtained;
+indeed, notwithstanding his want of optical aid, it has been estimated
+that the probable errors of his measures were not more than 24″ and 25″
+in right ascension and declination respectively. The last of the naked
+eye catalogues is that of Hevelius, giving the positions of 1,553 stars.
+
+Coming to more recent times, in which the employment of telescopes has
+vastly increased the power of accurate observation, there are the
+catalogues of Flamsteed, Halley, Lacaille, Lalande, Argelander, the
+British Association, and catalogues of the stars in particular parts of
+the sky which have been published by all the leading national
+observatories. Eighteen observatories are now taking part in the
+construction of an international star catalogue by means of photography,
+and this is intended to record with great accuracy the positions of
+nearly 3,000,000 stars. A modern star catalogue usually places the stars
+in the order of their right ascensions, and, in addition to the two
+co-ordinates, furnishes the necessary data for determining the exact
+situations of the stars at any particular time.
+
+
+
+
+                              CHAPTER IV.
+                           THE EARTH’S ORBIT.
+
+
+EXACT SHAPE OF THE ORBIT.—It will be clear that if we made our annual
+journey in a circle we should always be at the same distance from the
+sun, and the apparent size of that luminary would never vary. This,
+however, is not the case. Exact measurements, which are best made by
+means of the transit instrument, indicate variations which, though not
+perceptible to the unassisted eye, establish a want of circularity. The
+observations bearing on this point consist of a measurement of the time
+required for the sun to cross the meridian—the larger its apparent
+diameter, the longer it will obviously be in passing the meridian. An
+observation of the sidereal time at which the centre of the sun passes
+the meridian determines the right ascension, and from this one can
+calculate the sun’s longitude.
+
+[Illustration:
+
+  FIG. 13.—_Elliptic Form of Earth’s Orbit._
+]
+
+If such observations be made at intervals during a year, we can utilise
+them for determining the shape of the earths orbit independently of a
+knowledge of the actual size. In Fig. 13 let us suppose the sun to be
+situated at the point S; from S we draw a line, S A, representing the
+line joining the earth and sun at the vernal equinox when the sun’s
+longitude is zero. If our observations include a measure of the sun’s
+diameter on that day, let S A be drawn on some convenient scale. To plot
+the observations for other days, we must draw S F, S E, etc., at angles
+A S F, A S E, etc., equal to the sun’s longitude, and make the lengths
+inversely proportional to the apparent diameters, on the same scale as S
+A. The other observations can be plotted in the same way, and the earths
+orbit is then found to be an ellipse with the sun in one of its foci.
+Actually, the earth’s orbit is much more nearly circular than is shown
+in Fig. 13, and in illustration of this the following numerical data may
+be given:—
+
+ 1896. Jan. 1   Greatest apparent diameter of sun = 32′ 35″·2 in long. 281°
+       July 3   Least       „        „       „    = 31′ 30″·6    „     102°
+       March 29 Mean        „        „       „    = 32′ 4″       „       9°
+       Oct. 5      „        „        „       „    = 32′ 4″       „     193°
+
+It thus appears that in 1896 we were nearest to the sun on January 1, as
+on that day the sun’s apparent diameter was greatest, while we were
+furthest removed on July 3.
+
+[Illustration:
+
+  FIG. 14.—_The Ellipse._
+]
+
+The ellipse is a curve of such importance in astronomy that an
+understanding of some of its properties is essential for further
+progress. This beautiful closed curve lies in one plane, and its figure
+is such that the sum of the distances of any point upon it from two
+fixed points within the curve is constant. These two fixed points, F F′
+(Fig. 14), are called the foci of the ellipse, and we have, for example,
+the sum of the lengths P F and P F′, equal to the sum of P′ F and P′ F′.
+The line A B passing through the foci is the greatest distance across
+the ellipse, and is called the major axis; at right angles to this is
+the minor axis C D.
+
+Following our definition of the ellipse, we see that as B is a point
+upon its circumference, B F + B F′ must be equal to the sum of the
+distances of any point P from the foci. But since B F is of the same
+length as A F′, the sum of the distances of the point B from the foci,
+and therefore of all other points, is equal to the major axis. Hence the
+average or mean distance of the focus F from all points on the ellipse
+is half the length of the major axis. It follows also that C F is equal
+to the semi-major axis O B.
+
+At the point O, where the axes intercept each other, we have the centre
+of the ellipse, and the ratio between the distance from the centre to
+either of the foci and the semi-major axis, _i.e._, (O F)/(O B) is
+called the eccentricity of the ellipse. Thus, in an ellipse of
+eccentricity 0·5, the foci would lie midway between the centre of the
+ellipse and the extremities of the major axis. The eccentricity is
+always less than unity; if it become unity, the two foci merge together,
+and the curve becomes a circle.
+
+[Illustration:
+
+  FIG. 15.—_How to draw an Ellipse._
+]
+
+To draw an ellipse, two pins may be stuck into a piece of paper at the
+points intended as foci. A loop of thread is then made and thrown over
+the pins. A pencil placed inside the loop, so as to stretch it, and
+traced completely round, will outline an ellipse. The size and shape of
+the ellipse may be varied by changing the length of the thread and the
+distance between the pins. Such, then, is the curve in which our earth
+performs its annual journey round the sun, the sun being relatively
+fixed in one of the foci.
+
+APHELION AND PERIHELION.—When the earth is in that part of its orbit
+where it makes its nearest approach to the sun, it is said to be in
+_perihelion_; when at the point furthest removed from the sun it is in
+_aphelion_. The line joining these two points is obviously the major
+axis of the earth’s orbit, and when this is imagined to be prolonged
+indefinitely into space it is called the _line of apsides_, or _apse
+line_. When the earth is in perihelion, the sun’s apparent diameter will
+be the greatest possible, and when in aphelion it will be at a minimum.
+A knowledge of these limiting values of the apparent solar diameter
+enables us to determine the eccentricity of the orbit of the earth. The
+sun’s apparent diameter when the earth is in perihelion amounts to 32′
+35″·2, and to 31′ 30″·6, when the earth is in aphelion, from which it
+results that the value of e is 0·0167.
+
+UNEQUAL SPEED OF THE EARTH.—The observations by which we are enabled to
+determine the true form of the earth’s orbit are not quite exhausted of
+their usefulness; we can utilise them still further for studying the
+varying rate of the earth’s motion. If the earth moved through equal
+angles every day, the apparent movement of the sun would always be
+uniform, and in that case the sun’s daily increase of longitude would be
+constant.
+
+The following figures, however, prove that this uniformity does not
+exist:—
+
+                1896.   Sun’s daily motion in longitude.
+                Jan. 1          1°    1′    8″·5
+                Mar. 29         1°    0′    6″·7
+                July 3          0°   57′   12″·1
+
+Facts such as these led Kepler in 1609 to the discovery of his famous
+second law of planetary motion, namely, that the radius vector (the line
+joining the sun and earth in the case of the earth’s orbit) describes
+equal areas in equal times. For the sake of clearness, imagine the
+earth’s orbit to be represented by the elongated ellipse in Fig. 16,
+with the sun in the focus _S_. When the earth is near perihelion, it
+will move over a certain distance, _a b_, in a given time; some time
+afterwards it will be in another part of the orbit, and in the same
+interval as before it will traverse the distance _c d_; again, in
+another equal interval of time, it will move from the point _e_ to the
+point _f_. The law tells that the areas _S a b_, _S c d_, and _S e f_,
+are equal so long as equal times are in question; in different parts of
+its path, then, the earth’s rate of motion must vary, _c d_, for
+example, being smaller than _a b_. It will be seen that the motion is
+most rapid when the earth is in perihelion, and least rapid when in
+aphelion.
+
+[Illustration:
+
+  FIG. 16.—_Illustrating Kepler’s Second Law._
+]
+
+CHANGES IN THE EARTH’S ORBIT.—Owing to disturbances caused by the
+proximity of other bodies, the earth’s orbit is not always of the same
+shape. The eccentricity is steadily diminishing, and in about 24,000
+years the orbit will be very nearly a circle; it will afterwards become
+more elliptical again, until in another 40,000 years or so the
+eccentricity will be about 0·02. So far as our knowledge goes, the
+eccentricity will never exceed 0·07.
+
+The direction of the major axis of the earth’s orbit, that is, the line
+of apsides, moves forward at the rate of about 11″ per annum, so that at
+this speed a whole revolution will be made in a period of 108,000 years.
+
+On account of precession, the equinox moves backwards along the orbit at
+the rate of 50″·2 per annum, so that the movement of the apse line with
+regard to the equinox is 61′ in a year; or, in other words, the
+perihelion point of the earth’s orbit makes a complete revolution with
+respect to the equinoctial point in a little over 20,000 years. The
+earth at present passes through perihelion in our northern winter, but
+owing to this motion of the apse line it will in 10,000 years time be at
+aphelion in winter. Northern winters will then be somewhat colder than
+at present. The plane of the orbit itself is subject to changes, with
+the result that the obliquity of the ecliptic is variable in amount. In
+the course of ages the obliquity may oscillate between the limits 24°
+35′ 58″ and 21° 58′ 36″. The mean value during 1896 was 23° 27′ 9″·9.
+
+THE EARTH’S REAL PATH.—In this and preceding chapters, we have had
+occasion to consider various features of the earth’s orbit, but it must
+now be pointed out that what we call the orbit of the earth is not quite
+the same thing as the earth’s actual path in space. The earth, as we
+know, is accompanied by the moon, and these two bodies are bound
+together in such a way that it is really the centre of gravity of the
+earth and moon which describes an elliptic orbit round the sun; the moon
+is so small in relation to the earth that the centre of gravity of the
+two companions lies within the earth’s surface, but, nevertheless, an
+oscillatory displacement of the earth’s centre in space is produced by
+the moon’s monthly circuit round the earth. We judge of the earth’s
+movement by the apparent movement of the sun, and we actually find a
+monthly inequality in the sun’s apparent motion. A very good
+illustration of this may be found in the varying celestial latitude of
+the sun. It will be clear that if the earth always moved in the plane of
+the ecliptic, the sun’s latitude would always be zero. If, on the other
+hand, the earth has a motion round the common centre of gravity, it will
+be above the ecliptic when the moon is below, and _vice versâ_; the sun
+will, therefore, not always appear to be in the ecliptic, and its
+latitude will depend upon that of the moon. The following figures from
+the “Nautical Almanac” will illustrate this point:
+
+                       Sun’s apparent latitude. Moon’s latitude.
+        1896, April  1         0″·70 S.            5°  9′ S.
+                „   10         0″·01 N.            1° 41′ N.
+                „   16         0″·39 N.            5°  6′ N.
+                „   22         0″·07 S.            0° 48′ N.
+                „   29         0″·74 S.            5°  1′ S.
+
+The displacement in right ascension amounts to a little over 6″, and is,
+therefore, large enough to be directly measurable.
+
+On account of this association with her satellite, the earth’s centre
+moves some hundreds of miles above and below the plane of the ecliptic.
+
+The so-called “perturbations,” or disturbing effects of the other
+planets, also cause the earth to depart more or less from the plane of
+the ecliptic and from a geometrical elliptic path. Nevertheless, these
+disturbances can be calculated and allowed for, so that when we speak of
+the earth’s orbit we really mean the path which the centre of gravity of
+the earth and moon would traverse if subject only to the influence of
+the sun.
+
+
+
+
+                               CHAPTER V.
+                            MEAN SOLAR TIME.
+
+
+SUN-DIAL TIME.—The changing directions of shadows thrown by the sun have
+been utilised from very remote periods for the measurement of time, the
+instrument usually employed being a sun-dial. On account of the varying
+declination of the sun, it is necessary to employ as a time-measurer the
+shadow of a line which lies parallel to the earth’s axis, that is, if we
+wish the same hour marks to be permanently useful. Such a rod must lie
+in the plane of the meridian, and be inclined to the horizon at an angle
+equal to the latitude of the place. If the shadow be received on a
+horizontal dial, hours may be marked upon it corresponding to the
+duration of the longest day at the place where it is set up. Sometimes,
+as on old churches, one sees a vertical sun-dial, the rod, or _style_,
+as it is called, being still parallel to the earth’s axis, but as a dial
+facing the south is only serviceable for twelve hours, another on the
+north wall is necessary for times before six in the morning and after
+six in the evening. As indicated by the sun-dial, it will always be noon
+when the sun is on the meridian, that is, when it is due south.
+
+The time indicated by sun-dials is distinguished astronomically as
+_apparent time_, and an _apparent solar day_ is the time which elapses
+between two successive southings of the sun. It is longer than the
+sidereal day, for the reason that the sun moves eastward among the
+stars.
+
+NECESSITY FOR MEAN TIME.—The varying speed of the earth in its orbit, or
+what comes to the same thing, the variable rate of the sun’s apparent
+eastward movement, prepares us for the discovery that the intervals
+between successive noons as indicated by sun-dials are unequal. That is,
+the apparent solar day is not of uniform length, and our clocks could
+not be regulated to indicate noon at the same moments as the sun-dial
+unless they were rated afresh every day. All our daily actions are
+regulated by the sun, and our time-keepers must also be controlled by
+its movement if they are to be as convenient as is necessary for
+purposes of everyday life. Our clocks and watches are therefore
+regulated to measure twenty-four hours in the time corresponding to the
+average duration of the apparent solar day throughout a year. In other
+words, they are controlled by the movements of an imaginary sun, called
+the _mean sun_, which is supposed to come to the meridian after equal
+intervals, and in order that it may do this while having a uniform
+motion, it must of necessity move along the celestial equator. In this
+way the time shown by our clocks and watches never departs very greatly
+from that shown by sun-dials, the maximum discrepancy being little more
+than a quarter of an hour. A _mean solar day_ is thus the average length
+of the apparent solar days throughout a year.
+
+THE EQUATION OF TIME.—The difference between apparent and mean solar
+time is called the _equation of time_, and a knowledge of its amount
+enables us to determine mean time from an observation of apparent time.
+
+One of the causes of this difference we have already seen to be the
+varying speed of the earth in its orbital movement; this produces a
+correspondingly irregular motion of the sun amongst the stars, and in
+consequence the true sun comes to the meridian after unequal intervals.
+Neglecting for a moment another cause of the varying length of the day,
+the relation of the apparent and mean solar days would be somewhat as
+follows:—Let us suppose that when the earth is at perihelion, we set our
+clocks to the same time as the sun-dial. In the interval which elapses
+before noon next day the true sun will have moved faster than the mean
+sun, because the earth, which produces the apparent eastward movement of
+the sun, is then travelling at its greatest speed. Consequently, our
+meridian will overtake the mean sun before it comes up to the true sun,
+and mean noon will occur before apparent noon; the difference will be
+the equation of time for the day, and it must evidently be added to
+apparent time in order to give mean time. This will go on for a certain
+period, when, in consequence of the reduced rate of the earth’s orbital
+velocity, the suns eastward motion will be less than that of the mean
+sun, and the two will again come to the meridian at the same time when
+the earth reaches its aphelion point; clocks and sun-dials would then
+give identical times. After aphelion passage, the earth is moving
+slowly, and the apparent eastward velocity of the true sun will be less
+than that of the mean; our meridian will therefore come to the true sun
+before it overtakes the mean sun, so that apparent noon will precede
+mean noon, and the equation of time will have to be subtracted from
+apparent time to give mean time. The two suns would again come together
+when the earth reached perihelion, and the equation of time, so far as
+this cause was concerned, would vanish. As the earth’s orbit is only
+slightly elliptical, the equation of time due to this cause alone would
+never amount to more than seven minutes.
+
+This, however, is by no means the whole cause of the equation of time; a
+still greater source of variation is the obliquity of the ecliptic. To
+investigate the part played by this inclination of the fundamental
+planes, let us now suppose that the true sun has a uniform angular
+motion in the ecliptic, while the mean sun moves uniformly along the
+Equator. Both these fictitious suns would have the same rate of movement
+along their respective paths, since they come back to the same places
+after the lapse of a year. If, then, these two suns start together at
+the equinox, both would indicate noon at that time, and there would be
+no equation of time. The “ecliptic sun” would then be moving at an angle
+of 23½° to the Equator, as along _a b_ in Fig. 17. If the distance _a b_
+represents the average daily movement of the “ecliptic” sun, and _d c_
+the equal movement of the mean sun, it is clear that our meridian will
+overtake the true sun at _b_ before the mean sun at _c_, so that
+apparent noon will precede mean noon, and the equation of time must be
+subtracted from apparent time to give mean time. The difference becomes
+greater up to a certain limit, and then since both suns will traverse
+90° in the same time, they will pass the meridian together at the
+solstice.
+
+[Illustration:
+
+  FIG. 17.—_Effect of Obliquity of Ecliptic upon the Equation of Time._
+]
+
+In the next quarter of a revolution, from solstice to equinox the
+difference is similar, but in the opposite direction, and the same
+applies to successive quadrants described throughout the year.
+
+The net amount of the equation of time at any moment is thus the added
+effects due to two causes.
+
+In 1896 the greatest and least values of the equation of time at
+Greenwich mean noon were as follows:—
+
+                    M. S.
+          Feb. 11   14 27 to be added to apparent time.
+          April 14   0  7         „         „        „
+          May 13     3 50 to be subtracted from apparent time.
+          June 13    0  6         „         „        „
+          July 25    6 17 to be added to apparent time.
+          August 31  0  0         „         „        „
+          Nov. 2    16 20 to be subtracted from apparent time.
+          Dec. 24    0  7 to be added to apparent time.
+
+A somewhat notable effect, owing its origin to the equation of time, is
+seen in the times of sunrise and sunset given in our almanacs. On
+November 8, for example, the sun rises at Greenwich at 6h. 58m., and
+sets at 4h. 31m., thus apparently making the afternoon about half an
+hour longer than the morning. As reckoned by the sun-dial, however, the
+morning and afternoon would differ only by a few seconds, and the
+peculiarity noted arises simply from the fact that our clocks keep time
+with the mean, and not with the true sun.
+
+DETERMINATION OF TIME.—Although the sun-dial may be used to indicate the
+time of day with sufficient accuracy for some purposes, its use is
+limited by the fact that it can only be employed when the sun is visible
+at the place of observation. Other modes of measuring the flow of time
+have, therefore, long been adopted. In early days, the rate at which a
+candle burned, or at which water or sand escaped through a small
+aperture, was employed as a time-measurer. Coming to more recent times,
+clocks and watches serve a similar purpose, but from what has already
+been stated, it is evidently necessary to regulate them according to the
+results of astronomical observations.
+
+The most precise determinations of time are made by means of a transit
+instrument, that is, an instrument by which the exact moment at which a
+celestial body passes the meridian can be observed. The positions of
+certain fundamental stars called “clock stars” have been determined with
+great accuracy, and it is therefore known to within a very small
+fraction of a second at what sidereal time one of these stars will pass
+the meridian. If the sidereal clock does not indicate this time when the
+star is observed on the meridian, its error can be noted and corrected.
+In this way the sidereal time is ascertained, and its equivalent in mean
+solar time is only a matter of simple calculation.
+
+Another method is to observe, by means of a sextant, or an altazimuth,
+the time, by a clock, at which the sun or a star has a certain altitude
+before noon, and the time at which it has the same altitude after noon.
+Midway between these times marks the time at which the body passed the
+meridian; the true sidereal time of passage is furnished by the known
+right ascension, and the corresponding mean time can therefore be
+calculated.
+
+At sea, time is most frequently determined by observing the altitude of
+the sun in the morning or evening, when it is nearly in an east or west
+direction. The time by the chronometer corresponding to a certain
+altitude of the sun is noted, and by spherical trigonometry the apparent
+solar time is deduced; mean solar time is then obtained by correcting
+for the equation of time. The nearer the sun is to due east or west, the
+more accurate are the results obtained by this method.
+
+TIME AT DIFFERENT PLACES.—In all these methods of finding the time,
+_local time_ is alone determined, whether it be sidereal or solar. When
+solar time is in question, we have seen that mean noon is determined by
+the passage of the mean sun across the meridian. All places on the same
+meridian will thus have equal times; but at places on different
+meridians, the local times will be different. When it is noon at
+Greenwich, it will be something before noon at places to the west of
+Greenwich (for the reason that the sun has not yet crossed their
+meridians), while at places to the cast it will be afternoon, because
+the sun has already passed the meridian. As the earth rotates through
+360° in a day, it will turn 15° in an hour, or 1° in four minutes. Hence
+at places 15° east of Greenwich the time will be an hour in advance of
+Greenwich time, while at places 15° west it will be an hour earlier. For
+places in other longitudes, the difference of time is in the same
+proportion. The following are the local times at several places when it
+is noon at Greenwich:—
+
+                               A.M.            P.M.
+                     Dublin    11.35 Paris      0.9
+                     New York    7.4 Berlin    0.54
+                     Toronto    6.42 Calcutta  5.53
+                     Vancouver  3.38 Melbourne 9.40
+
+Throughout the whole of England and Scotland, Greenwich mean time is
+exclusively employed in preference to local times. This has the very
+practical advantage of uniformity; and as in no case does local time
+differ more than half an hour from Greenwich time, there is little
+inconvenience in regard to the beginning and end of day.
+
+Until recently, the time systems of other countries have been mainly
+based on the times corresponding to their various national
+observatories. At present, what is called “zone time,” in which the
+hours alone differ from Greenwich time, has been adopted in several
+European states, as well as in other parts of the world.
+
+The present state of time reckoning on this much improved plan is
+indicated by the following table:—
+
+           _Country._           │           _Standard time._
+ ───────────────────────────────┼───────────────────────────────────────
+ England                        │
+ Belgium                        │Greenwich time.
+ Holland                        │
+ ───────────────────────────────┼───────────────────────────────────────
+ Denmark                        │
+ Germany                        │Mid-European time, 1 hour fast on
+ Italy                          │  Greenwich.
+ Switzerland                    │
+ Norway and Sweden              │
+ ───────────────────────────────┼───────────────────────────────────────
+ Colony of Natal                │2 hours fast on Greenwich.
+ ───────────────────────────────┼───────────────────────────────────────
+ United States                  │4, 5, 6, 7, or 8 hours slow on
+ Canada                         │  Greenwich, according to longitude.
+ ───────────────────────────────┼───────────────────────────────────────
+ Japan                          │9 hours fast on Greenwich.
+ ───────────────────────────────┼───────────────────────────────────────
+ Western Australia              │8   „      „        „
+ ───────────────────────────────┼───────────────────────────────────────
+ South Australia                │9   „      „        „
+ ───────────────────────────────┼───────────────────────────────────────
+ Victoria, New South Wales,     │10  „      „        „
+ Queensland, and Tasmania       │
+ ───────────────────────────────┴───────────────────────────────────────
+
+TELEGRAPHING TIME.—An important part of the work of the chief national
+observatories is the determination of correct time, and its
+communication to the public at large. Railways have especially created a
+demand for a uniform and accurate system of time reckoning, and to meet
+this need there is usually an organised service providing an automatic
+distribution of time-signals by means of the electric telegraph. The
+transmission of such time-signals was first established on a large scale
+in connection with Greenwich Observatory, and at the present time
+signals are sent to the General Post Office, whence they are distributed
+automatically to post offices and subscribers throughout the kingdom. In
+addition, signals are sent direct to Westminster for the regulation of
+the great clock on the Houses of Parliament, and time-balls are dropped
+at certain hours at Greenwich and Deal, in order that navigators may
+have the opportunity of rectifying their chronometers.
+
+THE YEAR.—The day is too small an interval of time to be conveniently
+employed as a unit for chronological purposes, so that at present the
+count of time by days is practically limited to the number of days in a
+month. A greater unit, but still too small, is supplied by the month,
+and the necessity for a more serviceable unit early led to the adoption
+of the length of the year. This is at once a natural division of time,
+corresponding to the recurrence of the seasons, and sufficiently answers
+all requirements for measuring extended intervals.
+
+If we determine the exact time required by the sun to pass from one
+fixed point in the heavens to the same point again, we shall find the
+time in which the earth makes a complete revolution round the sun, that
+is, the time in which a line joining the earth and sun sweeps through an
+angle of 360°. This interval, which is called the _sidereal year_,
+amounts to 365 days 6 hours 9 minutes 9 seconds of mean solar time. It
+will be clear, however, that the most useful year is that which will
+give us the same day of the month at the same season in all years. If
+there were no precession of the equinoxes, this would be of the same
+length as the sidereal year, but on account of precession the passage of
+the sun from the vernal equinox to the same equinox again occupies less
+than a sidereal year. In fact, this equinoctial, or _tropical year_
+amounts to 365 days 5 hours 48 minutes 46 seconds; that is, about 20
+minutes less than the sidereal year. This is the year which is always
+understood, unless it is otherwise stated. If our calendars were
+regulated according to the sidereal year, the same day of the month
+would in time run through all possible changes of seasons, the 25th of
+December, for instance, occurring at one time in winter, and gradually
+changing through spring, summer, and autumn.
+
+THE CALENDAR.—The earlier calendars with which history acquaints us were
+mainly based on the lunar month of about 29½ days, twelve of which made
+up a lunar year of 354 days. The calendar year was thus more than 11
+days shorter than the actual year, and in order to bring the dates into
+agreement with the seasons, arbitrary intercalations were occasionally
+made by the authorities.
+
+In the year 45 B.C. a great reform was introduced by Julius Cæsar; 365¼
+days was adopted as the length of the year, and it was prescribed that
+ordinary years should be reckoned as consisting of 365 days, while every
+fourth year divisible by 4 without remainder should be a _leap year_ of
+366 days. Matters were so much simplified by this arrangement that the
+Julian calendar remained unaltered until 1582, and is even now retained
+throughout Russia.
+
+The tropical year, as we have seen, is less than 365¼ days, so that the
+Julian calendar does not quite keep course with the seasons. Although
+the difference is only 11¼ minutes, it amounts to an entire day in 128
+years, so that if the vernal equinox occurred on the 21st of March at
+one time it would occur on the 20th after 128 years. If, then, it be
+desired to bring the existing dates of any particular year into
+agreement with dates at a previous period, as regards the seasons, a
+correction in addition to that ordained by Cæsar must be introduced. In
+the time of Pope Gregory, in the year 1582, the vernal equinox fell on
+the 11th of March, and the necessity of a new calendar came to be
+recognised. The astronomer Clavius, with the authority of the Pope,
+devised our present “Gregorian” calendar. This arrangement, first of
+all, altered the actual date of the equinox from the 10th to the 21st of
+March, that is, to the day on which it occurred in the year of the great
+Council of the Church at Nicæa, 325 A.D. To bring about this alteration
+it was necessary to drop 10 days from the calendar, and it was therefore
+decided that the day following the 4th of October, 1582, should be
+called the 15th instead of the 5th. To prevent subsequent changes in the
+date of the equinox the Julian rule for leap year was slightly modified.
+If the date number of a year is divisible by 4 without remainder it is
+still to be a leap year, unless it be a century year, in which case it
+must be divisible by 400 without remainder if it is to be called a leap
+year.
+
+It was not until 1752 that the Gregorian calendar was adopted in
+England, and as 1700 was a leap year according to the Julian rule the
+old style date was 11 days behind the Gregorian date. An Act of
+Parliament decreed that the day following September 2, 1752, should be
+called the 14th. The Act was carefully planned so as to prevent
+injustice in the collection of rents and the like, but it was only
+accepted after considerable opposition.
+
+It has lately been pointed out that if we wish to make the day of the
+year correspond with the seasons for all time, a modification of the
+Gregorian calendar must be adopted. By the Gregorian rule, three leap
+years are omitted every four centuries; but Mr. W. T. Lynn has drawn
+attention to the fact that if one were dropped every 128 years instead,
+the calendar would be sensibly perfect, and the seasons would always
+commence on the same dates.
+
+
+
+
+                              CHAPTER VI.
+                       THE MOVEMENTS OF THE MOON.
+
+
+THE MOON’S REVOLUTION.—Apart from the changes in the appearance of the
+moon due to the ever-varying phases, the first fact which strikes the
+attentive observer is that the moon has an eastward movement among the
+stars, and that this motion is much more rapid than that of the sun.
+Indeed, the moon gains a whole revolution upon the sun in a period of
+about 29½ days, this being the interval between two successive new or
+full moons. As referred to the stars, however, it is found that the moon
+and any particular star which cross the meridian together at a certain
+time will again do so after the lapse of only 27⅓ days. Besides this
+eastward movement among the stars, the moon moves towards and away from
+the Pole; the full moon, for instance, is sometimes seen high in the
+heavens at midnight, and at other times very low. Indeed, the moon’s
+apparent movements resemble in a very general way those of the sun, but
+they cannot be attributed to a revolution of the earth round the moon,
+as those of the sun are to a real movement of the earth round the sun.
+We have seen that there are direct proofs of the earth’s revolution
+round the sun, and a revolution round the moon, even in a smaller orbit,
+would not be consistent with the observed movements of the greater
+luminary. Being convinced of the reality of the moon’s movements around
+the earth, we can next proceed to investigate the circumstances of its
+varied motions.
+
+Just as we learn the conditions of the earth’s movements by observations
+of the sun’s apparent movements which are their natural consequence, we
+can determine the moon’s motions by studying its varying situations with
+regard to the much more distant stars. We can measure the moon’s right
+ascension and declination at different times with the transit
+instrument, and, if desired, we can mark out the apparent path on our
+star charts or celestial globes. In this way it is found that the moon
+moves in a plane which is inclined at 5° 9′ to the plane of the
+ecliptic. As to the shape of the orbit, we have only to observe the
+changes in the moon’s apparent size; when it is nearest to us it will
+appear largest, and when furthest removed its apparent diameter will be
+least. Actual observations show that, like the orbit of the earth, the
+moon’s orbit is an ellipse, with the earth in one focus. Owing to
+various causes, the orbit is somewhat variable in shape, and its
+eccentricity ranges from 0·07 to 0·045. When the moon is at the point of
+its orbit nearest to the earth, it is said to be in _perigee_; and when
+at the most distant part of its orbit, in _apogee_.
+
+The earth’s orbit, as we shall see by and by, is very small as compared
+with stellar distances, and the moon’s apparent movement, with regard to
+the stars, is not affected by the revolution of the earth and moon round
+the sun; consequently the interval between its passing a star and
+overtaking the same star again is a measure of the time in which the
+moon’s movement round the earth is performed—this is 27 days, 7 hours,
+43 minutes, and is called the moon’s _sidereal period_. The direction of
+the moon’s motion is opposite to that of the hands of a clock, a
+movement which is said to be _direct_ (motion in the reverse direction
+would be _retrograde_).
+
+PHASES.—Two circumstances lead us to suppose that the light of the moon
+is borrowed from the vast store thrown out into space by the sun. First,
+the fact that it puts on _phases_, for if it were a body shining by its
+own light we should always see a full moon. Second, the fact that the
+phase we see depends absolutely on the moon’s situation with regard to
+the sun and earth.
+
+There is every reason to suppose that the moon is a dark globular body,
+so that the sun can only illuminate that hemisphere which is turned
+towards it. At new moon the illuminated part is turned directly away
+from us, and we are thus led to infer that when new the moon lies
+directly between the earth and sun. At full moon, on the contrary, the
+whole of the illuminated part is presented to us, and we therefore
+conclude that at this time the earth lies between the sun and moon. On
+account of the inclination of the moon’s orbit to that of the earth, the
+sun, earth, and moon do not always come exactly in a straight line at
+new or full moon; when they do, the interesting phenomena of solar and
+lunar eclipses occur. (Chapter VIII.)
+
+A diagram will help to elucidate the production of the moon’s
+intermediate phases. Supposing the sun’s rays to proceed from the left,
+the earth being at O, the moon will be at A when new. Proceeding towards
+B, a small portion of the illuminated side will be turned towards us,
+and the moon will be a crescent. On reaching the point C, exactly half
+of the sunlit hemisphere will be visible to us, and we have the moon’s
+_first quarter_. Passing to the point D we see more than half of the
+bright part of our satellite, and it appears gibbous in form, until it
+reaches E, where it becomes full. Similar phases occur in inverse order
+during the movement along the other part of the orbit.
+
+[Illustration:
+
+  FIG. 18.—_The Moon’s Phases._
+]
+
+Such would be the conditions as to the phases of the moon, if the earth
+were at rest.
+
+THE MONTH.—If the earth were fixed in space with regard to the sun, the
+moon’s phases would be repeated in the time corresponding to its period
+of revolution round the earth. This is 27 days 7 hours 43 minutes, and
+measures the length of a sidereal month.
+
+It is much more useful, however, to refer the month to the phases
+actually observed. If in Fig. 19 we have the sun, earth, and moon
+represented at a full moon by S, E, and M respectively, the next full
+moon will not occur until the three bodies occupy the positions S, E′,
+and M′, the earth having travelled about 30° along its orbit. Between
+two full moons, then, the moon must make a complete revolution round the
+earth, and through an additional angle, A E′ M′, which will be equal to
+the earth’s angular motion in the interval. This movement of the moon
+occupies 29 days 12 hours 44 minutes, and is the duration of a _lunar
+month_. It also determines the _synodic period_ of our satellite, a term
+which, taken generally, signifies the period in which a planet or
+satellite recovers the same position with respect to the sun when
+observed from the earth.
+
+[Illustration:
+
+  FIG. 19.—_The Lunar Month._
+]
+
+A calendar month, of which there are twelve in a year, must of necessity
+consist of a whole number of days, and the average duration of such a
+month is longer than that of a lunar month.
+
+A remarkable relation exists between the synodic month and the length of
+the year. In 19 Julian years of 365¼ days there are almost exactly 235
+synodic months, so that after the completion of this period full moons
+again occur on the same days of the month. The discovery of this cycle
+is usually ascribed to Meton, a Greek astronomer, 433 B.C. It is
+accordingly known as the _Metonic Cycle_, and is still used in the
+calculation of the moveable festival of Easter.[1]
+
+ROTATION AND LIBRATIONS.—Even observations made without instrumental
+assistance show that the surface of our satellite always presents the
+same face to us, and without further inquiry one might suppose that it
+had no axial movement corresponding to that of its primary. If there
+were no rotation, however, we should in turn see all parts of the moon,
+and the observed circumstances indicate that it must rotate on an axis,
+in the same direction as that of its orbital movement, and in the same
+time. In Fig. 20 let E represent the earth, and _a b c_ the part of the
+moon which is turned towards us when it is at M. When the moon arrives
+at M′, observations show us that the same part is presented to our view,
+so that the part corresponding to that we saw in position M is
+represented by _a′ b′ c′_. Now, if the moon had not rotated in the
+interval, the line joining _a_ and _c_ would have retained the same
+direction, and would have been in the position _d e_; the part _c′ e_
+would thus have been carried out of sight, while another part which was
+not seen when the moon was at M would have come into view. In order that
+we may see the same part of the moon in two different positions, M and
+M′, the dividing-line _a c_ between the visible and invisible portions
+must turn through an angle equal to that between the lines _d e_ and _a′
+c′_; and since this angle is equal to that described by the moon in the
+same time, the period of the moon’s rotation on its axis must be equal
+to that of its revolution round the earth.
+
+On account of the elliptical form of its orbit, the angular movement of
+the moon is not quite uniform; like the earth, it is subject to the law
+of areas. Hence, as the rotation is equable, the foregoing explanation
+does not strictly hold. In fact, this varying velocity results in a
+_libration in longitude_, which means that we sometimes see a little
+more of the western edge and sometimes of the eastern edge. There is
+also a _libration in latitude_ on account of the fact that the moon’s
+axis is inclined to the plane of its orbit, so that at different times
+we see more of the North or South Pole, as the case may be; in this
+respect the moon behaves to the earth somewhat as the earth does to the
+sun in regard to the seasons, but the inclination is not so great.
+
+[Illustration:
+
+  FIG. 20.—_The Moon’s Rotation._
+]
+
+The moon is so near to us that the portion of it which we see depends to
+a slight extent upon our terrestrial location. When the moon is rising
+we see a little more of its western edge than will be seen by an
+observer to the east of us, where the moon is in the south, and more
+than we ourselves shall see when it has come to our own meridian. Just
+before the time of setting we get to see a little beyond the eastern
+edge. This is called the _diurnal libration_, and never amounts to more
+than a degree.
+
+Thanks to these librations, we are enabled to make telescopic
+observations of 9 per cent. of the moon’s surface which would not
+otherwise be open to our investigations.
+
+CHANGES OF THE MOON’S ORBIT.—The moon’s orbit is by no means to be
+regarded as a hard and fast geometrical figure. Indeed, it is subject to
+such great distortions in consequence of “perturbations” that the
+computation of the moon’s position at any future time is one of great
+complexity. One of the most easily recognised changes in the orbit is
+the revolution of its _nodes_, that is, of the points where it crosses
+the plane of the ecliptic.
+
+[Illustration:
+
+  FIG. 21.—_The Moon’s Nodes._
+]
+
+The latter being a plane of indefinite extent, to which the moon’s orbit
+is inclined at 5° 9′, the moon will be alternately above and below the
+ecliptic for about half its period of revolution. The point where it
+passes from south to north of the ecliptic, A in Fig. 21, is the
+_ascending node_, and the corresponding point on its southward path is
+the _descending node_ of the orbit. Connecting these two points is the
+line of nodes (A B), and by observations of the points where the moon’s
+path intersects the ecliptic at different times it is found that the
+line of nodes _regredes_ or moves backwards. The rate of this revolution
+of the moon’s nodes is very irregular, but a whole revolution is made in
+18·6 years.
+
+This retrogression of the moon’s nodes may be well illustrated by the
+following heliocentric longitudes of the ascending node as given in
+recent “Nautical Almanacs”:
+
+                      1892 January 1  53° 51′·56.
+                      1893     „      34° 28′·69.
+                      1894     „      15° 19′·00.
+                      1895     „     355° 49′·31.
+                      1896     „     336° 29′·61.
+
+The line of apsides of the moon’s orbit joins the perigee and apogee;
+the direction of this line in space changes in a very variable manner,
+but in the long run it makes a complete revolution in 8·9 years.
+
+When the sun is passing through the moon’s line of apsides it
+temporarily increases the eccentricity of the orbit; when at right
+angles to this line, the orbit becomes more nearly circular. This
+disturbance of the moon has accordingly a period equal to that required
+for two successive passages of the sun over the apse line of the moon’s
+orbit.
+
+Such are a few of the movements which come within the province of the
+_lunar theory_, a fuller treatment of which is beyond our scope.
+
+THE HARVEST MOON.—The full moon which occurs nearest to the autumnal
+equinox is called the _harvest moon_, for the reason that it rises very
+nearly at the same hour for several nights together, and so gives us a
+greater share of moonlight, by which harvest operations may be extended.
+At that time the sun will be at the autumnal equinoctial point, and when
+it is setting in the west, the vernal equinoctial point, and the moon
+with it, must be rising due east. The part of the ecliptic then above
+the horizon will extend from the east to the west point, but will lie
+wholly below the celestial equator (Fig. 22). As the moon’s path is very
+slightly inclined to the ecliptic, its movement will thus make only a
+small angle with the horizon, and for several nights together it will
+rise at nearly the same time.
+
+In March, when the sun is near the vernal equinox, the full moon will be
+near the autumnal equinoctial point; when the sun is setting, the moon
+will be rising as before, but in this case the part of the ecliptic
+which is above the horizon lies wholly above the celestial equator. The
+ecliptic is thus inclined at an angle to the horizon greater by 47° than
+when the vernal equinox is rising in autumn; the moons path being near
+the ecliptic, its movement during a day will at this time carry it a
+long way below the Equator, and it will rise much later the following
+day.
+
+[Illustration:
+
+  FIG. 22.—_Position of Ecliptic at Sunset at Vernal Equinox_ (E A W)
+    _and Autumnal Equinox_ (E B W).
+]
+
+In the Southern Hemisphere, the conditions are reversed, the harvest
+moon occurring at our vernal equinox, which, however, is the
+commencement of the southern autumn quarter.
+
+The phenomena of the harvest moon recur, but are not so marked, in the
+month of October, and it is then called the hunter’s moon.
+
+It is important to bear in mind that this rising of the moon at nearly
+the same hour for several days occurs every month, but as the risings
+then occur either in daylight or after midnight, and the moon is not
+full, no special attention is drawn to them.
+
+Again, since the phenomenon of the harvest moon depends upon the small
+inclination of the path of the full moon to the horizon when it is at
+the equinoctial point, the circumstances will be modified by the
+latitude of the place of observation. At the Equator, for example, there
+will be no harvest moon, as there the ecliptic is always greatly
+inclined to the horizon; in fact, it will be inclined at the same angle
+in spring as in autumn.
+
+The moon’s path being inclined to the ecliptic, the conditions as to the
+harvest moon will depend to a small extent upon the position of the
+moon’s nodes, which, as we have seen, revolve in a period of a little
+less than 19 years. At times, then, the moon’s path will be inclined 5°
+more, and 9 years afterwards 5° less, than is the plane of ecliptic, and
+under the latter conditions the harvest moon will be most pronounced.
+
+HIGH AND LOW MOONS.—At the time of full moon, the moon is in the
+opposite part of the heavens to that occupied by the sun, sometimes
+being 5° above and other times 5° below. Manifestly, then, if the sun be
+high in the heavens at mid-day, it will be only a little below the
+northern horizon at midnight, and the moon, consequently, will be only a
+small distance above the southern horizon. In summer, then, quite apart
+from the fact that the nights are shorter, there is less moonlight. In
+winter, on the other hand, the sun descends far below the northern
+horizon at midnight, and the full moon has a high elevation in the
+southern part of the sky. By this happy arrangement, the full moon is
+longest above the horizon when its light is of greatest benefit to
+mankind.
+
+
+
+
+                              CHAPTER VII.
+             MOVEMENTS OF PLANETS, SATELLITES, AND COMETS.
+
+
+APPARENT MOVEMENTS OF PLANETS.—It has already been pointed out that like
+the sun and moon, the planets also have an apparent movement with
+respect to the more distant stars. Mercury and Venus are never seen very
+far from the sun, while other planets, among which are Mars, Jupiter,
+and Saturn, may be seen in the part of the heavens opposite to the sun.
+
+One point, and that a very important one, which we notice from our
+observations is that the planets never depart very far from the
+ecliptic, so that the planes in which they perform their movements are
+nearly coincident with the plane in which our own annual journey round
+the sun is performed. The apparent movements of the planets are such
+that it is quite impossible to regard these bodies as circulating in
+regular orbits round the earth itself. If they revolve round any other
+body it is manifest that their apparent or geocentric motions will be
+compounded of the real movements of the planets and that of the earth.
+It is not necessary here to trace the steps by which it has been
+determined that the planets revolve in regular orbits around the sun.
+Suffice it to say that their observed movements are simply and
+sufficiently explained by supposing that, like the earth, which may now
+be regarded as a planet, they travel in elliptic orbits with the sun at
+one of the foci. Besides this revolution, the planets have a rotatory
+motion about their axes, but this question cannot be studied apart from
+the telescopic features, and will therefore be treated in Section III.
+of the present work.
+
+The circumstance that the planets Mercury and Venus are never seen long
+after sunset or before sunrise, indicates that their orbits must lie
+between us and the sun. Hence, they are distinguished as the _interior
+planets_, while those outside the earth’s orbit are called the _exterior
+planets_.
+
+MOVEMENTS OF INTERIOR PLANETS.—Let us consider briefly the conditions
+under which we observe the interior planets. If such a planet be
+represented by M in Fig. 23, while the earth is represented by E
+traversing a larger orbit, the planet is said to be in _inferior
+conjunction_ with the sun, when it lies directly between the sun and
+earth. The actual movements of the planets being direct—that is,
+anticlockwise—the planet at M has an apparent westerly motion as seen by
+an observer situated on the earth, and from this we gather that it moves
+more rapidly than the earth. For simplicity let us regard the earth as
+being at rest at the point E. Then, as the planet reaches the position
+M′, where it is as far as possible to the west of the sun, it is said to
+be at its _greatest western elongation_. Proceeding in its orbit, the
+planet’s apparent movement is direct, and it eventually comes in line
+with the sun on the further side as seen from the earth; it is then said
+to be in _superior conjunction_. From this point the planet moves to the
+east of the sun until it comes to the point M, after which the motion
+becomes retrograde, and the planet proceeds to inferior conjunction
+again. When at its greatest distance to the east of the sun, as at M‴,
+the planet is said to be at its _greatest eastern elongation_. Taking
+the term _elongation_ in general, it may be regarded as a measure of the
+angular distance of a planet from the sun as observed from the earth.
+
+[Illustration:
+
+  FIG. 23.—_Movement of an Interior Planet._
+]
+
+If the orbits of the planets were perfect circles, the greatest
+elongation distances of an interior planet would always be the same;
+sometimes, however, we are nearer to the sun than at the other times,
+and the apparent separation of the planet from the sun would seem
+greater than at other times, even if there were no other cause at work.
+The variations of the elongation distances are greater than can be
+accounted for by our own varying distance, and are naturally attributed
+to the elliptical form of the orbits of the interior planets themselves.
+Mercury, for example, sometimes only departs 18° from the sun, while at
+other times it reaches as far as 28° east or west.
+
+When we take account of the fact that the earth has also a movement
+along its orbit, it will be seen that the same conditions hold good with
+regard to elongations and conjunctions, except that the intervals
+between them will be longer.
+
+[Illustration:
+
+  FIG. 24.—_Morning and Evening Stars._
+]
+
+MORNING AND EVENING STARS.—From superior to inferior conjunction an
+interior planet is to the east of the sun. It then rises after the sun,
+and sets after the sun, so that it is visible for a short time in the
+early evening; in other words, it is an _evening star_ during this part
+of its path. Between inferior and superior conjunctions, the planet is
+conversely a _morning star_. This is illustrated in Fig. 24, where the
+position of an observer towards whom the sun is rising is shown at A. An
+interior planet at P is above the horizon at sunrise, but will be below
+at sunset, the observer having been carried to A′ by the earths
+rotation; it will thus be a morning star. When the planet occupies the
+position P′ it is below the horizon at sunrise, but will remain in sight
+after the sun has set in the evening, the observer then having been
+transferred to A′ by the earth’s rotation.
+
+PHASES OF INTERIOR PLANETS.—From the conditions which have been stated
+with regard to the movements of the interior planets, one is not
+surprised to find that telescopic examination reveals that these bodies
+put on phases similar to those of the moon. At superior conjunction the
+planets exhibit a fully illuminated disc, at greatest elongations they
+appear as a half moon, while at inferior conjunction their dark sides
+alone are presented to us. The apparent sizes of the planets, as
+measured with the aid of a telescope, are also found to vary according
+to their positions; when at inferior conjunction, the planet is much
+nearer to us than at other times, and it consequently appears larger.
+The apparent brightness of an interior planet also varies. At superior
+conjunction the whole of the disc is illuminated, but the planet is then
+so far removed from us that its light is very feeble. On the other hand,
+at inferior conjunction, when it is nearest to us, the dark side of the
+planet is turned towards us. The greatest brightness thus occurs at some
+intermediate point. In the case of Venus this is between the greatest
+elongations and inferior conjunction, when it is 40° from the sun. It is
+then bright enough to be seen with the naked eye in full sunshine, and
+has sometimes, on such occasions, been erroneously regarded by ignorant
+persons as the Star of Bethlehem.
+
+TRANSIT OF VENUS.—If an inferior conjunction occurs when the planet is
+very near to a node—this term having the same significance as in the
+case of the moon (p. 94)—the planet, whether it be Mercury or Venus,
+will be seen projected as a dark spot upon the bright disc of the sun.
+Such an occurrence is called a _transit of Venus_ or of Mercury, as the
+case may be. Just as we do not get an eclipse of the sun every month, so
+we do not get a transit of Venus every time the earth and that planet
+have the same heliocentric longitude, and for the same reason, namely,
+that the plane of the orbit is inclined to the ecliptic. As we shall see
+in another chapter, a transit of Venus has a most important application
+in the determination of one of the fundamental constants of
+astronomy—the sun’s distance. The conditions as to the recurrence of
+transits are of great interest. In the case of Venus, the _synodic_
+period is 584 days, this being the time which elapses between two
+successive inferior conjunctions. Five synodic periods are thus very
+nearly equal to eight years, and 152 synodic revolutions are even more
+nearly equal to 243 years. As seen from the earth, the sun crosses the
+nodes of the orbit of Venus on June 5 and December 7, and since there
+can be no transit when the planet is more than 4½° from the node, the
+transits will all occur about these dates. A transit will be followed by
+another after the lapse of 8 years, if the planet is not too far from
+the node; but there can be no other transit with the planet at the same
+node until 243 years have elapsed. There are, however, transits
+occurring at similar intervals when the planet is at the other node. The
+following dates on which transits have occurred, or will occur, will
+illustrate the foregoing statements:—
+
+            8 years│December 7, 1631,│243 years.│——————————
+               „   │December 4, 1639,│    „     │243 years.
+            8 years│December 9, 1874,│    „     │    „
+               „   │December 6, 1882,│——————————│    „
+            8 years│June 5, 1761,    │243 years.│——————————
+               „   │June 3, 1769,    │    „     │243 years.
+            8 years│June 8, 2004,    │    „     │    „
+               „   │June 6, 2012,    │——————————│    „
+
+[Illustration:
+
+  FIG. 25.—_Movement of an Exterior Planet._
+]
+
+MOVEMENTS OF EXTERIOR PLANETS.—The exterior planets are at once
+recognised as such by their occasional appearance in the part of the sky
+opposite to that of the sun. They are then said to be in _opposition_.
+When in the same line as the sun, and on the remote side of it, as at P′
+in Fig. 25, the planet is in _conjunction_. The apparent movements of
+such a planet are very complex. Neglecting for a moment the earth’s
+motion, it is evident that the apparent rate of movement of the planet
+with reference to the stars will vary very considerably according as the
+planet is near opposition or near conjunction, the movement appearing to
+be most rapid when the planet is nearest to us. Upon this unequal rate
+of motion is superposed a varying direction of motion produced by the
+changing position of the earth. When the planet is at P, and the earth
+at E, both are moving in the same direction, but as the earth has the
+greater angular velocity, the apparent motion of the planet will be
+retrograde, that is, the planet will appear to go backwards in its path.
+If the earth be near the point E′, its orbital movement will be directed
+away from the planet, and will scarcely affect its apparent position;
+accordingly, about this time the planet has a direct movement in the
+heavens. Between these two points the direction of the apparent movement
+of the planet has changed, so that at some intermediate position it
+would seem to have suspended its wanderings; here we have a _stationary
+point_. For a certain time, before and after conjunction, the linear
+directions of movements of the earth and planet will be opposed to each
+other, and on this account the _direct_ apparent motion of the planet
+will be accelerated. Presently, as the earth gains on the planet,
+another stationary point will be reached, and with the approach to
+opposition the planet will again retrograde.
+
+If both orbits were in the same plane, these apparent movements would
+all be backwards and forwards along a great circle of the celestial
+sphere coincident with the ecliptic, the eastward movement
+predominating. The planes in which the planets perform their revolutions
+are, however, inclined to the ecliptic, and the result is that they
+appear to us to travel in loops, some of which are illustrated in Fig.
+26.
+
+[Illustration:
+
+  FIG. 26.—_Apparent Paths of Ceres, Pallas, Juno and Vesta, in 1896._
+]
+
+From the fact that we are constantly within the orbit of an outer
+planet, it is evident that we must always see more than half of the
+planetary hemisphere on which the sun is shining. Consequently, an
+exterior planet never puts on a crescent phase, or presents the
+appearance of a half moon. The nearer the planet the greater will be the
+dark area which it is possible for us to observe. In the case of Mars,
+for example, we sometimes see it gibbous like the moon about three days
+from full, but in the more distant planets this gibbosity is scarcely
+perceptible. The greatest phase of an exterior planet occurs when it is
+at _quadrature_, that is, when a line joining the earth and sun is
+perpendicular to one joining the earth with the planet.
+
+FAVOURABLE AND UNFAVOURABLE OPPOSITIONS.—A little consideration of Fig.
+25 will make it perfectly clear that an exterior planet is very much
+nearer to us at a time of opposition than at a conjunction. We are, in
+fact, then, nearer to the planet by the diameter of the earth’s orbit, a
+matter of some 186 millions of miles. Accordingly, the planets, more
+especially our neighbour Mars, are best studied in the telescope about a
+time of opposition. Now, if we had to deal with circular orbits, the
+distance of a planet at opposition would remain constant, and we should
+see the planet equally well at all oppositions. It is found, however,
+that this is not the case, and the ellipticity of the orbits of the
+earth and planets supplies a simple and sufficient explanation. Sir
+Robert Ball illustrates this in the case of Mars by a diagram similar to
+Fig. 27. It will be seen that, when the opposition occurs in August, the
+earth is much nearer to Mars than when it happens at other times. The
+least favourable oppositions are those which occur in February, the
+planet then being nearly twice as far removed from us as at the nearest
+approach during an August opposition.
+
+[Illustration:
+
+  FIG. 27.—_Opposition of Mars._
+]
+
+As regards the more distant planets, the diameter of the earth’s orbit
+and the variations of opposition distance are of less importance, since
+they form a much smaller proportion of the distances of those planets
+from the sun.
+
+ELEMENTS OF A PLANETARY ORBIT.—A complete study of the apparent
+movements of the planets with which we are acquainted shows that their
+real movements are performed round the sun in ellipses, the sun being
+placed at a focus. Each orbit, like that of the earth, has its
+perihelion and aphelion points, and its apse line; not being coincident
+with the ecliptic, it will have a line of nodes, and an ascending and
+descending node. Each planet will further have a particular inclination
+to the ecliptic, and a period of revolution peculiar to itself.
+Consequently, to systematise our knowledge of any particular orbit,
+certain conventions are adopted, and the seven things we must know, in
+order that we may specify the size of the orbit, its position in space,
+and the situation of the planet in its orbit, are as follows:—
+
+  _a_ = Semi axis major of elliptic orbit.
+
+  _e_ = Eccentricity.
+
+  _i_ = Inclination to ecliptic.
+
+   Ω  = Longitude of ascending node.
+
+   π  = Longitude of perihelion.
+
+   P  = Period of revolution. (_u_, the mean daily motion, sometimes
+          replaces P.)
+
+   E  = The epoch, giving the longitude of the planet at some particular
+          time.[2]
+
+The first two quantities indicate the size and shape of the orbit, the
+next three its position with regard to the ecliptic, and the last two
+are required to determine the situation of the planet in its orbit. Some
+of the elements are illustrated in Fig. 28.
+
+[Illustration:
+
+  FIG. 28.—_Elements of an Elliptic Orbit._
+]
+
+DETERMINATION OF A PLANET’S PERIOD.—Observations enable us to determine
+the synodic period of a planet, and knowing that the earth’s period is a
+year, it is a simple matter to determine that of the planet. In the case
+of an exterior planet, the interval from opposition to opposition
+furnishes the best means of determining the synodic period. The exact
+moment of an opposition cannot usually be directly observed, and what
+one actually does is to measure the R.A. and declination of the sun on
+several days about the time of opposition, as also those of the planet;
+then, by reducing these co-ordinates to celestial longitude and
+latitude, it is not difficult to determine at what moment the longitudes
+differed by 180°, that is, the moment at which opposition took place.
+The problem of finding the planet’s sidereal period, then, amounts to
+this: at what rate must the planet be moving in order that the earth may
+make a complete revolution, and move, in addition, through the same
+angle as the planet? In other words, what must be the period of the
+planet in order that the earth may gain a whole revolution in the
+interval corresponding to the synodic period? The daily movement of the
+planet will be 360°/P, and that of the earth 360°/365¼, if P denote the
+number of days in the planet’s sidereal period. The earth’s gain per day
+will thus be the difference between these two quantities, and since a
+whole revolution is gained in the synodic period, the gain per day can
+be expressed as 360°/S, where S represents the synodic period; thus we
+get
+
+                        360°/365¼ − 360°/P = 360°/S
+                     or
+                        1/365¼ − 1/P = 1/S
+
+The synodic period of Mars is 780 days, and the application of the
+foregoing formula leads us to 687 days as the time of its revolution
+round the sun.
+
+A single determination of a synodic period does not give precise
+results, for the reason that the orbits of the planets are elliptical,
+and the intervals consequently dependent upon whether the planet is near
+perihelion, or far removed from it when an opposition is observed. It
+is, therefore, necessary to determine the time of opposition at long
+intervals, and so reduce the errors in measuring the length of a single
+period.
+
+MOVEMENTS OF SATELLITES.—Telescopic observations show that some of the
+planets are accompanied by _satellites_, which revolve round their
+primaries as the moon revolves round the earth. The apparent movements
+of these bodies, with regard to the planets, are very similar to those
+of the interior planets with regard to the sun, having similar points of
+greatest eastern and western elongations. The facts which have been
+collected show that each satellite, like our own moon, moves in an
+elliptical orbit, with the planet in one of its foci. With one
+exception, the satellites attending the planets of our system have a
+direct movement; those of Uranus, however, have apparently a movement in
+the same direction as the hands of a watch, but this can be regarded as
+direct, if we consider the plane of the orbit to be inclined more than
+90° to the plane of the ecliptic.
+
+THE ORBITS OF COMETS.—Another class of bodies which circulate round the
+sun now claims our attention. These are the _comets_, some of which are
+never seen without the aid of telescopes, while others have been
+brilliant enough to excite a widespread wonder and interest. They
+usually have a very rapid movement relatively to the stars; and to learn
+something as to their real motions, we commence by measuring their right
+ascensions and declinations as frequently as possible. When such
+observations are plotted, they give us the geocentric movement of a
+comet, which generally seems very irregular, and gives one the idea that
+it is subject to no law. Unlike the planets, comets do not usually keep
+near the ecliptic, but move in planes inclined at all angles to it.
+Their rates of apparent movement also change very rapidly.
+
+When the effect of the earth’s movement upon that of a comet is
+eliminated, it is found that the movement of the comet is performed
+either in an ellipse, a parabola, or an hyperbola, the sun in each case
+occupying one of the foci.
+
+From our definition of the eccentricity of an ellipse, it will be seen
+that, when the eccentricity is zero, we have a circle. When the
+eccentricity becomes unity, the ellipse becomes a parabola, so that the
+latter curve may be regarded as part of an ellipse, of which the foci
+are at an infinite distance apart. In the case of the hyperbola, the
+eccentricity is greater than unity.
+
+Comets which move round the sun in ellipses are called _periodic
+comets_, for the reason that they return regularly into the sun’s
+neighbourhood. Those which traverse parabolic or hyperbolic paths will
+pass once round the sun and continue to journey into the depths of
+interstellar space until their movements are changed by the proximity of
+other bodies into the neighbourhood of which their wanderings may take
+them.
+
+When a new comet is observed, one of the things which astronomers
+endeavour to do is to determine its orbit, so that its path may be
+predicted with sufficient accuracy to enable it to be picked up readily
+with a telescope when it becomes so feeble that it is no longer visible
+to the naked eye. In the first instance, the motion is assumed to be
+parabolic, and any deviation from such an orbit forms the subject of a
+rigorous calculation by means of which the precise form is determined.
+
+
+
+
+                             CHAPTER VIII.
+                       ECLIPSES AND OCCULTATIONS.
+
+
+ECLIPSES OF THE MOON.—As the various members of the solar system shine
+only by virtue of the light which they receive from the sun, they will
+cease to be visible if by any means they are deprived of the sun’s rays.
+Each planet or satellite must evidently cast a shadow which is turned
+directly away from the sun, and any other body passing wholly or
+partially within such a shadow will be proportionately debarred from
+receiving the direct light of the sun.
+
+[Illustration:
+
+  FIG. 29.—_The Earth’s Shadow._
+]
+
+Were the sun a mere point of light these shadows would be parts of
+cones, the apex always being at the sun, and they would be prolonged
+indefinitely into space. As a matter of fact, every individual point
+upon the sun’s disc is competent to cast a conical shadow, and the net
+result is that only a relatively small space behind a planet or
+satellite is really in total darkness. This will be readily understood
+from Fig. 29, in which S is the sun, and E the earth. The total shadow
+now becomes a cone, with the apex turned directly away from the sun, but
+round this there is a region of partial shadow which is only illuminated
+by portions of the sun. If we imagine a section of the shadow across the
+line _a b_, we should find a central disc of total darkness called the
+_umbra_, and surrounding this a ring of half shadow called the
+_penumbra_.
+
+From the known dimensions of the sun and earth, and the distance between
+them, it is easy to calculate the size of the earth’s shadow-cone, and
+its length is found to be greater than the distance of the moon. The
+axis of this shadow will, of course, always be in the plane of the
+ecliptic. If, then, at the time of opposition, the moon is sufficiently
+near the plane of the ecliptic, it will pass through the shadow, and we
+shall have the phenomena of a _lunar eclipse_. When the moon is wholly
+immersed in the umbra, the eclipse is total, and if it further passes
+quite symmetrically through the shadow, the eclipse is said to be
+central. This would always be the state of affairs if the moon performed
+its monthly journey in the plane of the ecliptic, and a total eclipse
+would occur every month. The moon’s orbit, however, is inclined to the
+ecliptic, so that for a central eclipse, the moon must be simultaneously
+at opposition and at a node. If the moon be near the node when at
+opposition, a total eclipse may occur, but it cannot be central, and the
+duration of the total obscuration will be reduced. Still further from
+the node, the moon will be above or below the ecliptic, and will be only
+partially involved in the shadow-cone; such an eclipse is called a
+partial one. Beyond a certain distance from the node, the inclination of
+the moon’s orbit will take the moon entirely out of the umbral shadow,
+and no eclipse will be possible.
+
+The circumstances of an eclipse of the moon thus vary very considerably,
+and there is still another reason why we may expect them to be
+different. We have seen that the earth’s distance from the sun changes
+throughout the year, and, in consequence, its shadow will be of varying
+length, and the diameter of the shadow at any specified distance will
+not be constant. The moon, again, is not always at the same distance
+from the earth, and it will, therefore, pass through varying depths of
+shadow in different eclipses, and with different velocities.
+
+The breadth of the earth’s umbral shadow at the point where the moon
+passes through it is, on the average, about three times the moon’s
+diameter, and the time taken for the moon to traverse this distance is
+about two hours. The duration of totality in a central eclipse may,
+therefore, amount to two hours, while an additional two hours may be
+occupied by the partial phases.
+
+[Illustration:
+
+  FIG. 30.—_The Lunar Ecliptic Limit._
+]
+
+THE LUNAR ECLIPTIC LIMIT.—The greatest distance of the moon from a node
+at which a partial eclipse is possible, is called the _lunar ecliptic
+limit_, and is very easily calculated. In Fig. 30, let E N represent a
+part of the ecliptic, N being the node of the moon’s orbit, and E the
+centre of the earth’s shadow. As the orbit of the moon is inclined about
+5° 9′ to the ecliptic, it may be represented by the line N M, inclined
+at an angle to N E. If E A be the radius of the earth’s shadow, which,
+on the average, is about three-quarters of a degree, and M A the moon’s
+apparent semi-diameter (about a quarter a degree), it is clear that the
+point beyond which no eclipse is possible is that in which the line M E,
+perpendicular to N M, is equal to the sum of the semi-diameters. All the
+quantities for solving the triangle N E M are thus known, and it can be
+readily calculated that N M, the greatest distance of the moon from the
+node at which an eclipse would be possible, under average conditions is
+about 11°.
+
+Taking into account the varying distances between the sun, earth, and
+moon, it is found that an eclipse must always occur if the moon is
+within 9° of the node, and may occur if it be 12° from the node. These
+figures refer to the passage of the moon through the umbra, as the
+effect of its entrance into the penumbra is too slight to be observed.
+
+The entrance of the moon into the earth’s shadow is a definite
+phenomenon, which is independent of the observer’s position on the
+earth, and the phases of the eclipse are seen at exactly the same moment
+from all places where the moon is above the horizon. The computation of
+the circumstances at a given place is accordingly a simple one.
+
+When a lunar eclipse is not total at any of its phases, it is usual to
+specify its _magnitude_ by the ratio of the greatest measurement of the
+obscured part to the moon’s diameter. Thus the magnitude of the partial
+eclipse of February 28th, 1896, is given in the “Nautical Almanac” as
+0·870, the moon’s diameter being taken as unity.
+
+The conditions of lunar eclipses which have been stated have reference
+to the moon’s passage through the earth’s geometrical shadow, but the
+actual conditions are greatly modified by the fact that the earth is
+surrounded by an atmosphere which refracts the suns light so much that
+the moon is seldom quite obscured during totality. The commencement of
+the total phase is also rendered difficult of observation by the
+somewhat indefinite boundary between the umbra and penumbra.
+
+ECLIPSES OF THE SUN.—If the moon performed its revolution in the plane
+of the ecliptic, it is evident that it must always come between us and
+the sun once in each month. This it does not do, but occasionally it
+happens to be in the ecliptic when in conjunction, and the moon is then
+seen to be projected upon the sun. In other words, there is an eclipse
+of the sun. Let us consider the circumstances, in the first instance, to
+an observer placed at the centre of the earth. If the centres of the
+moon and sun appear in the same straight line, the eclipse will be
+_total_ or _annular_, according as the moon or sun has the greater
+apparent diameter. Both these forms of eclipses are possible, on account
+of the varying apparent diameters of the sun and moon consequent upon
+their variable distances from the earth. If the moon appear the larger
+it will evidently cover up the whole of the sun, but when it is the
+smaller, a ring of sunlight will be visible round the dark holy of the
+moon, and the eclipse will be an annular one. These conditions are
+illustrated in Fig. 31, _a_ and _b_ representing a total and an annular
+eclipse respectively. If the moon and sun be not quite in the same
+straight line, the moon may still be seen partially projected on the
+sun’s disc, in which case there will be a _partial eclipse_ of the sun,
+as in Fig. 31, _c_.
+
+[Illustration:
+
+  FIG. 31.—_Eclipses of the Sun._ (_a_) _Total Eclipse_, (_b_) _Annular
+    Eclipse_, (_c_) _Partial Eclipse._
+]
+
+In a total eclipse there are four so-called _contacts_: the first when
+the moon is seen to encroach upon the sun’s disc, the second when the
+advancing edge of the moon reaches the opposite limb, the third when the
+following edge of the moon first touches the sun’s boundary, and the
+fourth when the projected moon finally passes off the sun. The interval
+between the second and third contacts marks the duration of totality. As
+referred to our supposed observer at the centre of the earth, the
+duration evidently depends upon the apparent rate of the moon’s eastward
+movement as compared with that of the sun, as well as upon the
+differences of the apparent diameters of the two bodies.
+
+The production of eclipses of the sun may also be considered as arising
+from the immersion of an observer in the shadow of the moon. This shadow
+has its axis turned from the sun, but is so short that it does not
+always reach the earth. If an observer comes near the axis of the
+conical shadow, and within the apex, the eclipse will be total; if he is
+in the axis, but outside the apex, the eclipse will be annular.
+
+[Illustration:
+
+  FIG. 32.—_Duration of a Solar Eclipse._
+]
+
+The whole of the shadow of the moon is so small that only a few places
+on the earth’s surface can be simultaneously immersed in it, and when we
+come to discuss the conditions of an eclipse with regard to a particular
+observer, the problem becomes a complicated one. At some places the
+eclipse may be total, at others it will be only partial, while at others
+no eclipse will occur at all. These differences are due to the fact that
+the sun is scarcely appreciably displaced by the change of locality,
+while the apparent position of the moon may be affected to the extent of
+nearly a degree. Again, the observer situated on the earth’s surface has
+a movement of his own, produced by the earth’s rotation, and his rate of
+motion depends upon the latitude in which he is situated. The effect of
+this movement upon the conditions of the eclipse are very pronounced.
+Suppose for a moment that the sun, moon, and earth, are fixed along the
+same straight line S M E in Fig. 32, a terrestrial observer at _a_ on
+the earth’s Equator would see an eclipse at noon; if he were not in
+rotation, and the three bodies remained at rest, the eclipse would be a
+perpetual one. He is, however, carried onward by the earth’s rotation,
+and even if the moon were at rest, it would appear to him to pass over
+the sun in the reverse direction. This retardation of the moon will be
+less in amount for observers away from the Equator, and also for
+observers to whom the sun is not on the meridian when eclipsed. The
+effect of rotation on an observer at _b_ (Fig. 32), for example, is to
+move him almost in the direction of the line joining the moon and sun,
+and the backward tendency of the moon due to rotation is very slight. On
+account of the earth’s rotation, then, the duration of a solar eclipse
+is lengthened, the greatest increase occurring at those places where the
+sun is on the meridian at the time of eclipse.
+
+There is another source of gain of duration of an eclipse to the
+observer who sees the phenomenon about noon. The moon’s apparent
+diameter is then augmented by a greater amount than at other places,
+because the observer is then nearest to the moon; while the sun’s
+apparent diameter is not appreciably affected. The greater the
+difference in the apparent diameters of the sun and moon, the longer
+will totality last.
+
+These and other circumstances have all to be taken into account in
+computing the conditions under which an eclipse will be seen at any
+given place.
+
+According to an eminent authority, Professor Young, the greatest
+possible diameter of the moon’s shadow, where it strikes the earth, is
+167 miles. It may, however, cover a larger space on the earth’s surface,
+because the latter does not pass perpendicularly through the shadow. To
+all persons within the shadow, the eclipse will be total, but to those
+on its outer boundary the duration of totality will be for an instant
+only. The penumbral shadow has a cross section about 4,500 miles in
+diameter, covering sometimes a space on the earth’s surface 6,000 miles
+across. To all persons within this area, but not in the central shadow,
+the eclipse will be partial. The shadow spot travels over the earth’s
+surface, because of the moon’s movement, but its track and speed are
+greatly modified by the earth’s rotation. The movement of the shadow, as
+affected by the earth’s rotation, would be along a parallel of latitude;
+but its ultimate direction of movement, though trending eastwards,
+depends upon this, combined with the direction of the moon’s movement at
+the time of the eclipse. Thus, a portion of the track of the total
+eclipse of April 16, 1893, is as that shown in Fig. 33.
+
+[Illustration:
+
+  FIG. 33.—_Track of Eclipse of April 16, 1893._
+]
+
+These considerations will suffice to explain the necessity for very
+precise calculations as to the position of the central line of an
+eclipse, when observers are sent out for the purpose of recording the
+phenomena.
+
+Under the most favourable combination of conditions, that is, when the
+eclipse occurs at noon at a place on the Equator, an eclipse cannot be
+total for more than 7 minutes 58 seconds, nor be annular for a longer
+time than 12 minutes 24 seconds. From first to last contact may occupy
+as much as 2 hours, when all the circumstances are similarly favourable.
+(Loomis.)
+
+THE SOLAR ECLIPTIC LIMIT.—In order that an eclipse of the sun may occur,
+the moon must be so near the ecliptic that it can be seen projected on
+the sun, either wholly or partially, from some point on the earth. It
+must therefore not be very far from the node, and the distance it may be
+from the node, while still being seen upon the sun, is called the _solar
+ecliptic limit_. As in the case of lunar eclipses, this distance is
+determined by the inclination of the moon’s orbit, and the distances of
+the moon and sun from the earth. The latter being variable quantities,
+the limit is not always the same. It is calculated without much
+difficulty that an eclipse _must_ occur if the new moon happens when it
+is within 15° 21′ of the node, and may occur within 18° 31′. These are
+called the minor and major ecliptic limits respectively. For total or
+annular eclipses, the limits are respectively 9° 55′ and 11° 50′.
+
+NUMBER OF ECLIPSES IN A YEAR.—If the moon’s nodes were fixed, the sun
+would pass through the line of nodes twice a year. At such times an
+eclipse of the sun must necessarily occur if the moon were within 15°
+21′ of the node on either side. The sun requires more than a month to
+traverse this space of 30° 42′, and the moon must therefore pass through
+each node at least once while the sun is traversing these limits. It
+follows, then, that there must be at least two eclipses of the sun in a
+year. Since the line of nodes of the moon’s orbit revolves backwards in
+a period of about nineteen years, the sun returns to the same node after
+an interval of 346·6 days, and there must accordingly be two solar
+eclipses in this interval. If, then, there be an eclipse early in
+January, there will be another about the middle of the year, and another
+at the end of the year, so that on this ground alone there is a
+possibility of three solar eclipses in a year.
+
+Again, while the sun is passing through the ecliptic limits, it may
+happen that an eclipse occurs on its entrance, and then another will
+occur before it gets beyond on the other side of the line of nodes. In
+this way two eclipses may occur in the region of each node passage, and
+if the first of the series occurs early in January, five eclipses of the
+sun may occur in a single year.
+
+The sun, however, is not a month in traversing the lunar ecliptic limit.
+Consequently, a whole year may elapse without the moon being
+sufficiently near the node to pass within the earth’s shadow, and in
+many years there are accordingly no eclipses of the moon. Only one full
+moon can occur within the lunar ecliptic limits when the sun passes the
+node, but if there be an eclipse at one node, there may also be one six
+months later at the other node. As in the case of the solar eclipses,
+the “eclipse year” is one of 346·6 days, so that if there be an eclipse
+of the moon early in January, there may possibly be three altogether in
+the course of the year, but there could not be three lunar eclipses if
+the extra solar eclipse were possible. Altogether, then, there may be
+seven eclipses in the course of a year—five of the sun and two of the
+moon. Usually there are four or five, some particulars of which are
+furnished by all respectable almanacs. It will be observed that the
+number of solar eclipses is much larger than that of lunar ones, but as
+the latter are visible at all places having the moon above the horizon,
+while the former are restricted to small parts of the earth’s surface,
+more lunar than solar eclipses are visible at any specified place.
+
+RECURRENCE OF ECLIPSES.—We have seen that the sun requires only 346·6
+days to travel from one of the moon’s nodes back to the same node again,
+in consequence of the regression of the nodes, while the moon requires
+27·2 days. Suppose, then, that the moon and sun are at a node, and there
+is an eclipse at new moon; after 346·6 days the sun will return to the
+same node, but the moon will not be at the node, nor will it be exactly
+new. It will not be until the sun has returned nineteen times to the
+node that the moon is also very nearly new at the same node again.
+Nineteen returns of the sun to the moon’s nodes occupy a period of
+6,585·78 days; 223 intervals between successive new moons (synodic
+months) cover 6,585·32 days, while 242 node passages of the moon require
+6,585·357 days. In this period of 18 years 11⅓ days (or 10⅓ days if
+there are five, and 12⅓ if there are three leap years in the interval),
+the sun and moon thus return to nearly the same conditions as affecting
+the possibility of eclipses. This period was called the _Saros_ by the
+Chaldeans, by whom it was employed in the prediction of eclipses. The
+adjustment of periods, however, is not quite precise, so that
+predictions based upon the Saros are only approximations, which serve as
+a guide for more accurate computations.
+
+This eclipse period is still more remarkable from the fact that it
+almost exactly represents 239 passages of the moon through perigee, so
+that after the lapse of 18 years 11⅓ days the moon is almost at the same
+distance from the earth, as well as nearly at the same phase and the
+same distance from a node.
+
+As the Saros includes a fraction of a day, an eclipse is not necessarily
+repeated at the same place after the lapse of 18 years 11⅓ days, for the
+reason that the eclipse will not occur at the same time of day, and the
+sun may be below the horizon. After three Saroses, however, the eclipse
+will be repeated nearly at the same hour, but even then it will not be
+seen under the same conditions, because the track of the shadow will be
+in different latitudes, for the reason that the moon does not return
+_exactly_ to the node in the interval between 223 new or full moons, and
+eclipses can only occur when the moon is new or full.
+
+Beginning as a partial eclipse, an eclipse of the moon will gradually
+become of greater magnitude at successive intervals of 18 years 11 days,
+until it becomes a total eclipse, and will again gradually become of
+smaller magnitude, until it ceases to be reproduced at all. Altogether,
+it would be repeated once in every 223 months for 865 years.
+
+Since the solar ecliptic limit is greater than the lunar, a solar
+eclipse is repeated at similar intervals of 18 years for about 1200
+years. Most of these eclipses would be partial, 27 would be annular, and
+18 total. During this period, the track of the central eclipse would
+shift northwards if the moon were at a descending node, and southwards
+if at an ascending node, until finally it passed altogether clear of the
+earth.
+
+It must be remarked, however, that, in the period corresponding to a
+single Saros, about 28 eclipses of the moon, and 43 of the sun, usually
+appear, so that altogether about 71 series of eclipses are in progress.
+Of the solar eclipses which occur during a period of 18 years, about 12
+are total at some places upon the earth.
+
+OCCULTATIONS OF STARS AND PLANETS BY THE MOON.—In its monthly round, the
+moon is constantly passing in front of some of the stars which lie in
+its apparent path, and these luminaries will, therefore, at times, be
+hidden temporarily by the moons disc. Occasionally a planet may appear
+in the same line of vision as the moon, and that also will pass from
+view until subsequent motion again removes the intercepting body. These
+disappearances are closely allied to the phenomena of eclipses, and
+receive the name of _occultations_. On account of the moon’s eastward
+movement, it is evident that the disappearance of stars or planets when
+occulted will take place on the eastern edge of the moon; but since the
+moon trends north or south in some parts of its orbit, the disappearance
+near the northern and southern edges may occur slightly on the western
+side of the north or south point of the moons limb. Similarly, the
+reappearance generally occurs on the western side of the moon, but
+occasionally may occur on the eastern side—that is, when the northern or
+southern edge of the moon does not much more than appear to graze the
+stars.
+
+The calculation of the circumstances of an occultation is very similar
+to that involved in the computation of eclipses. (A simple graphical
+method for working out the conditions of an occultation is described by
+Major Grant, R.E., in the _Geographical Journal_ for June, 1896.)
+
+ECLIPSES AND OCCULTATIONS OF SATELLITES BY PLANETS.—Just as we find the
+moon eclipsed by passing through the earth’s shadow, we find the
+satellites of other planets to be at times invisible for a similar
+reason. We thus observe _eclipses_ of the satellites. The satellites may
+also be invisible to us for the reason that they are behind the planet,
+and they are then said to be _occulted_. These satellite phenomena are
+especially remarked in the case of Jupiter, and their observation is one
+of great interest. When a satellite passes between the sun and the
+planet it throws a shadow on the surface of the planet similar to that
+of the moon upon the earth. This is visible to us as a dark spot, and
+from the centre of that dusky patch an inhabitant of Jupiter would
+undoubtedly see a total eclipse of the sun. To us on the earth the
+passage of such a shadow across the planet’s disc is but a “transit of
+the shadow” with its “ingress” and “egress.”
+
+The times of all these appearances are computed from a knowledge of the
+movements of the satellites.
+
+
+
+
+                              CHAPTER IX.
+                HOW TO FIND OUR SITUATION ON THE EARTH.
+
+
+DETERMINATION OF LATITUDE.—In order that we may precisely define our
+situation upon the terrestrial sphere, we have seen that two
+measurements are necessary, namely, latitude and longitude. The first of
+these indicates the angular distance from the Equator, and the latter
+the angular distance east or west of an arbitrary initial meridian. It
+is necessary for us then to learn something of how these important
+co-ordinates can be determined.
+
+In considering the apparent movements of the heavenly bodies in
+different latitudes, we have already seen that at places on the earth’s
+Equator the north celestial pole is on the horizon, while at the North
+Pole it is in the zenith, and in other latitudes is elevated at
+different angles. If one sails from England to the Cape, for example,
+the Pole Star is seen to gradually get lower and lower in the sky,
+until, on crossing the Equator, it descends below the northern horizon
+and is no longer visible. Sailing northward, as to Norway, the Pole Star
+is seen to get higher in the sky.
+
+Now, although the Pole Star is not exactly at the north celestial pole,
+it is a convenient guide to the eye as to the location of that very
+important mathematical point, and what we learn from its behaviour as
+our latitude is changed is that the altitude of the Pole above the
+horizon is equal to the latitude of the place of observation.
+
+One of the methods employed for finding the latitude of a place is
+accordingly to determine the altitude of the Pole. This can be obtained
+by an instrumental measurement of the altitude of the Pole Star, from
+which, if the time of observation be known, the altitude of the true
+Pole, which occupies the centre of the small diurnal circle traversed by
+the star, can be computed. Tables which save an immense amount of labour
+in the calculations involved are given in the “Nautical Almanac,” and in
+“Whitaker’s Almanac.”
+
+Another method of finding the elevation of the Pole is to take advantage
+of the fact, that at intervals of twelve sidereal hours the Pole Star
+passes the meridian alternately above and below the Pole. If, then, one
+finds the altitudes at the upper and lower transits, and corrects them
+for refraction, the average of the readings is a measure of the altitude
+of the true Pole, and therefore of the latitude. Other stars which are
+circumpolar may be employed for the same purpose, and this method has
+the great advantage that a knowledge of the correct time, or of the
+exact position of the star observed, is superfluous. The disadvantage is
+that the correction for refraction, especially in low latitudes, cannot
+be made with the necessary degree of accuracy. It must be remembered
+that an error of only 1′ in latitude implies a mistake of a mile
+measured on the earth’s surface.
+
+Other methods, however, are available. As we go southwards, not only
+does the Pole Star become lower in the sky, other stars in the southern
+part of the sky become higher at the same rate that the Pole Star
+descends. Other stars can therefore be utilised, and in order that
+refraction may affect the observations as little as possible, stars of
+known declination near the zenith are observed. Suppose an observer,
+situated at O (Fig. 34) on the earth’s surface, observing a star S on
+his meridian, O Z will represent his zenith, and O E, parallel to the
+Equator, will be the direction in which he will see the celestial
+equator where it crosses his meridian. The declination of the star,
+represented by the angle S O E, has been previously determined with
+great accuracy, and the angle S O Z, the zenith distance of the star, is
+the angle which he measures. In the case illustrated by the diagram, the
+difference between the declination and the zenith distance will give the
+angle Z O E, which is evidently equal to the latitude O C Q. To get rid
+of the ever troublesome refraction of our atmosphere, stars which pass
+as nearly as possible through the zenith are selected for observation,
+and stars both to north and south are observed.
+
+[Illustration:
+
+  FIG. 34.—_Determination of Latitude._
+]
+
+Another way of determining the latitude, which is very commonly
+employed, is known as Talcott’s method. The observations are made with
+the aid of a zenith telescope. The latitude being approximately known,
+two stars are selected which transit nearly at the same time and nearly
+at the same distance from the zenith, one to the north and the other to
+the south. That which transits first is brought to the centre of the
+field of view, which is marked by a spider thread. The instrument is
+then reversed in its bearings so that it points at the same angle on the
+opposite side of the zenith. When the second star comes into the field,
+the telescope is kept fixed, and a moveable spider thread is made to
+coincide with the star passing through the field. The distance between
+the spider threads furnishes a measure of the difference in zenith
+distances. Half the sum of the declinations added to half the difference
+of zenith distances gives the latitude when this method is employed.
+
+Various other methods have been devised for the precise determination of
+latitude, but the foregoing will sufficiently serve to illustrate the
+processes followed when the observations are made on land.
+
+Before the invention of astronomical instruments, latitude was
+approximately measured by the lengths of shadows. At the summer
+solstice, at noon, the shadow of a vertical stick is at its shortest,
+while at the winter solstice it is longest. By measuring these lengths,
+a diagram can be made showing the altitude of the sun at noon on each
+occasion. Midway between these will be the altitude of the celestial
+equator where it crosses the meridian. Since the altitude of the Pole is
+equal to the latitude, the altitude of the Equator, subtracted from 90°,
+thus gives the latitude.
+
+[Illustration:
+
+  FIG. 35.—_Ancient Mode of measuring Latitude._
+]
+
+It will be noted that this _gnomon_ experiment also furnishes a measure
+of the obliquity of the ecliptic. The gnomon was in use by the ancient
+Chinese, and it is also believed that the Egyptian obelisks which are
+now embellishing various cities were originally erected for the same
+purpose.
+
+DETERMINATION OF LONGITUDE.—As we have imagined an observer travelling
+in a north or south direction in connection with the measurement of
+latitude, let us consider what will happen to an observer who travels
+only in longitude—that is, east or west. At the starting-point, he will
+see the Pole at a certain altitude, and the stars will perform their
+diurnal revolutions at a certain inclination to the horizon depending
+upon his latitude. If he travels towards the east, the Pole will remain
+at the same angle above the horizon, and he will detect no difference in
+the apparent movements of the stars. What then is there to indicate that
+he has changed his place at all? The answer is simple; he will find that
+the sun and stars cross the meridian earlier, and if he be 15° east of
+his first station they will transit an hour sooner, because it takes the
+earth an hour to turn through that angle. If he travel westward in the
+same way, the earth must turn through a greater angle to bring him back
+to the same star, so that the stars will appear to cross the meridian
+later.
+
+The determination of longitude is accordingly based upon a measurement
+of the difference in the times of transit of sun or stars at the place
+of observation, and the place from which longitude is reckoned.
+
+Let us take Greenwich as the start-point for our longitudes, and suppose
+we are in Dublin. The sun, or a star, will cross the meridian of Dublin
+at a certain interval after it has passed that of Greenwich, and if we
+measure this interval, the angle turned through by the earth in that
+time will determine the longitude. With a transit instrument one can
+readily tell the exact moment when the star crosses the meridian of
+Dublin, but how is one to know the exact moment at which the star
+crossed the meridian of Greenwich without going there?
+
+Looking at the question in another way, let us remember that the clocks
+in Dublin register local time, that is time reckoned from the passage of
+the sun over the meridian of Dublin, while the Greenwich clock indicates
+times based on the transit of the sun over the Greenwich meridian.
+Evidently the difference of these times is the difference of longitude,
+and our question becomes, how to find the time at Greenwich when
+stationed at the observatory in Dublin.
+
+In all modern work, the telegraph is employed whenever it is available,
+the two stations being directly connected. An observer at Greenwich is
+thus enabled to transmit a signal to the observer in Dublin at the exact
+moment a star passes through the centre of his transit instrument, and
+the latter observer then notes the interval which elapses before the
+same star passes the central line of his own instrument. If the signals
+were transmitted instantaneously, the interval elapsed from the
+reception of the signal to the observed transit of the same star would
+give the longitude as reckoned in time.
+
+Practically, what is done is for each observer to determine his local
+sidereal time very accurately, with the aid of his transit instrument,
+and in this way to find the error of his clock. It is then only
+necessary to compare the two clocks, and this is done in the following
+way: the clock at Greenwich has an attachment by which an electrical
+contact is made every second, and this is switched in to the telegraphic
+circuit, so that the Dublin observer receives a signal every second so
+long as the clock is connected. These signals are automatically recorded
+by a chronograph, together with similar signals from the Dublin clock,
+and the times to which each of them corresponds is easily identified.
+Immediately afterwards the Dublin clock is switched into the circuit,
+and records its beats on the chronograph sheet at Greenwich, alongside
+those sent by the Greenwich clock. In this way the differences between
+the clocks can be very accurately measured, and the longitude can then
+be reckoned in degrees and minutes by allowing 15° for each hour. Before
+the invention of the telegraph, less accurate methods were of necessity
+employed. Among others the entrance of the moon into the earth’s shadow
+during an eclipse was noted by an observer desiring to know his
+longitude. As we have already seen, this occurrence is independent of
+the observer’s position on the earth, so that if he records the local
+time of the observation and compares with the calculated Greenwich time
+of the commencement of the eclipse, he can find his longitude.
+Similarly, the eclipses of the satellites of Jupiter may be utilised to
+signal Greenwich time to an observer situated elsewhere. Unfortunately,
+the shadows are too ill-defined at the edges to permit very accurate
+determinations in this way.
+
+METHODS EMPLOYED AT SEA.—One of the most important applications of
+astronomy to the needs of everyday life is in enabling the navigator on
+the open ocean to determine the situation of his ship. Without the help
+supplied by astronomical predictions the sea would be truly trackless,
+and commerce by sea would be almost impossible.
+
+A sextant and two or three good chronometers, together with a copy of
+the current “Nautical Almanac,” furnish the means of ascertaining the
+geographical position of a ship. With the aid of the sextant, the sun’s
+greatest angular distance above the sea horizon—that is, its meridian
+altitude—is measured, and from the known declination of the sun at the
+time, the latitude is deduced in exactly the same way as in the case of
+an observation of a star (p. 124).
+
+The sextant also enables the observer, by measuring the sun’s altitude
+in the early morning or evening, to determine the local time, as already
+explained (p. 83). Greenwich time is kept by the chronometers, and the
+difference between this and the local time is a measure of the
+longitude. More than one chronometer is carried by a ship, for fear that
+a single one might fail, through accident or other causes, to give
+correct readings. The rate of each has been previously very accurately
+gauged, and by taking the average indications, Greenwich time is known
+with considerable accuracy.
+
+Should the chronometers fail, or any doubt be thrown upon their
+accuracy, there is another method by which the Greenwich time, and
+thence the longitude, can be ascertained. This is the _lunar method_, in
+which the heavens become the equivalent of the dial of a clock, while
+the moon, with its rapid easterly movement, plays the part of the hands.
+
+In the words of Dr. Lardner, this is “a chronometer of unerring
+precision; a chronometer which can never go down, nor fall into
+disrepair; a chronometer which is exempt from the accidents of the deep;
+which is undisturbed by the agitation of the vessel; which will at all
+times be present and available to him wherever he may wander over the
+trackless and unexplored regions of the ocean.”
+
+From the known movements of the moon, its position with regard to the
+sun, planets, or conspicuous stars, at definite Greenwich times, can be
+calculated in advance, and “lunar distances” are accordingly tabulated
+in our nautical almanacs. We find, for instance, that the apparent
+distances of the moon from the star Regulus, as they would appear from
+the earth’s centre, were as follows on Jan. 1, 1896:—
+
+                        6 P.M. G.M.T. 35° 50′ 22″
+                        9 P.M.   „    34°  3′ 23″
+                       12 P.M.   „    32° 16′ 12″
+
+To utilise these predictions for the purpose in hand, the observer would
+measure with the sextant the apparent distance of the moon from Regulus
+at a known local time, and he would then compute what the apparent
+distance would have been if his observation had been made from the
+earth’s centre. From the tabulated distances, he would then be able to
+find the Greenwich time at which his observation was made; and, as we
+have seen, the difference between this and local time is a measure of
+the longitude.
+
+
+
+
+                               CHAPTER X.
+                 THE EXACT SIZE AND SHAPE OF THE EARTH.
+
+
+GEODESY.—We have already seen that the earth is a sphere, or of some
+form which differs but little from a sphere, and a rough method of
+determining its size, on this supposition, has been indicated. Now we
+have to inquire more minutely into the size and shape of our planet,
+for, as we shall see presently, a knowledge of these facts is essential
+to the adequate explanation of the various movements of the heavenly
+bodies, besides forming the basis of all our knowledge of the distances
+which separate us from the other bodies which people space. As an
+illustration of the importance of an exact knowledge of the size of the
+earth, it may be remarked that Newton’s grand law of gravitation was
+kept from the world for ten years, owing to an error in the generally
+accepted value of the earth’s radius, which was afterwards rectified by
+the labours of a French astronomer, Picard.
+
+A great amount of labour has been expended in the endeavour to arrive at
+the true size and shape of the earth, and the name _geodesy_ is given to
+the science which deals with these operations. As a secondary object,
+geodesy is concerned with the measurement and description of tracts of
+country.
+
+AN ARC OF MERIDIAN.—The measurement of the size of the earth is
+accomplished by first measuring relatively small parts of its surface,
+and then applying geometrical principles, in order to determine the
+whole circumference. If the earth were a true sphere, and we could
+measure the exact distance in miles between two places on the same
+meridian, a subsequent determination of the difference of latitudes of
+the two places would enable us to find the length of a degree, measured
+on the earth’s circumference. As there are 360° in a circle, the
+circumference would be 360 times the length of a degree, and the
+diameter of the earth would be the length of the circumference divided
+by 3·14159, this number expressing the constant ratio which exists
+between the circumference and diameter of a circle of any size
+whatsoever.
+
+The determination of the size and shape of the earth thus involves two
+distinct sets of operations; first, measures of distances; and second,
+astronomical observations to determine the angular measurements of the
+arcs on the earth’s surface comprised between stations separated by
+known distances. When two such stations lie on the same meridian, the
+arc measured in this way is called an _arc of meridian_. We have already
+seen what means are available for finding the latitudes and longitudes
+of places on the earth, and it now remains for us to apply a yard
+measure, or its equivalent, to the precise measurement of the distance
+between places which are many miles apart.
+
+THE BASE LINE.—In the first instance a line of unimpeachable
+straightness is measured with scrupulous accuracy. The measuring-rod
+which has been most successfully employed is one consisting of a
+combination of brass and steel bars, which automatically corrects itself
+for changes of temperature in very much the same way that the
+balance-wheel of a chronometer, or of a good watch, corrects itself so
+as to perform its swing in equal periods at all temperatures. Several of
+these compensated rods are used, and they are enclosed in wooden boxes
+which are provided with levels and sights. When in use the outer boxes
+rest on adjustable trestles, and instead of putting the rods end to end
+they are placed a certain definite distance apart by the use of
+microscopes, which are themselves mounted on compensating bars. The
+first rod is put in position and levelled, and the others are
+successively placed in line with it by means of the sights. As the
+ground ceases to be perfectly flat it becomes necessary to raise the
+level of succeeding bars, but they are kept in the same vertical plane.
+Six bars are frequently employed in laying out a base line, and in order
+to protect them from extremes of temperature they are usually kept
+covered with long tents. In this way a distance of several miles can be
+measured with no greater probable error than a couple of inches, and the
+ends of such a measured base line are marked on metal plugs built in
+columns of masonry. The chief base lines measured in connection with
+British map construction were on the sandy shores of Lough Foyle in
+Ireland, 41,614 feet in length, and on Salisbury Plain, 36,578 feet
+long.
+
+TRIANGULATION.—When a base line has been accurately measured in this
+way, a distant object which is clearly visible from both ends is
+observed with the aid of an instrument called the _theodolite_, and the
+angles between the base line and the lines joining its ends with the
+object are very carefully determined. Thus if A B in Fig. 36 represent
+the base line, and C a conspicuous object several miles away, the angles
+C A B and C B A are measured, and then it becomes easy to determine the
+distances A C and B C by trigonometrical calculations. A check on the
+accuracy of the observations is obtained by transferring the theodolite
+to C and measuring the angle A C B. The sides of the triangle may then
+be employed as new base lines for the measurement of other distances.
+With the theodolite at C, another object, D, is sighted, and the angle D
+C A is measured; similarly, with the theodolite at A, the angle C A D is
+determined, and from these observations the distances of D from the
+points A and C are easily computed. These distances again become
+available for base lines, and so the triangulation can be extended
+indefinitely.
+
+[Illustration:
+
+  FIG. 36.—_Triangulation._
+]
+
+In a mountainous country, the sides of the triangles are often as much
+as 100 miles in length. Signals on the Wicklow Mountains in Ireland have
+been observed from Ben Lomond in Scotland and from Scafell in
+Cumberland. The stations are chosen so that none of the angles to be
+measured are very small, and in this way the chances of error are
+greatly reduced. Hence the triangles in the immediate neighbourhood of
+the base line are comparatively small, but the sides are gradually
+extended as the survey proceeds.
+
+The process of triangulation forms the basis of the construction of
+accurate _maps_, and for this purpose the great triangles are subdivided
+by a secondary triangulation, so that the exact situations of a very
+great number of places are determined. These, again, serve for another
+set of still smaller triangles, with sides perhaps a mile in length; and
+finally the details are filled in by local chain surveys and
+draughtsmanship.
+
+There is another point of some importance in connection with these
+triangulations when on a large scale. The larger triangles must be
+corrected for the curvature of the earth’s surface. The construction of
+the theodolite is such that two adjacent sides of any triangle, measured
+from their intersection, are referred to the same horizon; but when the
+instrument is transferred to another corner of the triangle, the
+adjacent sides are referred to a new horizon. The sum of the three
+angles of a triangle in these geodetical surveys thus exceed two right
+angles, whereas in plane triangles they are always equal to two right
+angles; the difference is called the _spherical excess_, and in the
+computations the observed angles have to be corrected on this account.
+
+Thus, after an extremely laborious survey, it becomes possible to
+determine with great accuracy the distance between any two places
+whatever, and so the number of miles between two places at the
+extremities of an arc of meridian is ascertained. An arc of meridian
+extending nearly 18° has been measured in India, and another over 25°
+long extends from Hammerfest in Norway to the mouth of the Danube.
+
+EXACT SHAPE AND SIZE OF THE EARTH.—From the facts which have been
+gleaned by the measurements of arcs of meridian in different parts of
+the world, it is found that the length of a degree of latitude as
+measured on the earth’s circumference increases towards the Poles. In
+latitude 66° N. a degree is about 3,000 feet longer than a degree near
+the Equator. This means that the curvature of a meridional arc is
+greatest at the Equator, whence it is concluded that the earth is
+flattened at the Poles. The figure which best accords with the
+observations is the ellipse, and thus it becomes possible to calculate
+the polar diameter, although no arcs have been measured in the immediate
+neighbourhood of the Poles.
+
+Arcs of longitude, extending between two places which have the same
+latitude, have also been measured and applied to the determination of
+the figure of the earth, and, indeed, any arcs between two places of
+known latitude and longitude can be utilised.
+
+When all the facts are brought together it is found that the earth’s
+polar diameter is about 26 miles shorter than the average equatorial
+diameter, while an equatorial section of the earth is also elliptical,
+the diameter passing through longitude 14° E, being two miles longer
+than the one at right angles to it. According to the calculations of
+Colonel Clarke, R.E., we have the following principal dimensions:
+
+    Earth’s mean        equatorial semi-diameter = 3,963·296 miles.
+    „            „      polar            „       = 3,950·738   „
+    Polar   compression                          ¹⁄₂₉₃.₄₆
+
+A solid which has a shape like that of the earth, with three axes of
+unequal lengths, is called an _ellipsoid_.
+
+A very important consequence of the ellipsoidal form of the earth is
+that lines which are vertical—that is, perpendicular to the surface of
+water—do not pass through the centre of the earth, unless they are at
+the Poles or at certain points on the Equator.
+
+There is every reason to suppose that at one time the earth was in a
+molten condition, and in response to physical laws, such a mass of
+matter could not retain a spherical form when set in rotation, although
+the sphere would be its natural shape if at rest. This has been
+demonstrated by a variety of experiments.
+
+Thus, taking it generally, the shape of the earth is very intimately
+associated with its rotation, and it will subsequently appear that the
+same holds good for the sun and planets. Those bodies which have the
+most rapid rotation show the greatest flattening in the direction of the
+polar diameter.
+
+In addition to direct measurements of the earth, there are other ways of
+studying the shape of our planet. One of these depends upon observations
+of the swing of a pendulum at different parts of the earth’s surface; as
+the time of oscillation of a pendulum depends upon the force of gravity,
+which itself varies with the distance from the earth’s centre, it is
+evident that this method is a practicable one. It is true that the
+matter is complicated in various ways, but after everything has been
+taken into account, these pendulum observations indicate, not only that
+the earth is flattened at the Poles, but they show further that the
+amount of polar compression deduced from geodetical work is in all
+probably very near the truth.
+
+Again, the movement of the moon around the earth is found to be subject
+to certain irregularities which would not exist if the earth were a
+perfect sphere. These inequalities being deduced from observations of
+the moon’s position, the amount of polar flattening necessary to produce
+them can be calculated, and this is found to agree very closely with the
+value derived from the measurements of arcs of meridian.
+
+DIFFERENT KINDS OF LATITUDE.—If the earth were a smooth spherical body,
+the latitude of a place would be simply equal to the angle made by a
+line joining it to the earth’s centre with the plane of the Equator.
+Owing to the bulging out of the earth in its equatorial part, however,
+it becomes necessary to distinguish between different kinds of latitude.
+If we adopt the definition given above, the name of _geocentric
+latitude_ is given to the angular measurement. Taking the earth as a
+smooth geometrical spheroid, and assuming it to have certain dimensions,
+the angle which a line perpendicular to the surface makes with the plane
+of the Equator determines the _geographical latitude_. As the line
+perpendicular to the surface does not pass quite through the centre of
+the earth, the geographical and geocentric latitude differ by as much as
+11′ in mid-latitudes, although nearly agreeing at the Poles and on the
+Equator.
+
+As there are no direct means of finding the direction of a line passing
+through the earth’s centre, or of one perpendicular to the imaginary
+standard spheroid, geocentric and geographical latitudes must be
+calculated from the _astronomical latitude_, which is determined by
+observations of the elevation of the Pole, or its equivalent. The
+astronomical latitude is the angle between the direction of gravity and
+the Equator, and is therefore to a small extent dependent upon local
+irregularities of the earth’s surface.
+
+A knowledge of geocentric latitude is chiefly of use in making
+corrections for parallax, in order that the data calculated for the
+earth’s centre may be precisely corrected for the place of observation,
+or _vice versâ_, as in the case of a lunar distance measured for the
+determination of longitude, or in the calculation of a solar eclipse.
+
+VARIATION OF LATITUDE.—For some years past a widespread interest has
+been taken in the question of a possible change in the position of the
+earth’s axis with regard to its surface. The subject is by no means a
+new one, for as far back as two thousand years ago, such variations were
+suspected. Changes amounting to several degrees were then believed to
+have occurred, but it is now certain that the supposed variation was due
+solely to the imperfection of the observations. As astronomical science
+became more and more precise, even before the discovery of aberration,
+it became evident that if any changes of latitude were taking place at
+all, they must be very minute.
+
+In its geological aspect, the possibility of great changes of latitude
+having occurred in the past history of our globe is evidently well worth
+serious investigation. Granted a sufficient change in the position of
+the earth’s axis, the climate of London might become Arctic, or that of
+Greenland tropical. From this point of view the subject has been
+mathematically investigated by Professor G. H. Darwin, and it appears
+that if only the varying distribution of land and sea indicated by the
+geological records be taken into account, past changes of more than
+about three degrees are very improbable. Admitting that at any time
+during the life-history of our globe the earth was sufficiently plastic
+to be deformed by earthquakes or other disturbances, it is possible that
+changes amounting to 10° or 15° may have occurred.
+
+Opinion is perhaps best reserved as to what has happened in the past. We
+are on surer ground when we consider the variations of latitude which
+are now going on.
+
+Many competent observers have investigated the present movements of the
+Pole, and it has been conclusively demonstrated that changes in the
+position of the earth’s axis do really occur. Dr. Küstner, of Berlin,
+commenced a series of observations for a different purpose in 1884, and
+found that some anomalous results could only be explained by supposing
+that the latitude of Berlin was from 0″·2 to 0″·3 greater from August to
+November, 1884, than from March to May in 1884 and 1885. Great interest
+was excited by this striking result, and steps were at once taken to
+test its truth. Old observations were re-discussed and compared, and new
+observations were made, with the final result that the movement of the
+earth’s axis of rotation was placed beyond dispute. It was not until Dr.
+Chandler attacked the problem, however, in 1891, that the nature of the
+changes became clear. His masterly analysis indicated that the observed
+variations in latitude arise from two periodic fluctuations superposed
+upon each other; one of these has a period of 427 days, and a
+semi-amplitude of 0″·12, while the other is an annual change which has
+ranged between 0″·04 and 0″·20 during the last fifty years. The
+resultant of the two movements produces changes which are seemingly very
+irregular in amount and of varying period, but a cycle is completed
+about every seven years. When the two sources of difference are at their
+maximum at the same time, the total range reaches about two-thirds of a
+second of arc. In consequence of the inequality of the annual part of
+the change, the apparent average period between 1840 and 1855
+approximated to 380 or 390 days; widely fluctuated from 1855 to 1865;
+from 1865 to about 1885 was very nearly 427 days, afterwards increased
+to near 440 days, and very recently fell to somewhat below 400 days.
+
+[Illustration:
+
+  FIG. 37.—_Movements of the Earth’s Pole, 1890–95._
+]
+
+At the present time the variation of latitude is being very carefully
+investigated by the International Geodetic Association, and the latest
+results obtained are illustrated diagrammatically in Fig. 37. The mean
+position of the Pole is at the centre of the diagram,[3] and the
+horizontal line to the right of this point is directed towards
+Greenwich. The remarkable spiral curve shows the wanderings of the Pole
+about its mean position during five recent years. To simplify matters,
+the amount of deviation is represented in feet instead of in angular
+measure, and it will be seen that although the variation of latitude may
+be of considerable interest and importance in astronomical matters, it
+really does not amount to very much in matters terrestrial, the greatest
+change in the position of the Pole not amounting to more than 20 yards.
+Nevertheless, it is not inconceivable that it may yet have to be
+reckoned with in questions relating to boundary lines which depend upon
+latitude determinations.
+
+
+
+
+                              CHAPTER XI.
+          THE DISTANCES AND DIMENSIONS OF THE HEAVENLY BODIES.
+
+
+PARALLAX.—The problem of determining the distance of a heavenly body
+resolves itself into a measurement of its _parallax_, that is, of the
+apparent change of its position brought about by a change in the
+situation of an observer. If one be seated in a room, about a yard from
+a window, a very simple experiment may be made to illustrate the meaning
+of this term. Closing one eye, the observer will see a vertical line,
+such as the partition between two panes, projected upon some particular
+part of an opposite building; when the other eye is used the line will
+apparently be displaced, and the nearer one is to the window the greater
+will be the displacement or parallax. As the heavenly bodies are so far
+away, each of our eyes sees them in the same directions. Indeed, the
+stars are so distant that to _all_ persons situated on our planet their
+apparent positions are identical. With the members of the solar system,
+however, the case is different; the earth has an appreciable size as
+seen from them, so that when viewed from different parts of the earth
+they will not appear in exactly the same part of the heavens.
+
+The earth’s rotation changes the relation of an observer’s position with
+regard to a heavenly body in pretty much the same way as a change in his
+actual position on the globe. When an object in the zenith is observed,
+it will appear in precisely the same part of the sky as if it were seen
+from the centre of the earth, but as it approaches the horizon it will
+be displaced. Hence the term _diurnal parallax_, meaning the
+displacement of a heavenly body depending upon the observer’s position
+as affected by the earth’s rotation. Taking it in its general
+astronomical sense, the parallax of a heavenly body is the angle between
+the two lines which join it to the observer and to the centre of the
+earth respectively. Thus, in Fig. 38, let O be an observer, Z his
+zenith, and C the centre of the earth; then the parallax of a body S is
+the angle O S C. As the observer’s position is changed to O′ by the
+earth’s rotation, the parallactic angle is increased to O′ S C. If S be
+on the horizon, that is, when O′ C is perpendicular to O′ S, the
+parallax is a maximum, and is then called the horizontal parallax. The
+_horizontal parallax_ of a body is therefore the greatest angle
+subtended by the earth’s radius as seen from the body. We have seen,
+however, that the earth’s radius is not of the same length in all parts,
+and it is therefore necessary to specify more particularly which radius
+is in question. The standard adopted is the equatorial radius, and, when
+this is employed, our greatest parallactic angle is called the
+_equatorial horizontal parallax_.
+
+[Illustration:
+
+  FIG. 38.—_Parallax of a Heavenly Body._
+]
+
+In the case of all the heavenly bodies the parallaxes are very small;
+that of the moon averages about 57′, while that of the nearest planet
+does not exceed 40″. The parallax of a body evidently diminishes as the
+distance increases.
+
+DISTANCE DEDUCED FROM PARALLAX.—When the parallax of a heavenly body has
+been determined, it becomes a simple matter to calculate the
+corresponding distance; thus, in Fig. 38, the distance C O′ represents
+the earth’s equatorial radius, O′ S C is the equatorial horizontal
+parallax, C O′ S is a right angle, and the required distance is C S. By
+a simple trigonometrical rule this distance is the earth’s radius
+divided by the sine of the parallax. In the case of a small angle, the
+sine is very nearly equal to the angle itself divided by the angle
+corresponding to an arc of a circle equal in length to the radius. As
+there are 206,265 seconds in an arc equal to the radius, the sine of a
+small angle may be taken as the angle itself, expressed in seconds,
+divided by this number. Thus, if _p_ be the equatorial horizontal
+parallax of an object reckoned in seconds of arc,
+
+   Distance = (earth’s equatorial radius)/(sine _p_)
+            = (206,265 × earth’s equatorial radius)/(_p_)
+
+We shall see presently that the average parallax of the sun is 8″·80,
+and its average distance, as derived from the application of this
+formula, is accordingly about 92,790,000 miles.
+
+DIAMETERS.—It is a familiar fact that the further an object is removed
+from us the smaller it appears. The ascent of a balloon at once suggests
+itself as an excellent example. It is necessary, therefore, to
+distinguish very carefully between the apparent and the true size of an
+object. A halfpenny at a distance of nine feet from the eye will just
+cover the moon if the line of sight be directed towards that body, but
+we should not say the moon is the size of a halfpenny, because we know
+perfectly well that a disc twice the size would produce just the same
+appearance if removed to double the distance. Apparent size must,
+accordingly, be reckoned in angular measure, and we say, for example,
+that the moon has an apparent diameter of a little more than half a
+degree.
+
+When the angular diameter and distance have both been measured, the real
+diameter, in miles, can at once be deduced by a simple inversion of the
+process of determining the distance of an object from its known
+parallax. Thus, in Fig. 39 let A B represent the moon or other heavenly
+body, and E the centre of the earth. The angle M E A is the angular
+semi-diameter, and E M the required distance; then, since the angle E A
+M is a right angle,
+
+                         A M = M E × sine M E A
+
+That is,
+
+      Semi-diameter in miles = distance in miles × sine of angular
+                             semi-diameter.
+
+Or,
+
+     Diameter = twice the distance × sine of angular semi-diameter.
+
+[Illustration:
+
+  FIG 39.—_Diameter of a Heavenly Body._
+]
+
+Since the apparent diameters are always small, the sine may be taken as
+equal to the circular measure; that is, the number of seconds which the
+angle contains divided by 206,265.
+
+DISTANCE AND SIZE OF THE MOON.—If the moon were a fixed body outside the
+earth, its parallax could be easily determined by a single observer,
+who, in that case, would note the apparent displacement produced by his
+rotation. It has, however, a very complex movement, and it is therefore
+difficult to separate the real change of position from the parallactic
+change. The best method is one in which two observers, far removed from
+each other, can observe the moon’s position at nearly the same instant,
+so that the effect of its movement is very small and can be sufficiently
+allowed for. A necessary consequence of this condition is that the two
+observers should be placed as nearly as possible on the same meridian.
+Observations with the object of determining the lunar parallax have
+accordingly been made at Greenwich and the Cape of Good Hope. From the
+known positions of these places and the size of the earth, the distance
+between them is very accurately known, and this serves as a base line in
+a triangulation of the moon.
+
+[Illustration:
+
+  FIG. 40.—_Measurement of the Moon’s Distance._
+]
+
+If G and C, in Fig. 40, represent Greenwich and the Cape respectively,
+the celestial equators at the two places will be in the directions G E
+and C E. M being the moon, its declination, as measured at G, will be
+the angle M G E, and as measured at C it will be the angle M C E′. Since
+G E is parallel to C E′, the difference of these declinations (when both
+are north declinations, as in the diagram) will be the value of the
+parallactic angle G M C, which is about 1½°. From these data it is easy
+to calculate the distance of the moon either from Greenwich, the Cape,
+or the earth’s centre. In this way the distance of the moon is found at
+some particular moment, and the additional knowledge of the shape of its
+orbit enables us to determine the semi-major axis of the orbit, which is
+nothing more than the average or mean distance of the moon. The mean
+equatorial horizontal parallax of the moon is 3,422″·5, and the
+corresponding mean distance from the earth is 238,855 miles.
+
+The average apparent diameter of the moon, as it would appear from the
+centre of the earth, is 31′ 7″, from which it results by the method
+already stated that the true diameter is 2,162 miles.
+
+The apparent diameter of the moon is affected by the observer’s position
+upon the earth, as well as by the situation of the moon in its orbit. An
+observer to whom the moon is directly overhead is nearly 4,000 miles
+nearer to it than another observer who has it on his horizon. Tables
+have accordingly been drawn up to indicate the _augmentation_ of the
+moon’s apparent diameter as it rises above the horizon. The greatest
+possible apparent diameter is about 36″.
+
+Everyone must have noticed that when the moon is rising or setting, it
+looks much larger than when it is high up in the sky, an appearance
+which does not seem to accord with the fact that its measured angular
+diameter is least when on the horizon. It is evident, however, that the
+seeming increase of size is a subjective phenomenon, due to our
+incapacity to correctly judge distances.
+
+RELATIVE DISTANCES OF PLANETS.—The relative distances of the planets
+from the sun were found long before any of the actual distances were
+known with any reasonable degree of accuracy. Kepler discovered the
+relation which exists between these distances, and expressed it in his
+third or harmonic law, which states that “the squares of the periodic
+times of the planets are proportional to the cubes of their mean
+distances from the sun.”
+
+In the case of the interior planets, the angles of greatest elongation
+furnish the means of finding their distances from the sun as compared
+with that of the earth. Thus, if V in Fig. 41 represents Venus, E the
+earth, and S the sun, the angle E V S is a right angle when Venus is at
+greatest elongation. The observed value of the angle S E V is 46°, and
+this definitely determines the shape, though not the size, of the
+triangle S E V. The distance of Venus from the sun, S V, is thus found
+to be 0·72 times the distance of the earth from the sun, S E. If Venus
+be at inferior conjunction, that is, at V′, its distance from the sun
+will be represented by 72, if the earth’s distance from the sun be
+denoted by 100.
+
+This method can also be applied in the case of Mercury, but as the orbit
+is so eccentric, it is necessary to take the average of a large number
+of greatest elongation angles.
+
+The process of determining the relative distance of an exterior planet,
+such as Jupiter, is a little more complex, but involves no considerable
+difficulties.
+
+[Illustration:
+
+  FIG. 41.—_Relative Distance of Venus._
+]
+
+There is a curious relationship between the relative distances of the
+planets, which is commonly known as _Bode’s law_. A series of figures,
+0, 3, 6, 12, 24, 48, 96, 192, 384, each, with the exception of the
+second, being double the preceding one, is written down, and the number
+4 added to each. Then the resulting numbers approximately represent the
+relative distances of the planets from the sun. Thus:—
+
+       4      7    10    16     28       52     100    196     388
+    Mercury Venus Earth Mars Asteroids Jupiter Saturn Uranus Neptune
+
+It is interesting to note that this law was announced in 1772, when the
+asteroids and the planets Uranus and Neptune were still unknown, so that
+there was a break in the series corresponding to the number 28. The
+discovery of Uranus in 1781, and the fact that its distance agreed
+roughly with Bode’s law, strengthened the conviction that an unknown
+planet revolved round the sun in an orbit between those of Mars and
+Jupiter. An association of astronomers was then formed to search
+systematically for the missing planet; but the actual discovery was made
+in 1801 by Piazzi, the Sicilian astronomer, who had not joined the
+association. The new planet was a very small one, and its discovery was
+rapidly followed by the detection of several others. At the present
+time, more than 400 of these asteroids, or minor planets, are known, and
+their average distance fits in very well with Bode’s law.
+
+THE SUN’S DISTANCE.—One of the grandest problems which astronomical
+science requires us to solve is the determination of the sun’s distance.
+Starting with a knowledge of the earth’s dimensions, the subsequent
+measurement of the sun’s distance enables us to get a clear idea of the
+scale, not only of the solar family to which we ourselves belong, but of
+the whole sidereal universe. No wonder then that a vast amount of
+astronomical energy has been expended on this investigation.
+
+The problem, however, is beset with many practical difficulties, and the
+greatest possible skill is required to cope with it. In the first place,
+the parallax of the sun is so small that the method employed for the
+moon fails, and it can only be determined by indirect means.
+
+We have already seen that the constant of aberration gives us a means of
+determining the size of the earth’s orbit, and consequently the distance
+of the sun. When proper allowance is made for the eccentricity of the
+orbit, this method is a very valuable one.
+
+Other methods which have been employed depend upon the measurement of
+the parallax of one of the nearer planets, from which the distances of
+all the planets, including the earth, from the sun, can be found from
+our previous knowledge of the relative distances. Mars and some of the
+asteroids have been thus utilised at their oppositions, and Venus when
+at inferior conjunction.
+
+[Illustration:
+
+  FIG. 42.—_The Parallax of Mars._
+]
+
+The parallax of Mars can be determined in the same way as that of the
+moon, either by concerted observations at two distant places, or by a
+single observer who utilises the earth’s rotation to provide him with a
+base line. The actual measurements do not consist of direct estimations
+of the right ascension and declination of the planet, but of its angular
+distances from stars among which it appears, the measurements being made
+with micrometers or heliometers. In this way certain errors due to
+refraction, etc., are minimised. To take an extreme case, let the planet
+M (Fig. 42) be rising to an observer at O; it will then be seen in the
+direction O M, while a neighbouring star will be seen along the line O
+S. After twelve hours the rotation of the earth will have carried our
+observer to O′, and he will now see the planet in the direction O′ M,
+while the star will remain in the same direction, O′ S′. In each case he
+would measure the angle separating the planet from the star, and would
+thus obtain the values of the angles S O M and S′ O′ M, which, in the
+case shown in the diagram, would be together equal to the angle O M O′.
+When corrected for the observer’s latitude, and for the planet’s change
+of place in the interval, the equatorial horizontal parallax of Mars
+would be determined. Then the distance of Mars from the earth would be
+known, and at opposition this is the difference between the distances of
+the earth and of Mars from the sun; the ratio between the latter is
+already known, and their actual distances at once follow.
+
+[Illustration:
+
+  FIG. 43.—_The Transit of Venus._
+]
+
+TRANSIT OF VENUS.—The planet Venus at inferior conjunction is near
+enough to the earth to have a considerable parallax, but the method
+employed in the case of Mars cannot be used, as the planet is not
+visible when between us and the sun, except on the very rare occasions
+when it transits across the sun’s disc. When a transit occurs, the
+distance of the planet from the earth can be measured in essentially the
+same way as that of Mars at opposition, when two observers work
+together. The difference is that the apparent place of the planet is
+referred to the sun’s disc instead of to neighbouring stars. Suppose the
+conditions to be as represented in Fig. 43, E being the earth, V the
+planet, and S the sun. Two observers on the earth, at _a_ and _b_, will
+see the planet projected on different parts of the sun’s disc. If we at
+first regard them as being at rest, the observer at _b_ would see the
+planet cross the sun along the line C D, while to the one at _a_ it
+would appear to cross the line F G. The times of crossing would, under
+the assumed conditions, depend upon the orbital velocity of Venus, and a
+measure of these times at the two stations would determine the relative
+lengths of the chords C D and F G. We already know that the distance of
+Venus from the sun is to its distance from the earth at inferior
+conjunction in the proportion 72 to 28. (See p. 145.) The rectilinear
+distance between the two places is also known, and the distance _x y_
+between the chords is ⁷²⁄₂₈ of that from _a_ to _b_, whatever the actual
+distance of the sun may be. We thus know the ratio of the lengths of two
+parallel chords, and the distance between them in miles, from which it
+is a simple matter to find the diameter of the sun’s disc in miles. The
+angular diameter of the sun is measured with a transit instrument, and
+to find the sun’s distance we have simply to calculate the distance at
+which a body of known size subtends a known angle.
+
+We have supposed the observers at rest, but they are in reality carried
+forward by the earth’s orbital motion, and are turned about the earth’s
+axis. The first of these movements will affect both observers in the
+same degree, and will simply lengthen the duration of the transit. The
+effect of rotation, however, depends upon the position of the sun and
+planet, with regard to the observer’s meridian. At sunrise, an observer
+is carried by the rotation of the earth almost directly towards the sun,
+while at sunset he is carried away from it. The rate at which the planet
+traverses the sun’s disc would, therefore, be little affected by the
+earth’s rotation at sunrise or sunset. About mid-day, however, the
+effect of the earth’s rotation is to accelerate the apparent motion of
+the planet, and to shorten the time of transit. If the beginning of the
+transit be observed at sunset, and the end soon after sunrise, as it may
+well be in high latitudes, the duration of the transit is retarded by
+the earth’s rotation. Corrections for rotation, however, are not
+difficult to apply.
+
+In this method of observing a transit of Venus, which was suggested by
+Halley, when it was impossible that he would live to see it carried out,
+the places of observation must be widely separated in latitude, and the
+beginning and end of the transit must both be observed.
+
+Another method of utilising a transit of Venus is known as Delisle’s
+method. In this case the two stations are near the Equator, and each
+observer notes the Greenwich time of internal contact, when the planet
+fully enters upon the sun’s disc.
+
+Owing to various causes, chief among which is the so-called “black
+drop,” the time of ingress and egress cannot be actually recorded with
+the desired degree of accuracy, and the transit Venus is no longer
+looked upon as the best method of determining the distance which
+separates us from the sun.
+
+Some of the results which have been obtained for the solar parallax are
+as follows:—
+
+         Transit  of Venus,  1874, contact observations, 8″·859
+         „            „        „   photographs,          8·859
+         „            „      1882, contact observations, 8·824
+         „            „        „   photographs,          8·842
+         Gill’s  observation  of   Mars, 1877,           8·780
+         Galle’s      „            Flora, 1873,          8·873
+         Gill’s       „            Juno, 1874,           8·765
+         „            „            minor planets, 1896,   8·80
+
+From a discussion of all the available data, Professor Harkness
+considers the most probable value of the solar parallax to be 8″·80905,
+with a probable error of 0·00567″. Turning this into miles, we find the
+distance of the sun to be 92,796,950 miles, and this is in all
+probability not more than 60,000 miles in error. This agrees very
+closely with Dr. Gill’s latest value, which has been accepted by the
+superintendents of the British and American nautical almanacs.
+
+THE SUN’S DIAMETER.—The real diameter of the sun is found from the
+parallax, and its mean angular diameter in the manner already explained
+(p. 142). Taking the distance as 92,780,000 miles, and the mean apparent
+semi-diameter as 962″, we have
+
+ Sun’s diameter = (2 × 92,780,000 × 962)/(206,265)
+                = 865,400 miles.
+
+The sun’s diameter is the same in all directions, so far as our
+measurements give any information on the point, so that there is no
+appreciable polar flattening corresponding to that of the earth and some
+of the other planets. This result is what we should expect from the
+relatively slow rate at which the sun turns upon its axis.
+
+DISTANCES AND DIAMETERS OF PLANETS.—It has already been pointed out that
+our knowledge of the relative distances of the planets from the sun
+enables us to determine their absolute distances when the distance of
+one of them has been ascertained. In this way the determination of the
+earth’s distance leads us to those of the other planets.
+
+Our additional knowledge of the planetary orbits further permits the
+calculation of the distance of any planet from the earth at a stated
+time. If, then, the angular diameter of a planet be measured with a
+micrometer attached to a telescope, the absolute diameter in miles can
+be determined in the same way as that of the sun or moon.
+
+To take an actual example, the equatorial angular diameter of the globe
+of Saturn, as measured by Prof. Barnard with the great telescope of the
+Lick Observatory on April 14, 1895, was 19″·4. It was then computed that
+if the observation had been made from the sun this would have been
+reduced to 17″·9. The distance of Saturn from the sun being 9·5388 times
+the earth’s distance, it results from this measurement that the true
+equatorial diameter of the ball of Saturn is 76,500 miles. A number of
+independent measures made at intervals from March to July gave an
+average value of 76,470 miles for the diameter.
+
+
+
+
+                              CHAPTER XII.
+                    THE MASSES OF CELESTIAL BODIES.
+
+
+MASS AND WEIGHT.—As a matter of daily experience, we know that a certain
+effort is required to prevent a body from falling to the ground, and the
+larger the bulk of any particular kind of matter, the greater is the
+effort demanded. Again, equal bulks of different kinds of matter require
+unequal efforts to sustain them in the hand. From facts such as these we
+get the idea of _weight_, and we say that one body is heavier than
+another when it has the greater tendency to fall to the ground. For the
+purposes of everyday life, the weight of a body is used as a measure of
+the quantity of matter which it contains, and the standard of weight in
+our own country is that of a certain piece of platinum kept at the
+Exchequer Office, in London, which is called a _pound_. The weight of
+the same piece of matter varies at different parts of the earth’s
+surface, and also at different distances from the ground, and it is
+evident, therefore, that weight is not a very scientific measure of the
+quantity of matter which a body contains. The standard of comparison
+must be one which is invariable not only in all parts of the earth, but,
+if we wish to investigate the quantity of matter in the celestial
+bodies, it must be unalterable through all parts of the universe.
+
+One’s first idea is that the bulk, or space which a body occupies, will
+furnish a means of measuring the quantity of matter which it contains,
+but here again we find that the volume of a body can be varied without
+either adding to or subtracting from it, its weight remaining constant.
+A piece of ice, for example, occupies a greater space than an equal
+weight of water.
+
+It is evident then that some other property of matter must be used as a
+measure of quantities. Now, there is every reason to believe that the
+same piece of matter, in whatever part of space it may be situated,
+requires the same force to set it moving with the same speed in a given
+time. By the continued application of a force, a body will first be set
+in motion, and at the end of a second it will have a certain speed; in
+the next second the velocity will have increased by an amount equal to
+that acquired at the end of the first second, and so on for subsequent
+intervals. For example, if at the end of a second the velocity were 3
+feet per second, at the end of the next second it would be 6 feet per
+second, and after other equal intervals it would be successively 9, 12,
+15, and so on. In this way the velocity is increased uniformly, and is
+said to be uniformly accelerated, while the gain per second is called
+the _acceleration_. The greater the force applied, the greater will be
+the acceleration it produces, and the acceleration can be used as a
+measure of the force at work.
+
+If the same force be applied to different quantities of the same
+substance, the acceleration produced will be in inverse proportion to
+the quantities. We thus arrive at the important result that two bodies,
+whatever their nature, contain equal quantities of matter, or have equal
+_masses_, when equal forces give them the same acceleration. The mass of
+a body can thus be ascertained by observing the acceleration due to the
+action of a known force.
+
+As a matter of observation, it is found that all bodies, whatever their
+composition or size, fall to the ground from the same height in the same
+time if the observations be made at one place. This means that the
+forces corresponding to weights produce equal accelerations in all
+bodies at the same place, and it follows, therefore, that the weights of
+bodies at the earth’s surface, are proportional to their masses. Hence,
+it is that weight can be practically employed in comparing masses, or
+quantities of matter, for the purposes of everyday life. It must be
+clearly understood, however, that a _mass_ of a pound is in reality
+quite distinct from a _weight_ of a pound, the former specifying a
+certain quantity of matter, and the latter its tendency to fall towards
+the earth.
+
+THE LAW OF GRAVITATION.—The idea that weight is due to the attraction of
+the earth for all bodies in its neighbourhood was first suggested by
+Newton, and an extension of this idea led him to formulate the great law
+which underlies the whole science of astronomy. All bodies near the
+earth’s surface are acted upon by forces proportional to their masses,
+and the same acceleration is produced in all of them if they are allowed
+to fall to the ground. Falling freely for a second, all bodies
+whatsoever, when the resistance of the air is eliminated, pass through a
+little over 16 feet, and acquire a velocity of just over 32 feet per
+second. The acceleration due to gravity is thus 32⅙ feet per second for
+bodies near the earth’s surface. If the experiment be made at the top of
+a high mountain, the distance fallen through and the acceleration
+acquired in a second is found to be less.
+
+If we could ascend still higher, the acceleration produced in falling
+bodies would be again reduced, and, in the light of what has gone
+before, it is evident that the force with which bodies tend to fall to
+the earth is diminished as the distance from the earth’s surface is
+increased. It was such considerations as these which led Sir Isaac
+Newton to formulate the law that _the force with which a body is
+attracted towards the earth diminishes in inverse proportion to the
+square of the distance from the earth’s centre_. Terrestrial means of
+testing the truth of this statement are obviously very limited, and
+hence it was that Newton looked to the moon for its verification. If the
+law holds good at the distance of the moon, an object so far removed and
+not acted upon by other forces, should fall towards the earth, and as
+its distance is about sixty times that of a body at the surface from the
+centre of the earth, the acceleration produced should be only ¹⁄₃₆₀₀th
+part of that imparted to bodies near the surface. In other words, since
+a body near the surface falls through 16 feet in the first second, one
+at the moon’s distance should only fall through about ¹⁄₂₀th of an inch.
+If, then, the moon be subject to the earth’s attraction, this fall
+towards the earth must be exhibited in some form or other, although the
+fact that the moon does not fall down upon the earth shows that there is
+some counteracting tendency.
+
+Observations have shown us that the moon moves in a curved path. It has
+been put in motion somehow, and since there is no reason why it should
+turn to one side or the other, or come to rest, unless some forces are
+acting upon it, it would tend to go on uniformly in a straight line for
+ever. That its movement is curvilinear is at once an indication of the
+action of a force besides that which originally set it in motion. This
+force is directed towards the earth, and the moon is drawn out of its
+rectilinear path just as far in any specified time as it would fall
+towards the earth if at rest.
+
+Let E and M in Fig. 44 represent the earth and moon respectively. Then,
+if the moon were not hindered in any way, it would move in the direction
+M _b_, and would reach the point _b_, let us say, at the end of a
+second. It is, however, found to be at the point _a_, and it has
+therefore fallen towards the earth through the distance _b a_. The size
+of the moon’s orbit and the angle through which it moves in a second
+being known, it is easy to calculate the distance _a b_, which is found
+to be about ¹⁄₂₀th of an inch, as demanded by Newton’s law.
+
+[Illustration:
+
+  FIG. 44.—_The Moon’s Curvilinear Path._
+]
+
+In his first attempt to thus verify the law of gravitation, Newton
+failed for the want of a sufficiently accurate knowledge of the earth’s
+diameter, but a few years later a new arc of meridian was measured, and
+he had the untold satisfaction of demonstrating its truth.
+
+The curved path of the moon is, indeed, similar to that of a projectile.
+A cannon ball thrown out horizontally will reach the ground after
+describing a curved path; but if it could be projected from a great
+elevation, with sufficient velocity, its forward movement would prevent
+its ever reaching the earth’s surface at all, and a new satellite of the
+earth would have been manufactured.
+
+The same kind of reasoning can be applied to the paths of the earth and
+planets around the sun, and Newton demonstrated that the laws of Kepler
+were a necessary consequence of the law of gravitation extended beyond
+the system of the earth and moon. By mathematical reasoning it was
+proved that if one body describes an elliptic orbit around another, and
+the line joining them describes equal areas in equal times, the
+attractive force must be directed to the central body, and, moreover,
+must vary inversely as the square of the distance between the two
+bodies. In this way the movements of the planets round the sun are
+perfectly explained by supposing that an attractive force, similar to
+that which causes bodies to fall to the earth’s surface, is exerted
+between all masses of matter, and hence the origin of the term
+_Universal Gravitation_. In its complete form, the law of gravitation
+states that “any particle of matter attracts any other particle with a
+force which varies directly as the product of the masses, and inversely
+as the square of the distance between them.”
+
+Confirmation of this grand law, which controls the movements of all the
+vast array of heavenly bodies, is furnished by many other phenomena. We
+see one of its effects in the tides, and another in the disturbances of
+the movements of planets brought about by their mutual attractions. Even
+in the depths of stellar space the same law holds good for those systems
+of stars which are sufficiently close together for their attractions to
+produce effects which we can study at our immense distance from them.
+
+The cause of gravity is still one of the greatest mysteries of physical
+science, although many ingenious attempts have been made to furnish an
+explanation of its mode of action.
+
+MASS OF THE SUN.—When we know the distance of the sun, and the time in
+which the earth travels completely round it, it is easy to calculate the
+fall of the earth towards the sun in the same way that the moon’s fall
+towards the earth is determined.
+
+The distance which a body 93,000,000 miles distant falls towards the sun
+in a second is thus found to be 0·116 of an inch. A body at the earth’s
+surface is about 4,000 miles from the centre, and it falls 16¹⁄₁₂ feet
+in a second; if removed to a distance of 93,000,000 miles, its fall
+towards the earth would be reduced inversely as the squares of 4,000 and
+93,000,000, and would amount to ·000,000,349 of an inch. This is only
+1/332,000th part the fall due to the sun’s attraction, and hence it is
+concluded that the mass of the sun is 332,000 times that of the earth.
+
+Strictly speaking, the accelerations produced by the sun and earth
+should be compared, but the fall during the first second is proportional
+to the acceleration due to gravity, and the same result is therefore
+obtained. It may be observed also that the fall of the earth towards the
+sun would not be appreciably effected if it were twice the size. All
+bodies fall towards the earth at the same rate, whatever their weights,
+and so in the case of a planet, the distance fallen towards the central
+sun is independent of the planet’s mass; the greater the mass the
+greater the attractive force.
+
+The sun occupies about 1,300,000 times the space occupied by the earth,
+and as its mass is only 332,000 times that of the earth, it follows that
+the sun’s density is only about a quarter that of the earth.
+
+MASSES OF PLANETS.—The process employed for the determination of the
+sun’s mass can be utilised for finding the masses of those planets which
+are accompanied by satellites. From the known distance of the planet,
+the size of the orbit of a satellite can be calculated in miles, and
+knowing the period of revolution of the satellite, its fall towards the
+planet can be determined. This fall is then compared with that of the
+planet’s fall towards the sun, and the mass of the planet in terms of
+the sun’s mass is thus arrived at.
+
+A convenient way of employing this method is to make use of a
+modification of Kepler’s third law. If _m_ be the mass of a planet in
+terms of the sun’s mass, M, _a_ and T respectively denote the semi-axis
+major of the orbit of the planet and its time of revolution round the
+sun; _a′_ and T similar quantities pertaining to the satellites’
+revolution round the planet: The following formula gives the relation of
+the masses:—
+
+                    _m_/M =  (_a′_/_a_)^3  (T/T′)^2
+
+This formula can be applied in the case of Mars, Jupiter, Saturn,
+Uranus, and Neptune, but fails in the case of Mercury, Venus, and the
+asteroids, which, so far as we know, have no satellites.
+
+The mass of Jupiter obtained in this way can be further checked by the
+influence of this giant planet upon other bodies in its neighbourhood.
+This planet has such an enormous mass that it produces very notable
+effects on the motions of Saturn, the asteroids, and of comets which
+travel in its neighbourhood, and, by measuring the amounts of these
+_perturbations_, the mass of the planet can be deduced.
+
+This method of perturbations is at present the only one by which we can
+obtain a knowledge of the masses of those planets which have no
+satellites. The motion of Mercury is disturbed by its nearest
+neighbours, Venus and the earth; that of Venus by the earth and Mercury.
+The differences between the observed positions of the planets and those
+calculated on the supposition that the others did not affect them, give
+the necessary data for the computation of the masses. The process,
+however, is one requiring profound mathematical knowledge, and even yet
+the mass of Mercury is not very certainly known.
+
+The asteroids, again, present no little difficulty. Their feeble light
+and small size point to small masses, and their mutual perturbations are
+almost insensible, except when two of them come into line with the sun.
+They produce no appreciable effects upon the movements of comets, so
+that it is almost impossible to determine their individual masses. Each
+asteroid, however, tends to produce a revolution of the major axis of
+the orbit of the nearest planet, Mars, and all tend to give it a motion
+in the same direction. If the total mass of all the asteroids put
+together were a quarter of the earth’s mass, a measurable displacement
+of the position of Mars would be produced. Professor Newcomb has
+recently shown that such a displacement actually occurs, but cannot
+amount to more than 5″·5 per century. From this it has been recently
+calculated that the total mass of the asteroids is probably about
+¹⁄₁₁₅th that of the earth’s mass.
+
+MASS OF THE MOON.—As the moon has no satellite, we must again have
+recourse to indirect methods if we wish to know anything as to its mass.
+Various processes are open to us; but although the moon is so near to
+us, it is more difficult to determine its mass than that of the most
+remote planet in our system.
+
+It has already been explained (p. 77) that as the earth is accompanied
+by the moon, it is really the centre of gravity of the two bodies which
+obeys the laws of planetary movement. As this point lies between the
+centres of the two bodies, at distances which are in inverse proportion
+to the masses, the centre of the earth describes a small monthly orbit,
+which, as we have already seen, produces a small monthly inequality in
+the sun’s apparent movement.
+
+By a careful investigation of this monthly oscillation of the sun, it
+has been found that the centre of gravity of the earth and moon must lie
+within the earth at a distance of about 2,900 miles from the centre.
+This is about ¹⁄₈₁th of the moon’s distance, whence it follows that the
+mass of the moon is about ¹⁄₈₁th that of the earth.
+
+Other methods of ascertaining the moon’s mass are also available. Among
+these are the investigation of the parts played by the moon in the
+production of the tides which swell our shores, and in the displacement
+of the earth’s axis which causes “nutation.”
+
+MASSES OF SATELLITES.—The earth’s satellite is of exceptional magnitude
+in comparison with its primary, and the method of finding its mass from
+the situation of the centre of gravity cannot be applied to the
+satellites attending other planets. In the case of the satellites of
+Jupiter and Saturn, the masses have been approximately determined by
+their mutual perturbations, these generally resulting in a revolution of
+the major axes of the orbits. Even this method fails for the satellites
+of Mars, Uranus, and Neptune, so that practically nothing is known with
+regard to their masses.
+
+MASS AND DENSITY OF THE EARTH.—So far we have been concerned entirely
+with relative masses, referring the masses of the various orders of the
+heavenly bodies either to the earth or sun. Although this is usually all
+that is required for astronomical purposes, it is of great interest to
+determine the absolute mass of the earth, and from this the absolute
+masses of the heavenly bodies can at once be deduced.
+
+We already know the dimensions of the earth, and therefore the number of
+cubic miles or feet which it occupies. We know also the weight or mass
+of a cubic foot of water or lead, and if the earth were of uniform
+specific gravity throughout its bulk, and composed of water or lead, we
+could at once calculate its total mass. It is, however, neither water
+nor lead; but if we can compare the mass of the earth with what it would
+be if composed of either of these substances, we can deduce either its
+mass or its specific gravity.
+
+A very simple method of “weighing” the earth has been employed with much
+success by Professor Poynting. The experiment was carried out at the
+Mason Science College, Birmingham, with a large bullion balance in which
+the beam was 123 centimetres long. Two spheres of lead and antimony,
+each weighing about 21 kilograms, were suspended from the arms of the
+balance. Another sphere of lead and antimony, weighing 153 kilograms,
+was successively brought by means of a turn-table under each of the two
+smaller weights. The alteration in the weights of the attracted balls
+were measured by observing the deflection of the beam, this being
+immensely magnified by a simple optical arrangement in which a mirror
+reflecting a pencil of light was made to turn through 150 times the
+angle moved through by the beam itself. The weight corresponding to a
+given deflection of the beam was determined by observing the disturbance
+produced by the addition of “riders” of known weights. In order to
+reduce the chances of error, the large weight was balanced on the
+turn-table by another mass of half the weight and at twice the distance
+from the centre, this being necessary in order that the attracting
+weight should rotate horizontally. The effect of this additional mass
+was calculated and allowed for, and the weighings were also repeated
+with the weights in various positions. The principle of the subsequent
+calculation is briefly as follows:—A mass A of lead and antimony of
+known bulk attracts another mass B with the force measured; if A were of
+the same size as the earth, the attraction would be increased by as many
+times as the earth is larger than A. If the average specific gravity of
+the earth were the same as that of the mass A, this calculated
+attraction would be equal to the weight of B. The ratio of this
+calculated weight of B to the actual weight accordingly gives the
+proportion between the specific gravity of the experimental ball and the
+average specific gravity of the whole earth. From this experiment it was
+estimated that the mean density of the earth is 5·4934 times that of
+water.
+
+The same principle is applied in the case of the famous Cavendish
+experiment, and its subsequent modifications by Baily, Cornu, and Boys.
+
+Another method of finding the earth’s density, and therefore its mass,
+is chiefly of historical interest. This is known as the “mountain
+method,” and was carried out in 1774 by Maskelyne, Hutton and Playfair
+on the Schiehallion Mountain, in Perthshire. A plumb-line suspended at
+the north side of the mountain is drawn towards the mountain, and so
+will not hang quite vertically. If removed to the opposite side of the
+mountain it will be deflected in the reverse direction. The amount of
+this deflection can be measured by reference to the stars, the positions
+of which are in no wise influenced by the attraction of the mountain. A
+survey of the mountain was next made in order to determine its bulk, and
+then the average specific gravity of the rocks composing it was
+determined with the greatest possible accuracy.
+
+The volume of the earth is 9,933 times that of the mountain, and its
+attraction would be this number of times greater if it were composed of
+the same materials as the mountain throughout. It was found to be in
+reality 17,781 times as great as the attraction of the mountain, and as
+this is 1·79 times 9,933, it follows that the average specific gravity
+of the matter composing the earth would be 1·79 times that of the rocks
+which build up Schiehallion. The mean specific gravity of the rocks
+being 2·8, the mean density of the earth was thus found to be 5·012
+times that of water.
+
+As a general result of all the observations which have been made, the
+value of the earth’s density may with much probability be considered to
+be not far from 5·576, or a little over 5½ times that of water.
+
+Whatever may be the composition of the earth’s interior, it is clear
+that the density must increase as the centre is approached.
+
+This knowledge of the earth’s density, in conjunction with the
+known number of cubic miles occupied by the earth, readily enables
+us to determine that the total mass of the earth is about
+6,000,000,000,000,000,000,000 tons.
+
+
+
+
+                             CHAPTER XIII.
+         GRAVITATIONAL EFFECTS OF SUN AND MOON UPON THE EARTH.
+
+
+THE TIDES.—The familiar phenomena of the tides are of such importance to
+commerce in so many parts of the world that they have been carefully
+investigated from very early times. The necessities of coast navigation
+would soon lead to the recognition of a periodic character in the tides,
+as well as to their association with the age and position of the moon.
+With the march of science, an explanation of tidal phenomena was
+therefore sought in the motion of the moon. A great impetus was given to
+this inquiry by Newton’s generalisation, and the tides were shown to be
+a necessary consequence of the gravitational attraction of the sun and
+moon. Regarding the earth merely as a cosmical particle, we have seen
+that its orbital motion is perfectly explained by the gravitational
+attraction of the sun, and some of its minor movements by the
+attractions of other members of the solar system. The law of
+gravitation, however, compels us, in a closer investigation of these
+mutual attractions, to regard each globe as an assemblage of particles,
+each of which individually influences and is influenced by other
+particles. If such a collection of particles be spherical and perfectly
+rigid, it will behave precisely as a simple particle in which the whole
+mass is concentrated.
+
+When we cease to consider the earth as a mere particle, we must regard
+the waters of the oceans as being free to move over the more rigid crust
+of the globe. Imagine our globe to be a spherical mass completely
+surrounded by a liquid envelope. At any moment one half of this is
+presented towards the moon. The solid earth we may conceive to be
+attracted by the moon as a simple particle; but the water on the side
+nearest to the moon is attracted with a greater force than the solid
+globe, because of its greater proximity to the attracting body, and it
+has therefore a tendency to heap itself up directly under the moon.
+Being free to move, the water thus remains heaped up under the moon,
+notwithstanding the earth’s rotation, and if there were only one such
+elevation, there would only be one tide a day. Observation shows us that
+there are two high tides a day, and the water must therefore be heaped
+up on the side of the earth which is turned away from the moon. This is
+perfectly true, though seemingly at first sight inconsistent with the
+moon’s attraction. The fact is that the solid earth is attracted by the
+moon with greater energy than the water on the side most remote from it,
+so that the heaping up of the water on the side away from the moon is to
+be regarded as due to the earth having left it behind.
+
+[Illustration:
+
+  FIG. 45.—_The Tides._
+]
+
+There is thus a double tidal wave produced by a spheroid of water which,
+in the simple case we have considered, has its axis directed towards the
+moon, as in Fig. 45. The earth, rotating within this liquid shell,
+successively brings different parts of the solid earth to the points of
+high and low water. If the moon were fixed, we should then experience
+two high and two low waters every day, but as it revolves in the same
+direction that the earth rotates, the average interval between two
+successive meridian passages is 24 hours 51 minutes. This, then, is the
+period in which alternate high waters or alternate low waters are
+experienced.
+
+A similar train of reasoning applies to the attraction of the sun upon
+different parts of our planet, so that there are solar as well as lunar
+tides. Nevertheless, the moon is the dominating cause, for although the
+total attraction of the sun upon the earth is about 200 times that of
+the moon, its differential attraction upon the opposite sides of the
+earth, which is alone effective in producing tides, is only about ⅖ths
+that of the moon.
+
+A simple mathematical investigation shows that the tide-raising force of
+a body is proportional to its mass, and approximately in inverse
+proportion to the cube of its distance from the affected body. Thus, it
+appears that if the moon were removed to 1·36 times its present
+distance, solar and lunar tides would be equal.
+
+At the times of new and full moon, the sun and moon will produce two
+tidal spheroids of water upon our imaginary earth, having their axes
+coincident, and an exceptionally high tide will occur. This is a _spring
+tide_. When the moon is at its quarters the two ellipsoids tend to
+neutralise each other, and an exceptionally low or _neap tide_ results.
+Two spring tides and two neap tides thus occur in each synodic month of
+29½ days.
+
+The height of the tide will also be affected by the variations in the
+distance of the moon. If the moon be at perigee the tide will be greater
+because of the smaller distance, and if this occur at new or full moon
+there will be a very high spring tide, while a less notable spring tide
+will occur when the new or full moon is at apogee.
+
+The combination of the solar and lunar tides gives rise to what is
+called the _priming_ and _lagging_ of the tides. At new and full moons
+the combined tides will produce a spheroid of water with its axis
+directed towards the moon. When the moon is a few days old however, the
+crest will take up a position intermediate between the direction of the
+moon and that of the sun, and high water will therefore be accelerated.
+The same thing will happen during three or four days after full moon.
+Three days before full or new moon the combination of the two tides will
+displace the crest towards the sun, and therefore in advance of the
+moon, so that high water will be retarded. The retardation and
+acceleration correspond to lagging and priming respectively.
+
+At the quadratures the combined tides simply reduce the height of the
+crest, since there is no reason why the deviation should be to one side
+any more than to the other. On account of priming and lagging, the tides
+on successive days are accelerated or retarded by as much as 13 minutes
+when the effects are greatest.
+
+Sufficient has been said to indicate that tidal phenomena are very
+complex even when we suppose the earth to be very simply constituted.
+When we take into account the actual configuration of the land and the
+consequent restrictions in the movements of the water, these
+complications are increased tenfold. Yet, by continued observations, the
+recurrence of tides at any port can be predicted with tolerable
+accuracy. It is observed that there is a certain pretty regular interval
+of time between the moon’s meridian passage and the time of next high
+water; this is different at different ports, but is so nearly constant
+at a given place as to be called _the establishment of the port_.
+Observations being made at a great many places, the peculiar movements
+of the tidal wave can be investigated. For this purpose, it is
+convenient to draw on a map what are called _co-tidal lines_ that is,
+lines passing through places at which high water occurs at the same
+moment. It then appears that it is only in the Southern Pacific where
+the water is of sufficient extent to permit the formation of the tide
+crest. The effect of this wave, which commences twice a day, is
+gradually spread over different parts of the world, but before it
+reaches most places other waves have commenced a similar journey. The
+tide at London, for example, coming round the north of Scotland and down
+the North Sea, really started in the Southern Pacific 66 hours before,
+and in the same way the tide at New York is a little over 40 hours old.
+
+The height of a tide is thus regulated by the conditions of the sun and
+moon with regard to the earth when the primary tide was formed, and not
+by their relation when a tide is actually observed.
+
+In the Pacific Ocean the tides are very feeble, but near the coast they
+vary enormously, and sometimes reach great heights. At Bristol the
+difference between high and low water sometimes amounts to fifty feet,
+and in the Bay of Fundy, Nova Scotia, it has been as much as a hundred
+feet.
+
+The peculiarities of the tides at many places are due to interference.
+The primary tidal wave striking the British Islands travels partly up
+the English Channel, and partly round to the North Sea by the north of
+Scotland. At some places on the east coast the two waves almost
+neutralise each other, while at others there are even four high tides in
+a day.
+
+The circumstances under which tides occur at a given place can only be
+determined by actual observations, as theory is at present utterly
+inadequate to deal with the manifold complications brought about by the
+configuration of the land, and the varying depth of the water.
+
+TIDAL FRICTION.—The regular influx of the tide supplies us with a source
+of mechanical energy, which in the future will no doubt become of
+immense importance to mankind. A great mass of water is raised to a
+higher level, and by suitable contrivances it can be made to do useful
+work during its subsequent flow to the ocean from which it came.
+Ordinarily, however, the water simply rushes back without its energy
+being utilised, and the potential power is merely transferred to another
+locality. It is manifest, however, that a certain amount of tidal energy
+is lost by friction as the water rolls to and from the rocky shores.
+This energy is converted into heat, and finally radiated into space, or
+dissipated. Now, the principle of the conservation of energy tells us
+that energy can neither be created nor destroyed, although its form may
+change from a useful to a useless one. It follows, therefore, that the
+energy lost through the tides must be abstracted from one source or
+another, and it has been shown that this energy is really derived from
+the earth’s rotation. As the earth steadily ploughs its way through its
+liquid envelope, the tides act as a break, and its rotational velocity
+is reduced; it is part of this lost energy of rotation which is
+dissipated by the tides.
+
+One tendency of tidal friction is accordingly to lengthen the period of
+the earths rotation, and, therefore, to increase the length of the day.
+There are, however, counteracting causes, so that there is no certain
+direct evidence that the day has actually lengthened in historical
+times.
+
+All the energy of rotation which is lost by the earth is not, however,
+dissipated by the tides. Some of it is transferred to the moon, with the
+result that the velocity of our satellite, and consequently the size of
+its orbit, must be increasing. From this it is inferred that the moon
+was formerly very much closer than at present, and an elaborate
+investigation of the conditions of its retreat has led Professor G. H.
+Darwin to his interesting theory of “tidal evolution.” (See p. 236.)
+
+Professor Darwin has shown that if the term “tide” be extended to
+include distortions of the earth and moon at an earlier stage of their
+history, when both were fluid or viscous, a similar grinding down of the
+energies of rotation of both bodies must have taken place. The axial
+rotation of the moon, under these circumstances, would be retarded by
+the attraction of the earth on the tides raised in the moon, while that
+of the earth would also be slowed down, but in a less degree because of
+the moon’s smaller mass.
+
+CAUSE OF PRECESSION.—On account of the spheroidal form of the earth, we
+may regard it as a sphere which is surrounded by a ring of protuberant
+matter at the Equator. Now the attraction of the sun upon the spherical
+part will be quite independent of the position of its axis of rotation,
+and will, therefore, not affect the position of the Equator. It is
+different, however, with the ring; at the solstices the ring is inclined
+to the line joining its centre with the sun, and the near side is
+subject to a greater attraction than the side more remote from the sun.
+On account of this difference of pull, there is a tendency for the ring
+to move into the plane of the ecliptic, and this is what would happen if
+the ring were not in rotation. The practical outcome of this tendency,
+combined with the rotation, is to produce the twisting of the plane of
+the ring, and, therefore, of the plane of the Equator. At the equinoxes
+the plane of the ring passes through the sun, and although there is
+still a difference of attraction on opposite sides of the ring, the
+differential force is entirely directed to the sun, and therefore cannot
+produce any precessional effect.
+
+The ultimate tendency to turn into the plane of the ecliptic thus
+depends upon the _difference_ of the attractions on opposite sides of
+the ring, or rather that part of the difference which acts in a
+direction perpendicular to the Equator.
+
+The terrestrial ring cannot change the position of its plane without
+taking the whole earth with it, and the rate of precession is thus very
+slow. The effect of solar precession alone would cause the equatorial
+plane to twist round with but little change of inclination; or the
+earth’s axis would travel with a conical movement round a perpendicular
+to the ecliptic passing through the earth’s centre.
+
+It will be remarked that as the force-producing precession is identical
+with that which is effective in producing the tides, the moon must have
+a greater precessional effect than the sun. This is quite true, and on
+the average the precession-producing force of the moon is 2½ times that
+of the sun. When the moon is on the celestial equator, as it is twice a
+month, the differential force acts in the plane of the ring, and no
+precessional effect results. On the other hand, the greatest effect is
+produced by the moon when the earth’s Equator is most inclined to the
+line joining the earth and moon. The amount of this greatest inclination
+is different in different months according to the position of the moon’s
+nodes. In consequence of the revolution of the moon’s nodes, the moon’s
+orbit is inclined to the Equator at all angles from 18° to 28°, and back
+again to 18° in a period of 19 years. The precessional effect of the
+moon thus has a principal period of 19 years, while that of the sun has
+a period of a year during which it has two maxima and two minima. The
+summation of the effects of the sun and moon gives us the _luni-solar
+precession_, which is very variable in its actual rate, but averages
+about 50″·2 per annum.
+
+[Illustration:
+
+  FIG. 46.—_Nutation._
+]
+
+NUTATION.—If the precession-producing force were of constant amount,
+there would be no change in the inclination of the earth’s axis to the
+ecliptic. When the force is increasing, the equatorial ring is slightly
+tilted towards the ecliptic, and when it is decreasing the converse
+takes place. As the moon has the preponderating effect, these changes in
+the inclination will evidently depend mainly upon the changing value of
+the moon’s precessional force; that is, they will have a period of 19
+years. Thus, if _P_, Fig. 46, represents the pole of the ecliptic, the
+north celestial pole would travel in a circle of 23½° radius about _P_
+if precession were uniform. Suppose, then, the celestial pole to be at
+_a_ when the moon’s node is on the Equator—that is, when the inclination
+of the moons orbit to the Equator is greatest—from this time the
+integrated effects of the moon’s precessional force will be decreasing,
+and the inclination of the Equator to the ecliptic will be increased;
+the celestial pole will consequently recede a little more than the
+average from the pole of the ecliptic, so that after 9½ years it will be
+at _b_ instead of _c_. During the next 9½ years the inclination of the
+moon’s orbit to the ecliptic will be gradually getting smaller, the
+precessional force will be proportionately reduced, and the obliquity of
+the ecliptic will be increased, so that the north celestial pole will
+have arrived at _d_ after the lapse of 19 years. The prolongation of the
+earth’s axis thus describes a wavy curve, each wave extending over 19
+years, so that there are about 1,400 waves during the great precessional
+cycle. This approach and recession of the two poles is called
+_nutation_, or nodding of the earth’s axis. The most recent
+investigation of its maximum amount, by Dr. Chandler, gives it as
+9″·202. Besides the principal nutation there are others of very much
+smaller amount, due to the monthly changes of the moon’s declination and
+to the annual change of the sun’s declination.
+
+The most obvious effect of nutation is that upon the inclination of the
+earth’s axis to the ecliptic—the “nutation in obliquity.” There is,
+however, a displacement of the equinoctial point, and corresponding
+nutations in longitude and right ascension.
+
+As pointed out by Sir John Herschel, we have in nutation a splendid
+example of a periodical movement in one part of a system giving rise to
+a motion having the same precise period in another.
+
+EFFECTS OF PRECESSION.—The effects of precession may be conveniently
+summarised here, although some of them have necessarily been mentioned
+elsewhere:
+
+(1) The first point of Aries revolves completely round the ecliptic, so
+that it passes through all the constellations of the zodiac in a period
+of 25,800 years. The “signs” of the zodiac, accordingly, no longer
+correspond with the constellations after which they are named.
+
+(2) The Pole Star is constantly changing, since the north celestial pole
+travels round the pole of the ecliptic at a distance of about 23½° in a
+period of 25,800 years. About 14,000 years ago the bright star Alpha
+Lyræ was the Pole Star.
+
+(3) The position of the north celestial pole is in time changed by 47°,
+and there may accordingly be this change in the north polar distances or
+declinations of all stars whatsoever. As the position of the ecliptic is
+almost constant, the celestial latitudes of stars will be but little
+affected by precession.
+
+(4) The right ascensions and longitudes of stars, being reckoned from
+the shifting first point of Aries, are themselves changeable, passing
+through all possible values in the precessional period.
+
+(5) The tropical year is shorter than the sidereal year by the time
+taken for the earth to travel through 50″·2—that is, 20 minutes 23
+seconds.
+
+(6) Celestial globes and maps, as well as star catalogues, can only
+represent the right ascensions and declinations of stars at a specified
+epoch.
+
+
+
+
+                              CHAPTER XIV.
+              INSTRUMENTAL MEASUREMENT OF ANGLES AND TIME.
+
+
+GRADUATED CIRCLES.—Astronomy is essentially a science of precision, and
+the progress of our knowledge has to a large extent been dependent upon
+the increasing power of accurately measuring angles and time.
+
+Let us see, first of all, how to measure angles.
+
+A circle is divided into 360 degrees, each degree again into 60 minutes,
+and each minute into 60 seconds of arc; and yet, a second of arc is not
+a small enough quantity for many astronomical purposes. Now, unless a
+very large circle be employed, it is mechanically impossible to even
+mark the minutes of arc directly upon it, and if a very large circle
+were constructed, the distortion of its shape produced by its own weight
+would be sufficient to mar its accuracy.
+
+What is actually done then is to get a circle of convenient size, and to
+graduate it, as well as the highest mechanical skill is capable of, into
+such parts as may leave distinct and equal spaces between the separate
+divisions. A competent instrument maker would, for instance, put 4,320
+divisions on the _limb_ of a circle 16 inches in diameter, two
+consecutive divisions thus being 5′ apart. For work of the highest
+precision it is necessary to strictly investigate the errors of the
+divisions and to correct for them in all observations.
+
+For the further subdivision of these graduations, verniers or reading
+microscopes are introduced.
+
+THE VERNIER.—A graduated circle being attached to an instrument, what
+one has to do is to take a _reading_ with reference to some fixed mark.
+If the fixed mark is seen to fall precisely on one of the divisions of
+the circle when observed with a magnifying-glass, the reading can be
+written down exactly. If there be no such coincidence, some means are
+required for accurately reckoning the fraction of a division. One method
+in general use on small instruments, and where extreme precision is
+unnecessary, is to employ a subsidiary scale which is called a
+_Vernier_, in honour of the Frenchman who invented it. This can be
+applied indifferently to a scale of degrees and parts of degrees on a
+graduated circle, or to a straight scale. With the aid of this device it
+becomes possible to measure angles with no greater probable error than a
+few seconds of arc.
+
+[Illustration:
+
+  FIG. 47.
+]
+
+THE READING MICROSCOPE.—If a greater degree of accuracy than 10″ be
+required, the vernier is superseded by a _reading microscope_. This is a
+compound microscope (Fig. 48) by which the scale can be observed, and at
+the focus of its eye-piece is a pair of spider threads which can be
+moved by a fine screw S. Looking into such a microscope, one sees a
+magnified picture of a very small part of the scale running through the
+field of view, as in Fig. 47. Running across the field, in the same
+direction as the marks on the scale, are the spider threads _a b_, which
+can be given a right and left movement by means of the screw. At the top
+of the field is the part called the “comb,” having its edge cut with
+saw-like teeth; like the threads, this is at the focus of the eye-piece.
+The scale is divided so that the smallest part is 5′, and in that case
+the teeth of the comb are arranged so that five of them equal a scale
+division. The reading microscope is a fixture, and the circle is brought
+into the position in which its reading is required by moving the
+instrument with which it is connected. The zero of the microscope is a
+point at the middle of the comb, and one has to determine what part of
+the scale corresponds with it. In order to do this, the threads or
+“wires” are moved until the next division lies between them, and the
+amount which the screw has been turned from the position of zero is read
+off on the graduated head of the screw. The dimensions of the parts, and
+the magnifying power of the microscope, are adjusted so that the screw
+must be turned five times to carry the wires through a space equal to a
+division on the scale. One division, therefore, will move the wires
+through 1′, and as the screw head is divided into 60 parts, a movement
+of ¹⁄₆₀th of a revolution will shift the wires through a second of arc.
+Even fractions of a second can be thus measured.
+
+[Illustration:
+
+  FIG. 48.—_The Reading Microscope._
+]
+
+The introduction of this method of measuring minute angles is due to
+Ramsden, who first applied it at the end of the last century. The
+microscopes themselves are used for measuring fractional parts of the
+graduations of the circles, and usually four to six of them are applied
+to different parts of the same circle. In this way, errors arising from
+flexure of the circle, fluctuations of temperature, want of exact
+circularity, etc., are eliminated, so that finally, after taking every
+conceivable precaution, the astronomer can measure angles with the
+accuracy which is absolutely necessary in many branches of research.
+
+ASTRONOMICAL CLOCKS.—Means for the exact estimation of time are of no
+less importance in an observatory than arrangements for the accurate
+measurement of angles. Astronomical clocks are constructed with extreme
+care, but in principle they do not differ from ordinary time-keepers. As
+sidereal time is of the greatest use in an observatory, the hour hand
+only makes one revolution a day, and the face is provided with a seconds
+hand, which is plainly visible. The pendulum is of such a length that it
+performs its swing in a second. One of the most important improvements
+in clocks was the introduction of the “compensation” principle, whereby
+the equivalent length of a pendulum remains constant in spite of
+fluctuations of temperature. The mercurial pendulum which one very
+frequently sees in a watchmaker’s establishment has a glass or steel
+cylinder near the bottom partly filled with mercury; as the rod
+lengthens by increased temperature, the centre of gravity is raised by a
+corresponding amount, on account of the upward expansion of the mercury,
+and the rate of swing remains constant when the quantity of mercury is
+properly adjusted. The chief defect of this plan is that the mercury and
+the steel rod do not respond equally well to a change of temperature.
+
+In the most approved clocks the pendulum rod is a compound one,
+consisting of rods, or concentric tubes, of zinc and steel. The pendulum
+bob is hung on a steel rod suspended from the top of a zinc tube, which
+in turn is fixed at the bottom end to a larger tube of steel; a rod
+attached directly to the latter is suspended by a flat spring in the
+usual manner. By this arrangement the unequal expansions or contractions
+of the different parts due to changes of temperature neutralise each
+other, so that a constant rate is the result. The tubes are pierced with
+numerous holes so that the inner and outer ones acquire the same
+temperature almost at the same time.
+
+The rate of a clock is disturbed slightly by changes in the pressure of
+the atmosphere. When the air is densest there is a greater resistance to
+the swinging of the pendulum, and the clock will go more slowly.
+Although this only amounts to a small fraction of a second a day, it
+must necessarily be taken into account in such an establishment as that
+at Greenwich, to which all the country looks for the precise control of
+time-keepers. In the standard clock at Greenwich a magnet is raised or
+lowered by the changing height of a barometer, and its varying
+attraction upon a certain piece of iron attached to the pendulum
+compensates for the differences produced by change of pressure.
+
+Pendulum clocks are obviously unsuitable for use at sea, so that
+_chronometers_ are usually employed on ships. These are like large
+watches, very carefully constructed, with “compensation” balance wheels,
+and can generally be relied upon as good time-keepers.
+
+After all precautions, however, no astronomer would put his faith in any
+clock for any length of time, as the best of them is liable to change
+its rate rather irregularly. The “error” of the clock is therefore very
+frequently determined by the observation of certain standard stars with
+the transit instrument. The stars can be relied upon to come to the
+meridian at the proper time, and any apparent departure from this time
+must be set down to the account of the clock.
+
+THE CHRONOGRAPH.—A good clock, however, is not the only requirement of
+an observatory. It is necessary further to be able to record very
+precisely the moment at which an observation is made. If the clock be in
+the immediate vicinity of the observer, the time can be noted by
+counting the beats of the pendulum, and a practised observer will, by
+this “eye and ear” method, record times to the nearest tenth of a
+second. Mere estimation, however, is not very reliable, so that a
+mechanical method, which also permits greater subdivision of the second,
+is very generally adopted. The instrument is called a _chronograph_,
+and, although constructed in various forms, its function is to record on
+a sheet or strip of paper the regular beats of the clock, as well as the
+signals made by the observer. In one form of the instrument the
+recording sheet is fixed on a cylindrical drum which is made to revolve
+once a minute by a small clock. Beneath the drum is a pair of prickers
+worked by the armatures of electromagnets. One of these magnets is in
+connection with the clock, and a simple arrangement sends an electric
+current through it every second, with the result that the seconds are
+marked by small punctures on the paper. As the cylinder revolves, the
+marker travels slowly lengthwise, so that the clock record runs spirally
+from one end to the other. To facilitate the identification of the
+punctures, one is omitted at the end of every minute. When an
+observation is made, the observer presses a button, and a current is
+sent through the second magnet, with the result that a puncture is made
+alongside those made by the clock. In this way the exact moment at which
+an observation is made can be easily registered, and read off at any
+convenient time.
+
+At Greenwich a room is set apart for a number of chronographs, each in
+communication with an instrument in the various observatories.
+
+
+
+
+                              CHAPTER XV.
+                              TELESCOPES.
+
+
+THE REFRACTING TELESCOPE.—The function of a telescope is two-fold.
+First, to magnify the heavenly bodies, or, what comes to the same thing,
+to make them look as if they were nearer to us, so that we can see them
+better. Second, to collect a much greater number of rays of light than
+the unassisted eye alone can grasp, so that objects too dim to be
+otherwise perceptible are brought within our range of vision.
+
+There are two forms of telescope, distinguished as _Refractors_ and
+_Reflectors_. The simplest form of refracting telescope is exemplified
+by the common opera-glass, and large refractors are not essentially
+different. Such instruments depend for their action upon the formation
+of an image by a lens. One can easily illustrate this by producing upon
+the wall of a room an inverted image of a candle or gas flame with a
+spectacle lens (one adapted for a long-sighted person), or with one of
+the larger lenses from an opera-glass. Having such an image, it may be
+magnified by means of another lens, just as one may magnify a photograph
+with an ordinary reading glass. Technically, the lens which forms the
+primary image is called the _object-glass_ of the telescope, and that
+which is used to magnify this image is called the _eye-piece_. The
+object-glass is usually a large lens, which is placed at one end of a
+tube, while the eye-piece is a much smaller lens, placed at the other
+end. Means are provided for adjusting the distance between the two
+lenses so as to admit of distinct vision.
+
+[Illustration:
+
+  FIG. 49.—_The Achromatic Object-Glass._
+]
+
+Matters are, however, not quite so simple as has been stated. There is a
+very great difficulty introduced by the fact that a lens made out of a
+single piece of glass gives an image which is surrounded by fringes of
+colour, so that some device has to be adopted in order to destroy, as
+far as possible, this enemy of good definition. In the early history of
+the telescope, this so-called _chromatic aberration_ was considerably
+reduced by making small object-glasses of very great focal length.[4]
+
+Lenses of 100-feet focus, however, are not easy to employ as
+object-glasses, and astronomy was, therefore, greatly benefited by
+Dollond’s invention of the _achromatic lens_ in 1760. This is a compound
+lens, usually consisting of a double convex crown-glass lens and a
+concavo-convex, or double concave, lens of flint glass. The curvatures
+of the lenses, and the optical properties of the two kinds of glass
+composing them, are such that the colour due to one of them is
+practically neutralised by that due to the other acting in opposition. A
+section of such an object-glass, with the “cell” in which it rests, is
+shown in Fig. 49.
+
+In this way the focal length of the lens, and, therefore, the length of
+the telescope tube, can be kept within reasonable dimensions, while the
+definition is improved. There is, however, usually a little outstanding
+colour, due to the imperfect matching of the two lenses, and if one
+looks through a large refractor, even of a good quality, a purple fringe
+will be noticed round all very bright objects. This only affects a few
+of the brighter objects, while millions of others which are dimmer may
+be seen free from spurious colour.
+
+It may be remarked that the curved surfaces of the lenses forming
+telescopic object-glasses must not be parts of spheres. If they are, the
+images will be rendered indistinct by _spherical aberration_, and the
+optician has to design his curves to get rid of this defect at the same
+time as chromatic aberration.
+
+A new form of telescopic objective, consisting of three lenses, which
+has many important advantages, has recently been invented by Mr. Dennis
+Taylor, of the well-known firm of T. Cooke & Sons, York.
+
+Such a lens as this illustrates the perfection which the optician’s art
+has now attained. Six surfaces of glass have to be so accurately figured
+that every ray of light falling upon the surface of the lens shall pass
+through the finest pinhole at a distance of eighteen times the diameter
+of the lens.
+
+THE REFLECTOR.—In a reflecting telescope, the object-glass of the
+refractor is replaced by a concave mirror. In order that such a mirror
+may reflect all the rays from a star to a single point, its concave
+surface must be part of a paraboloid of revolution, that is, a surface
+produced by the revolution of a parabola on its axis. If a spherical
+surface be employed, all the rays will not be reflected to a single
+point, and the images which it gives will be ill-defined. Yet it is
+astonishing to find that the difference between a parabolic and
+spherical surface, even in the case of a large mirror, is exceedingly
+small. Sir John Herschel states that in the case of a mirror four feet
+in diameter, and forming an image at a distance of forty feet, the
+parabolic only departs from the spherical form at the edges by less than
+a twenty-one thousandth part of an inch.
+
+[Illustration:
+
+  FIG. 50.—_The Newtonian Reflector._
+]
+
+An image being formed by a mirror, it is next to be viewed with an
+eye-piece just as in the case of a refracting telescope. Here there is a
+little difficulty, for if the eye-piece be applied in the direct line of
+the mirror, the interposition of the observer’s head will block out the
+light. Several ways of overcoming this have been devised, but the plan
+most generally followed is that which Newton adopted in the first
+reflecting telescope which was ever constructed. With his own hands
+Newton made a small reflector, 6¼ inches long and having an aperture of
+1⅓ inches, with which he was able to study the phases of Venus, and the
+phenomena of Jupiter’s satellites. This precious little instrument is
+now one of the greatest treasures in the collection of the Royal Society
+of London. The general design of this telescope is shown in Fig. 50. The
+concave mirror is at the bottom of the telescope tube, and normally it
+would form an image of a star near the end of the tube. A plane mirror,
+however, of small size intercepts the rays and reflects them to the
+side, where they converge to a focus. This image is observed and
+magnified by an eye-piece, as in the refractor. It is true that in this
+arrangement the plane mirror, or _flat_, renders the central part of the
+principal mirror ineffective, but the loss of light is very much less
+than would be the case if the eye-piece were placed in position to view
+the image centrally.
+
+In the hands of Sir William Herschel the reflecting telescope was
+greatly developed. The great telescope with which he enriched
+astronomical science had a mirror four feet in diameter, and its tube
+was 40 feet in length. With the view of utilising the whole surface of
+the mirror and dispensing with a second reflecting surface, the four
+foot mirror was placed at a small angle to the bottom of the tube, so
+that its principal focal point was no longer at the centre, but at the
+side of the tube.
+
+In practice, however, it is found that the Herschellian form of
+reflector does not give the best definition, and it is now very seldom
+seen.
+
+Among other forms, the “Cassegrain” is perhaps the most important.
+During the last year or two this form has received a great deal of
+attention, more especially in regard to its special adaptability for
+photographic purposes.
+
+In the Cassegrain telescope, the plane mirror of the Newtonian form is
+replaced by a small convex mirror which is part of a hyperboloid of
+revolution, its axis and focal point being coincident with those of the
+primary mirror. The rays are in this way reflected back to the mirror at
+the bottom of the tube, and in order that the image may be seen, it is
+necessary to cut out the middle part of the mirror to admit the
+eye-piece.
+
+Although the small mirror must theoretically be hyperbolic, tolerable
+definition is obtained even if it be spherical or ellipsoidal, and its
+actual departure from these forms is so slight as to be beyond detection
+by measurement, so that the figuring of such mirrors can only be tested
+in the telescope. For photographic purposes this telescope has the very
+important advantage that a short telescope is equivalent to a very long
+one of the Newtonian form, or refracting telescope, so that the image of
+sun, moon, or planets formed at the focus is very large in comparison
+with the size of the telescope. A modification of this form of
+telescope, in which the small mirror is out of the path of the rays
+falling upon the larger one, and no longer obstructing the central part,
+has been recently revived by Dr. Common, and has become generally known
+as the “Skew Cassegrain.”
+
+In reflecting telescopes the mirrors were formerly made of _speculum_
+metal (an alloy of copper and tin), and the word speculum is even now
+commonly employed to signify a telescopic mirror, although it is usual
+to make the mirror of glass, with the concave surface silvered and
+highly polished.
+
+[Illustration:
+
+  FIG. 51.—_The Cassegrain Reflector._
+]
+
+One is frequently asked for an opinion as to which is the better form of
+telescope, the reflector or refractor, and it is a question that one
+finds some little difficulty in answering. On one point, however, all
+are agreed, namely, that the reflector has the advantage in regard to
+its achromatism; it is indeed perfectly achromatic, while the so-called
+“achromatic” refractor is at best only a compromise. For the rest, one
+cannot do better than quote the evidence of Dr. Isaac Roberts before the
+International Astro-photographic Congress:—“The reflector requires the
+exercise of great care and patience, and a thorough personal interest on
+the part of the observer using it. In the hands of such a person it
+yields excellent results, but in other hands it might be a bad
+instrument. The reflector gives results at least equal, if not superior,
+to those obtained with the refractor, if the observer be careful of the
+centering, and of the polish of the mirror, and keeps the instrument in
+the highest state of efficiency; but when entrusted to an ordinary
+assistant the conditions necessary for its best performance cannot be so
+well fulfilled as the same could be in the case of the refractor.” One
+great practical advantage of the reflector is that there are fewer
+optical surfaces, so that a large reflector may be obtained for the
+price of a much smaller refractor.
+
+EYE-PIECES.—So far we have regarded the eye-piece of a telescope as a
+simple lens, but it is evident that the spherical and chromatic
+aberration of such a lens will interfere with its performance. For
+occasional use, however, even a simple lens is very serviceable if the
+object observed is brought to the centre of the field of view.
+
+Compound eye-pieces are of various forms, each having certain
+advantages, the desiderata being freedom from colour and “flatness of
+field”—that is, stars in different parts of the field are to be equally
+well in focus. Those most commonly employed are the Ramsden and
+Huyghenian eye-pieces. The former consists of two plano-convex lenses of
+equal focal lengths, having their curved faces towards each other, and
+being placed at a distance apart equal to two-thirds of the focal length
+of either lens. Such an eye-piece can be used as a magnifying-glass, and
+it is therefore placed outside the focal image formed by the telescope
+with which it is used; on this account it is called a _positive
+eye-piece_. This kind of eye-piece is not quite achromatic, but its flat
+field of view gives it a special value for many purposes.
+
+In the Huyghenian eye-piece there are again two lenses, made of the same
+kind of glass. That which comes nearest to the eye has a focal length of
+only one-third that of the _field_ lens, and the distance between the
+two lenses is half the sum of the focal lengths. This form of eye-piece
+cannot be used as a magnifying-glass in the ordinary sense, and as the
+field lens must be placed on the object-glass or mirror side of the
+focus, it is called a _negative eye-piece_. The Huyghenian eye-piece is
+more achromatic than the Ramsden, and is more widely used when it is
+only required to view the heavenly bodies. In instruments employed for
+purposes of measurement, a positive eye-piece is essential in order that
+the spider threads may be placed at the focus of the telescope. The
+images formed by an astronomical telescope are upside down, and neither
+of the eye-pieces described reinverts them.
+
+A special form of eye-piece is therefore used when a telescope is
+employed for terrestrial sight-seeing. The desired result is obtained by
+the introduction of additional lenses, but there is a corresponding
+reduction of brightness.
+
+For viewing the sun some device is necessary to reduce the quantity of
+light entering the eye. To look at the sun directly, even with a small
+instrument, is very dangerous. The arrangement usually adopted is a
+_solar diagonal_, in which the light is reflected from a piece of plane
+glass before entering the eye-piece; the piece of glass is wedge-shaped,
+so that the reflection from one surface only is effective; if the glass
+had parallel sides, the solar image would be double.
+
+MAGNIFYING POWER.—The magnifying power of a telescope depends upon the
+focal length of the object-glass, or speculum, and that of the
+eye-piece. Optically, it is equal to the former divided by the latter,
+so that the greater the focal length of an object-glass, or the smaller
+the focal length of the eye-piece, the greater will be the magnifying
+power. In a given telescope, the object-glass, or speculum, is a
+constant factor, and the magnifying power can only be varied by changing
+the eye-piece. The focal length of the Lick telescope, for example, is
+about 600 inches; with an eye-piece which is equivalent to a lens of
+one-inch focus, the magnifying power would be 600; with a lens of half
+an inch focus, it would be 1,200, and so on.
+
+The magnifying power which can be effectively employed, however, depends
+upon a great variety of circumstances. First, the clearness and
+steadiness of the air; then there is the quality of the object-glass, or
+speculum, to be considered; and also the brightness of the object to be
+observed, for when the object is very dim, its light will be spread out
+into invisibility if too high a power be used.
+
+In practice, good refractors perform well with powers ranging up to 80
+or 100 for each inch in the diameter of the object-glass. Thus, on
+sufficiently bright objects, a six-inch telescope will work well with a
+power of about 500, while a 30-inch may be effectively employed with
+powers between 2,000 and 3,000.
+
+ILLUMINATING POWER.—It has already been pointed out that magnification
+is not the only function of a telescope. As a matter of fact, the most
+powerful telescopes in the world fail to produce the slightest increase
+in the apparent size of a star, for even if these objects be brought to
+apparently a 3,000th part of their real distances, they are still too
+far away to have any visible size. But although a star cannot be
+magnified, it can be rendered more visible by the telescope, for the
+reason that the object-glass collects a greater number of rays than the
+naked eye. The pupil of the eye may be taken to have a diameter of
+one-fifth of an inch; a lens one inch in diameter will have 25 times the
+_area_ of the pupil, and will therefore collect 25 times the amount of
+light from a star; a two-inch lens will grasp 100 times, and a 36-inch
+32,400 times as much light as the pupil alone. Practically all these
+rays collected by the object-glass, or speculum, of a telescope cannot
+be brought into the eye; some are lost through the imperfect
+transparency of the glass, or the imperfect reflecting power of the
+speculum. Still, allowing a considerable percentage for loss, there is
+an enormous concentration of light when a large telescope is employed.
+
+THE ALTAZIMUTH MOUNTING.—Having got a telescope, we have next to see how
+it can be best supported, for unless it be a very small instrument
+indeed, it will be impossible to hold it in the hand like a spy-glass.
+However a telescope be mounted, provision must be made for turning it to
+any part of the sky whatsoever. Very frequently one of the axes on which
+the instrument turns is vertical, while the other is horizontal. Such a
+stand for a telescope is called an _altazimuth mounting_, for the reason
+that it permits the instrument to be moved in altitude and in azimuth.
+
+As a rule, one finds only small telescopes mounted in this manner. The
+objection to it is that, as one continues to observe a heavenly body,
+two independent movements must be given to the telescope in order to
+follow the body in its diurnal movement across the heavens. If we
+commence observing a star newly risen, for example, the telescope must
+trace a stair-like path in order to follow it, as it ascends into the
+heavens.
+
+THE EQUATORIAL TELESCOPE.—A much more convenient method of setting up a
+telescope is to mount it as an _equatorial_. The essential feature of
+this instrument is that one of the axes of movement, instead of being
+vertical, is placed parallel to the axis of the earth. This is called
+the _polar axis_, and, when the telescope is turned around such an axis,
+it traces out curves in the sky which are identical with those described
+by the stars in their diurnal motions. If, then, the telescope be
+directed to a star or other heavenly body, it can be made to follow the
+object and keep it in view by a single movement. The axis at right
+angles to the polar axis is called the declination axis, and is
+necessary in order that the telescope may be moved towards and from the
+Poles so that all the heavenly bodies above the horizon may be included
+in its sweep.
+
+One very important advantage of the equatorial is that as only one
+motion is required to keep a star in view, so long as it is above the
+horizon, the necessary movement may be furnished by clock-work. A good
+equatorial is accordingly provided with a driving clock, which is
+regulated so that it would drive the telescope through a whole
+revolution once a day. Unlike an ordinary clock, the driving clock of a
+telescope is regulated by a governor, in order that the instrument may
+have a continuous and not a jerky movement.
+
+The telescope is also provided with clamps and fine adjustments, one
+each in R. A. and declination, in order that it may be under the control
+of the observer. It is evident that the telescope must be capable of
+moving independently of the driving gear, so that it may first be placed
+in the desired direction; when this is accomplished, the R. A. clamp is
+used to put the telescope in gear with the clock. The declination clamp
+is them made to fix the telescope firmly to the declination axis. Fine
+adjustments in both directions are necessary, because it is impossible
+to sight a large instrument with such precision as to bring an object
+exactly to the centre of the field of view.
+
+Some of the driving clocks fitted to equatorials are very elaborate. As
+clocks regulated by governors are not such reliable time-keepers as
+those regulated by pendulums, arrangements are made by which the
+accuracy of a pendulum can be electrically communicated to a governor
+clock. One of the best forms of electrically-controlled clocks is that
+devised by Sir Howard Grubb.
+
+Another important feature of an equatorial is that it can be provided
+with circles which enable the telescope to be pointed to any desired
+object of known right ascension and declination. One of these is the
+declination circle, attached to the declination axis and read by a
+vernier fixed to the sleeve in which the axis turns; this is adjusted so
+as to read 0° when the telescope points to any part of the celestial
+equator, and 90° when it is directed to the Pole. The other circle is
+attached to the polar axis, and determines the position of the telescope
+with regard to the meridian; this is called the _hour circle_, and is
+divided into 24 hours. When the telescope is on the meridian, the hour
+circle reads zero, so that its reading in any other position gives the
+hour angle of the telescope. Having given the right ascension and
+declination of a heavenly body which it is desired to observe, the
+telescope is turned until the declination circle reads the proper angle,
+and the hour circle indicates the hour angle which is calculated for the
+particular moment of pointing the telescope. [The hour angle is the
+difference between the right ascension of the object and the sidereal
+time of observation.] In this way it is easy to find objects of known
+position which are invisible to the naked eye, and one can even pick up
+the planets and brighter stars in full sunshine. Conversely one can
+determine from the circles the right ascension and declination of any
+object under observation, but for various reasons only approximate
+results can be obtained in this way. The chief use of the circles on an
+equatorial is therefore to provide a means of pointing the telescope.
+
+Telescopes of 4 inches aperture and upwards are usually provided with a
+smaller companion called a _finder_. This has a larger field of view
+than the main telescope, so that objects which are of sufficient
+brightness can readily be picked up and brought to the centre of the
+finder, the adjustments being such that the object is then also at the
+centre of the field of the large telescope.
+
+There are, of course, many practical details connected with the working
+of an equatorial with which space does not permit us to deal. It may be
+remarked, however, that the adjustment of the polar axis is very simply
+performed by first inclining it at an angle approximately equal to the
+latitude of the place where it is set up, and setting it as nearly as
+possible in the meridian by means of a compass or by observations of the
+sun at noon. The final adjustment is then made by a series of
+observations of stars of known position.
+
+SOME OF THE WORLD’S GREAT TELESCOPES.—Thanks to the wide public interest
+taken in astronomical matters, a large number of powerful telescopes has
+been set up in various parts of the world. To the British Islands
+belongs the honour of possessing the largest telescope in the world.
+This is the giant reflector erected by Lord Rosse, in 1842, at
+Parsonstown, the mirror being 6 feet in diameter, and the focal length
+60 feet. Many very valuable observations were made with this instrument
+in its early days, but of late years it seems to have fallen into
+disuse. One reason may be that the mounting is not of the most
+convenient form, and makes the telescope unsuitable for photographic
+work.
+
+Coming next in point of size to the Rosse telescope is the reflector
+erected at Ealing, by Dr. A. A. Common. The glass mirror of this
+telescope is 5 feet in diameter, 5 inches thick, and weighs more than
+half a ton. Dr. Common aimed specially at constructing the largest
+possible telescope which could be equatorially mounted and provided with
+a driving clock, and he was only limited to an aperture of 5 feet by the
+impossibility of obtaining a glass disc of larger size. He has attained
+such great skill in this work that he was able to produce a perfect
+mirror 5 feet in diameter in three months time, although no less than
+410,000 strokes of the polishing machine were required.
+
+The telescope is of the Newtonian form, and the mounting is quite
+unique. The polar axis consists of an iron cylinder, made up of boiler
+plates, 7 feet 8 inches in diameter, and about 15 feet long. From the
+top of the cylinder, near its outer edge, two horns, each 6 feet long,
+project outwards, and the tube of the telescope swings on trunnions
+attached to the ends of the horns. The main part of the telescope tube
+is square, built up of steel angle iron, and carries the mirror at its
+lower end; the upper part of the tube, which carries the “flat” and
+eye-piece, is round, and of tinned steel strengthened by a skeleton
+framework.
+
+It is evident that such an enormous instrument as this cannot be made to
+travel by clock-work with the necessary uniformity without some very
+efficient arrangement for reducing friction. Dr. Common’s plan—and it is
+here that his instrument is unlike others—is to make the hollow polar
+axis water-tight, and to fix it in a tank of water. At the bottom of the
+polar axis is a ball and socket joint to keep it in position, and at the
+top is another bearing, which can be adjusted so that the polar axis
+lies truly in the meridian. It was found necessary to introduce 9 tons
+of iron into the bottom of the hollow polar axis in order to sink it to
+the proper angle, and to put sufficient weight on the bearings to give
+stability to the instrument. In this way the great mass is brought into
+the region of manageability, and the driving clock, which is driven by a
+weight of 1½ tons, is able to do its work efficiently. Such, in general
+outline, is this wonderful telescope, which, although not so large as
+Lord Rosse’s famous instrument, is undoubtedly its superior in
+light-grasping power and general utility, and more especially in its
+adaptability for photographing the heavens.
+
+Among other large reflecting telescopes now in use are the four-foot
+reflectors at Melbourne and Paris, and the three-foot reflectors at
+South Kensington and the Lick Observatory, California.
+
+The largest refracting telescope yet constructed is one of 40 inches
+aperture for the new Yerkes Observatory of the University of Chicago. It
+is interesting to note here that Professor Keeler, in his report as an
+expert upon the performance of the object-glass, considers that there is
+“evidence for the first time that we are approaching the limit of size
+in the construction of great objectives.” Unlike a mirror, a lens can be
+supported only upon its circumference, and it is the bending by its own
+weight that proves detrimental to its defining power. If the lens be
+made thicker with a view of overcoming this defect, the absorption of
+light by the glass increases, so that there is in the end no special
+gain by increasing the size.
+
+The length of the Yerkes telescope is 62 feet, and it will be provided
+with all accessories pertaining to astrophysical research. The Yerkes
+telescope, however, is not yet in actual use, and meanwhile the
+world-renowned Lick telescope, of 36 inches aperture, keeps the lead
+among active big refractors. The story of the foundation of this monster
+instrument is not much less wonderful than the telescope itself. Brought
+up in poor circumstances, with few opportunities for intellectual
+development, James Lick, nevertheless, amassed a fortune in business,
+and having few relations, he was anxious to dispose of his wealth in
+such a way as to bring him that fame which he had failed to achieve in
+other directions. Although it is very probable that he had never looked
+through a telescope in his life, the idea of a large telescope had taken
+a very firm hold upon his mind, and, thanks to the influence of his
+advisers, it was definitely announced in 1873 that Mr. Lick’s bid for
+immortality was to take this form. Several sites were examined by
+experts, and finally Mount Hamilton, California, 4,200 feet above
+sea-level, was selected. An excellent road, 26 miles in length, made at
+the cost of the county authorities, connects the observatory with the
+nearest town, San José, 13 miles distant.
+
+Owing to various delays, operations were not commenced until 1880, and
+five years were consumed in clearing away 72,000 tons of rocks and in
+erecting the buildings.
+
+Mr. Lick had stipulated for the erection of “a telescope superior to and
+more powerful than any telescope yet made,” and Messrs. Alvan, Clark &
+Co. contracted to supply a lens of 36 inches aperture for the sum of
+50,000 dollars. It turned out, however, that it was much easier to make
+such a contract than to fulfil it. To produce large discs of optically
+perfect glass, even in the rough, requires the greatest possible skill
+and patience, and this part of the work was undertaken by Feil & Co. of
+Paris. The flint glass disc was safely delivered in America in 1882, but
+the crown disc was cracked in packing. The elder Feil having retired
+from business, the duty of providing a new block of crown glass devolved
+upon his sons, who, after two years spent in vain attempts, ended in
+bankruptcy, and it was only through the elder Feil again resuming
+business that the much-required disc was finally completed in 1885.
+After the lapse of another year, the rough discs were fashioned, in the
+workshops of the Clarks, into the most marvellous of telescopic lenses.
+
+The mounting of the object-glass is worthy of the occasion, as will be
+seen from our illustration (see page 40). The tube is no less than 57
+feet long, and 4 feet in diameter in the middle part. An iron pier, 38
+feet high, beneath which lie the remains of Mr. Lick, supports the
+equatorial head, and a winding staircase enables the observer to reach
+the setting circles. Inside the hollow pier is the powerful driving
+clock which turns the telescope to follow the heavenly bodies in their
+apparent movements. Finders of 6, 4, and 3 inches diameter, rods for the
+manipulation of the instrument, and all necessary accessories, complete
+what must long remain one of the most perfect instruments at the service
+of astronomical science. The 200,000 dollars expended upon it have
+already been amply justified by the work accomplished, while Mr. Lick’s
+dream of immortality has become a reality.
+
+The following list indicates some of the large refractors now (Feb.,
+1897) doing active service:—
+
+                   _Aperture._     _Observatory._
+                   36 inch     [Lick] California.
+                   30  „       Pulkowa, Russia.
+                   30  „       [Bischoffeim] Nice.
+                   28  „       Greenwich.
+                   27  „       Vienna.
+                   26  „       Washington.
+                   25  „       [Newall] Cambridge.
+                   24  „       [Lowell] Mexico.
+                   23  „       Princeton, New Jersey.
+
+It is right to add, however, that opinion is still greatly divided as to
+whether these telescopes of large aperture really repay the expense and
+labour involved in their erection and use. On the very rare occasion
+when the “seeing” is practically perfect—which occurs perhaps only a few
+hours in a year—it is probable that the superiority of a large telescope
+is very marked, but under average conditions there seems to be little
+advantage over instruments of moderate size for many classes of
+observations.
+
+Certain it is that a great deal of valuable work is done with
+comparatively small telescopes, ranging from six to fifteen inches
+aperture, and this in all departments of astronomical research. Hence,
+some of the most active observatories do not figure in the above list;
+among them may be mentioned the observatories of Harvard College
+(U.S.A.), Potsdam, Paris, Heidelberg, Cape of Good Hope, Edinburgh,
+South Kensington, Stonyhurst College, and the observatory of Dr. Isaac
+Roberts at Crowborough, Sussex.
+
+HOUSING OF EQUATORIALS.—The building which accommodates an equatorial
+telescope must evidently be designed to admit of giving a clear opening
+to any part of the sky. Usually this is accomplished by making the roof,
+or _dome_, with a circular base, provided with wheels, which run on
+rails. It is then only necessary to open a narrow portion of the dome,
+extending from top to base, and to turn the dome until this aperture is
+in the required direction. One of the most elaborate domes now in
+existence is that built by M. Eiffel for the great refractor of the Nice
+Observatory. The lower part of the building is in the form of a square
+(see Frontispiece), having a side of about 87 feet, and a height of
+about 30 feet. The dome itself is 74 feet in diameter, and the moving
+parts alone weigh 95 tons.
+
+As will be seen from the illustration, there are two shutters, each a
+little wider than half the possible opening: these run on short rails,
+and are moved simultaneously by means of an endless rope. The whole of
+the dome is built up of steel angle iron, covered with very thin sheet
+steel. In order to facilitate the manipulation of the dome, its great
+weight is buoyed up by means of a float attached to its base and
+immersed in a circular tank of water of a little greater size than the
+base of the dome. If any mishap occurs with this gigantic tank, the dome
+rests on wheels which run on a circular rail, so that the work need not
+be interrupted. The whole arrangement is very easily turned with the aid
+of a winch by one man when the dome is floating, but when resting on the
+wheels several men are required at the winch.
+
+This brief description will serve to illustrate some of the problems
+which confront the possessor of a very large telescope. For smaller
+instruments, the observatories follow pretty nearly the same plan,
+except that it is unnecessary to provide an arrangement for floating the
+dome.
+
+The observatory which shelters a reflecting telescope need not differ
+very greatly from one which contains a refractor. If the instrument be a
+Newtonian, it is generally convenient to sink the polar axis below the
+level of the floor in order that the observer may not be at too great a
+height from the ground, and in that case, the dome, or its equivalent,
+is all that is necessary. For his five-foot reflector, Dr. Common
+designed an observatory which is not of the ordinary form, but gives the
+necessary opening partly by means of large shutters, and partly by a
+revolution of the whole house. It is not everyone who is able to lay out
+£8,000 on such a dome as that erected at Nice by M. Bischoffeim.
+
+The varying position of the eye end of a telescope, when it is turned to
+different parts of the sky, makes it necessary to provide comfortable
+and safe seating accommodation for the observer, more especially when
+the telescope is a very large one. In the case of the Yerkes telescope,
+the eye-piece will be 30 feet higher when observing near the horizon
+than when observing near the zenith, and the observer must necessarily
+follow the telescope. The most convenient arrangement in such a case is
+to raise or lower the floor of the observatory as occasion demands. The
+floor of the Yerkes Observatory is 75 feet in diameter, and by means of
+electric motors it can be given a vertical motion of 22 feet. A similar
+arrangement was provided for the Lick telescope from the designs of Sir
+Howard Grubb. With smaller instruments, observing ladders and adjustable
+chairs of various forms are employed.
+
+THE EQUATORIAL COUDÉ.—A form of equatorial telescope which has possibly
+a great future before it is one introduced at Paris under the name of
+the _equatorial coudé_, or elbowed telescope. Its practical advantage is
+that the observer remains in a constant and comfortable position, so
+that revolving domes and elevating floors, or other arrangements serving
+similar purposes, are no longer necessary. The telescope tube is of two
+parts of nearly equal length, and what is ordinarily the lower half of
+the tube forms part of the polar axis, while the other half is attached
+to it at right angles. At the point of intersection of the two halves of
+the tube is a plane mirror, and there is another mirror in front of the
+object-glass. If the latter mirror were removed, such a telescope would
+only enable the observer to see objects lying along the celestial
+equator, but by its means objects in all parts of the heavens can be
+brought within range to an observer gazing down the hollow polar axis.
+The largest instrument is that at the Paris Observatory, which has an
+object-glass 23½ inches in diameter for visual observations, and another
+of the same size for photographic purposes.
+
+FIXED TELESCOPES.—There is still another method of using a telescope.
+The telescope itself may be fixed, and the light of the heavenly bodies
+may be reflected into it by means of a mirror which is made to revolve
+so as to keep pace with their movements. Foucault devised an instrument
+called the _siderostat_ for this purpose, and although it is not largely
+employed for telescopic observations, it is very widely utilised for
+spectroscopic work, where the spectroscope is of a kind not readily
+attached to a telescope.
+
+Another instrument used for the same purpose has recently been brought
+forward under the name of the _coelostat_. This is simply a mirror which
+is made to turn on a polar axis in its own plane, and since a reflected
+ray of light moves through twice the angle that the reflecting surface
+turns through, the mirror is made to revolve at the rate of one
+revolution in two days. As the name indicates, the whole heavens appear
+stationary in such an instrument, whereas in a siderostat, only one star
+at a time appears at rest, while its neighbours slowly revolve round it.
+
+PHOTOGRAPHIC TELESCOPES.—The application of photography to the study of
+the heavenly bodies marks one of the greatest advances of the present
+century. The instruments which are employed for this purpose range from
+the ordinary tourist camera to the largest telescope. Unlike a person
+sitting for a portrait, the heavenly bodies cannot be made to stand
+still for the purpose, and as instantaneous photographs can only be
+obtained in the case of the sun and moon, it is usually necessary to
+make the camera follow the stars very exactly during the time of
+exposure, in order that the images may fall on precisely the same parts
+of the photographic plate.
+
+Some guiding arrangement is, therefore, essential, and generally the
+photographic camera or telescope is attached to an ordinary equatorial
+which is driven by clock-work, or very carefully by hand if the camera
+be a small one. In the guiding telescope are two spider threads at right
+angles to each other, and it is by constantly keeping the image of a
+star at the intersection of these “wires” that the operator ensures the
+images remaining in a constant position upon the sensitive plate.
+
+An ordinary portrait camera, in the hands of a skilled observer, yields
+very beautiful pictures, but they are naturally on a small scale. The
+field of view of such an instrument is so large that a whole
+constellation may be photographed with a single exposure.
+
+Portrait lenses of 6 inches aperture in the hands of Dr. Max Wolf and
+Professor Barnard have given magnificent delineations of the Milky Way,
+and of the extremely faint nebulosities which are to be found in many
+parts of the heavens.
+
+For many purposes, however, telescopes of greater power are required,
+and here it may be remarked that the distance between the images of any
+two adjacent stars will vary in direct proportion to the focal length of
+the telescope. In the same way the size of the image of a planet, the
+moon, or a comet, increases as the focal length of the objective is
+increased.
+
+Refracting telescopes which are employed for photography require
+object-glasses which are specially “corrected” for the photographic
+rays. White light is compounded of light of all colours, but it is the
+blue and violet constituents which are effective in producing
+photographic action on an ordinary sensitive plate. Now, an object-glass
+which is intended for visual purposes is made to focus at the same point
+as many as possible of the rays which are most effective to the human
+eye, that is the green, yellow, and red, and usually there is a blue or
+purple halo round the images of the brighter objects, which is, however,
+too feeble as a rule to interfere with visual observations. This blue
+halo, will evidently result in defective definition if the lens be
+employed for photography. By putting the plate at the point where the
+blue rays are most nearly focused, a better image is obtained; but for
+really good work a photographic object-glass must be so designed that
+all the blue and violet rays are brought to one and the same focus. Such
+a lens will consequently be a very poor one for visual observations. At
+the present time, 18 photographic telescopes, each of 13 inches
+aperture, and corrected in this way, are at work in various parts of the
+world for the international star chart.
+
+[Illustration:
+
+  FIG. 52.—_The Photographic Telescope employed by Dr. Isaac Roberts._
+]
+
+The new “photo telescopic” object-glass now manufactured by Messrs.
+Cooke appears to be full of promise. In this lens all the colours of the
+spectrum are brought to almost exactly the same focal point, so that it
+serves equally well for photographic or visual purposes.
+
+This difficulty in regard to achromatism does not exist in the case of
+the reflecting telescope, since rays of light of every colour are
+reflected at precisely the same angles. For this reason reflectors, when
+properly managed, give the best photographic results. Dr. Isaac Roberts
+and Dr. Common are especially identified with the application of the
+reflecting telescope for celestial photography. The instrument employed
+by the former consists of a 20-inch reflector, and a 7-inch guiding
+telescope of the refracting form. The two telescopes are mounted on the
+extreme ends of the declination axis of an equatorial, a photograph of
+which we owe to the kindness of Dr. Roberts.
+
+Dr. Common does not employ a guiding telescope at all. The photographic
+plate which he places at the focus of the reflector is smaller than the
+field of view, so that by means of an eye-piece fitted with a cross wire
+at the side of the dark slide, he is able to watch a star near the edge
+of the field. Both eye-piece and dark slide are attached to a frame
+which can be controlled by two screws at right angles to each other. If
+the guiding star leaves the cross wire through errors in driving, or
+other causes, the eye-piece and dark slide are bodily moved after it by
+means of the adjusting screws. This method not only has the advantage of
+saving the cost of a guiding telescope, but reduces the effects of
+vibration consequent upon the correction of errors by moving the whole
+telescope.
+
+For photographing the sun a special instrument called a
+_photoheliograph_ is usually employed. This differs only from an
+ordinary photographic telescope in being provided with a secondary
+magnifier, by which means the focal image formed by the object-glass is
+amplified before falling upon the photographic plate. On a bright clear
+day, pictures of the sun 8 inches in diameter can be taken with an
+exposure of about ¹⁄₅₀₀th of a second, and such a photograph will
+frequently record more facts as to the state of the solar surface than a
+whole day’s observation. Lenses or mirrors of very long focus are also
+occasionally employed in solar photography, and in this way a large
+image is obtained without the use of a secondary magnifier.
+
+Photographs of the moon and planets may be taken either with or without
+a secondary magnifier, but in either case the exposures are longer than
+for the sun.
+
+Finally, it may be added that the sensitive plates and processes used in
+astronomical photography do not differ from those employed by ordinary
+photographers.
+
+
+
+
+                              CHAPTER XVI.
+                       INSTRUMENTS OF PRECISION.
+
+
+THE MERIDIAN CIRCLE.—The accurate registration of the positions of the
+heavenly bodies is one of the most important functions of an
+astronomical observatory. When the apparent places of an object at a
+sufficient number of different times have been duly recorded, it becomes
+possible to investigate the laws upon which its changes of position
+depend, and to predict its positions at subsequent times for the benefit
+of navigators and others to whom such predictions are of practical
+utility. For this purpose various instruments have been devised, but in
+all cases where it can be employed, the _transit circle_, or _meridian
+circle_, as it is indifferently called, is generally conceded to give
+the most trustworthy results.
+
+With this instrument the observations are made when the celestial body
+under observation is crossing the meridian of the place where the
+instrument is set up, that is, when it “transits,” or “souths.” At this
+time the accuracy of the observations is least impaired by the
+ever-varying effects of atmospheric refraction.
+
+The meridian circle consists of a refracting telescope—seldom exceeding
+6 inches in aperture—which is fixed to a hollow axis at right angles to
+itself, and this axis is supported horizontally in an east and west
+direction, so that the telescope is only free to move in the plane of
+the meridian. A large graduated circle—or frequently two such
+circles—attached perpendicularly to the hollow axis, and read by
+microscopes fixed to the walls or iron pillars which support the axis,
+completes the essential parts of the instrument.
+
+As the field of view of the telescope covers a considerable area, it
+becomes necessary to provide some means of marking the exact point
+within it which represents the meridional axis of the instrument. This
+is accomplished by placing at the common focus of the object-glass and
+the positive eye-piece a system of “cross wires,” consisting of
+tightly-stretched spider threads, two of which are fixed horizontally
+and nearly in contact, and five or seven vertically at equal distances
+apart. What the observer has actually to do is to incline the telescope
+at such an angle that the star is seen to traverse the space between the
+two horizontal threads, and then to record the exact times, by means of
+a chronograph and sidereal clock, at which the star appears to cross
+each of the equidistant vertical threads. By thus making five or seven
+observations and taking the average, greater accuracy is attained.
+
+The time observations, as we have already seen, determine the right
+ascension of the star under observation, while the declination is
+indicated by the readings of the graduated circle, if the latter is so
+placed as to read 90° when the telescope is directed to the Pole.
+
+The ideal meridian circle is thus simplicity itself, but the mechanical
+difficulties encountered in making such an instrument are insuperable.
+Perfect right angles and perfect circles exist only in our minds, so
+that after all the undoubted skill and care bestowed on its
+construction, the actual meridian circle is only an approximation to the
+ideal. Still, when the instrument is provided with levels and other
+means for estimating its deviation from the meridian plane in which it
+ought to move, the actual observations are capable of correction by
+mathematical processes, so that the final statements of positions
+sensibly represent those which would follow from the use of a perfect
+instrument.
+
+The greatest possible care is taken to secure rigidity in all parts of
+the meridian circle. The hollow horizontal axis is supported on bearings
+which rest either on heavy piers of iron or walls of masonry, and the
+axis and telescope tube are firmly joined together at their
+intersection. The bearings for the axis are turned with extreme
+precision, and, to reduce the friction upon them, the pressure of the
+instrument is counterpoised by an arrangement of balancing weights.
+
+Adjustments are provided for every needful purpose. The cross wires are
+fitted in a small frame which can by suitable fittings be given a small
+movement in the field of view until the right place for them is found,
+while the horizontality of the axis and its correct direction can be
+secured by other adjusting screws.
+
+Since most of the observations have to be made at night, the field of
+view will generally be dark, and the exceedingly delicate spider lines
+will be invisible unless some means of illuminating them be provided.
+Usually a very tiny mirror is fixed diagonally at the intersection of
+the axis and the telescope, where it is held in position by a stiff
+wire. A light shining through the hollow axis is thus reflected into the
+field of view, and the threads are rendered visible. The intensity of
+this illumination of the field can be regulated in accordance with the
+brightness of the star under observation.
+
+The instrument having been erected, one of the first tests applied to it
+is to see that it is correctly _collimated_, or, in other words, that
+the optical axis of the telescope is perpendicular to the axis of
+movement. For this purpose the telescope is directed to some distant
+object, such as a building, and some mark which falls on the
+intersection of the central spider threads is noted. The axis is then
+reversed end for end by a mechanical arrangement, and the telescope
+again pointed at the same object. If the mark again falls on the
+intersection of the cross wires, the collimation is correct; if not, the
+wires are moved with the frame containing them until the error is
+corrected.
+
+To test the horizontality of the axis, a spirit-level long enough to
+stretch across the bearings, and called the “striding level,” is
+provided.
+
+Various methods are employed for adjusting the instrument so that the
+telescope moves as truly as possible in the plane of the meridian.
+Collimation and level being correct, the telescope will move in a
+vertical plane, whatever may be the error in the direction of the
+horizontal axis, and therefore any star passing through the zenith will
+cross the centre of the instrument at the same moment that it crosses
+the meridian. A star away from the zenith, however, will not be seen on
+the cross wires when it crosses the meridian, unless the axis be truly
+east and west. Hence, by taking the difference of time between the
+observed transits of a star near the zenith and one a long way from the
+zenith, and turning the whole instrument in azimuth until this
+difference is equal to the difference of right ascensions of the two
+stars, the instrument is readily placed in the meridian.
+
+Another useful method of adjustment is to observe the upper and lower
+transits of a circumpolar star. If the instrument moves truly in the
+meridian, the interval between the two transits will evidently be twelve
+sidereal hours.
+
+Next, the declination circle has to be adjusted so that it reads 90°
+when the telescope is directed to the celestial pole, or zero when an
+equatorial star is under observation. An obvious way of doing this is to
+take the readings when Polaris, or other circumpolar star, is at upper
+and lower transits; the celestial pole lying midway between these
+positions, the average of the two readings, when corrected for
+refraction, should be 90°, and the circle would be shifted round in its
+fittings until this was the case.
+
+Such, in mere outline, are the processes by which the meridian circle is
+set up. In actual practice, the greatest possible refinement is brought
+to bear on the adjustments, and every precaution taken to estimate the
+various errors so that due allowance may be made for them in the
+reduction of the observations. It has even been shown that the heat of
+the observer’s body, by affecting the lower side of the telescope tube
+more than the upper, introduces sensible errors in the measures of
+declination. Hence it is important to use metals of high conductivity in
+the construction of meridian instruments, so that errors due to the
+varying temperatures of different parts may be reduced to a minimum.
+
+As an illustration of a modern meridian circle, we select that of the
+Lick Observatory. (Fig. 53.) This instrument has an aperture of six
+inches, and embodies all the improvements which have been introduced by
+the Berlin firm of Repsöld & Co.
+
+The observatory containing a meridian circle is usually a very simple
+structure, as it is only necessary to provide an opening to the sky
+along a north and south line. This is sufficiently provided for by a
+series of narrow shutters in a building of ordinary construction.
+
+To prevent confusion it may be pointed out that the term “transit
+instrument” is frequently restricted to a meridian instrument which is
+not supplied with large circles for the accurate measurement of
+declinations, although it may have a small circle to assist in directing
+the telescope. The use of such an instrument is evidently limited to the
+determination of time and right ascension.
+
+[Illustration:
+
+  FIG. 53.—_The Meridian Circle of the Paris Observatory._
+]
+
+THE ALTAZIMUTH.—Although the meridian circle furnishes us with the most
+accurate method of determining celestial positions, its use is somewhat
+restricted by the fact that it can only be employed for the observation
+of objects on the meridian. It sometimes happens, however, that bodies
+cannot conveniently be so observed, and other methods become necessary.
+This is especially the case with the moon during the first and fourth
+quarters, when it crosses the meridian in daylight, and it is then that
+an instrument called the _altazimuth_ is of special value. This is
+something like a transit circle in which the base supporting the piers
+is made to turn on a vertical axis, so that the telescope can be
+directed to any part of the heavens whatsoever. A fixed horizontal
+graduated circle, read by verniers or microscopes attached to the
+revolving part, gives the azimuth of the telescope when an observation
+is made, and the altitude is furnished by the vertical circles. The
+azimuth circle is adjusted to read zero when the telescope is pointed
+due north, and the altitude circle to zero when the telescope is
+horizontal. To secure the first adjustment, after correcting level and
+collimation, a star may be observed before it crosses the meridian, and
+again when it has exactly the same altitude after passing to the west;
+midway between the two positions would be due south, and the circle
+should read 180°. In adjusting the vertical circle, the telescope is
+made to point downwards to a trough of mercury, and it is known that the
+telescope is truly vertical when the reflected image of the cross wires
+is coincident with the wires themselves; the circle should then read
+90°.
+
+From a knowledge of the sidereal time at which a celestial body has an
+observed altitude and azimuth, the more useful co-ordinates of right
+ascension and declination can be calculated by spherical trigonometry.
+
+One of the largest instruments of this class has recently been erected
+at Greenwich Observatory. The aperture of the telescope is 6 inches, and
+the rigidity of the various parts may be gathered from the fact that the
+instrument weighs something like six tons.
+
+A _theodolite_ is a small portable form of altazimuth specially adapted
+for the needs of surveyors, but occasionally employed in astronomical
+work.
+
+THE WIRE MICROMETER.—Notwithstanding that an equatorial telescope is
+usually furnished with circles for estimating the positions of objects
+observed, or to serve as a guide in directing the telescope to objects
+of known position, it is not entitled to be called an instrument of
+precision in the sense we are now considering. The provision for driving
+by clock-work and other causes are antagonistic to constancy of
+adjustment, and hence determinations of positions by the circles alone
+might be many seconds in error. Most large telescopes, however, are
+provided with some form of micrometer which not only serves for the
+measurement of planets, lunar craters, and the like, but may also be
+used to measure the angular separation of adjacent stars. In this way,
+by making a “triangulation” of stars visible in the field of view, and
+including at least two which have had their precise positions determined
+by the meridian circle, the positions of objects can be measured with
+great accuracy.
+
+This method is especially valuable in the case of comets, which may
+cross the meridian in daylight, and are often too dim to be seen with
+the altazimuth.
+
+Several forms of micrometers are in use, but the so-called _wire_ or
+_filar micrometer_ is most commonly seen in our observatories. The
+essential parts are very similar to those of the reading microscope (p.
+172). Two parallel spider threads are so arranged on sliding frames that
+they may be brought into coincidence, or separated, by means of very
+finely-cut screws. Perpendicular to these are two fixed threads almost
+close together. The system of “wires” is viewed by a positive eye-piece,
+and the whole is attached to a draw tube so that it may be placed in
+position at the eye end of the telescope. In order that the wires and
+telescopic images may be sharply defined at the same time, the plane of
+the wires must be at the principal focus of the object-glass. The screws
+are provided with large heads which are graduated so as to show the
+hundredth of a revolution, and counting wheels register the numbers of
+complete turns.
+
+Matters are so arranged that when both counting wheels indicate zero,
+the spider threads are coincident. Then, supposing one of the screws be
+turned through a revolution, the threads will be separated by a definite
+amount; an equal and opposite movement of the other screw will double
+the separation, and in all cases the distance between the threads will
+be registered in turns, and fractions of turns of the screws.
+
+The next proceeding is to ascertain what is called the “value,” in
+angular measure, of the micrometer screw. This value will evidently
+depend upon the pitch of the screw and the focal length of the telescope
+to which the micrometer is applied, so that measurements merely stated
+in terms of revolutions of the screw would serve no useful purpose. It
+can easily be calculated that the images of two stars which are 28′ 39″
+apart will be separated by an inch at the focus of a telescope of 10
+feet focal length; then, if the screws have 100 threads to the inch, the
+angular separation of the wires corresponding to a single revolution
+will be one-hundredth part of 28′ 39″, that is, 17″·15, and the latter
+would be the value of that particular micrometer when used with the
+telescope in question. If the focal length of the telescopic
+object-glass were 20 feet, the linear separation of the images of two
+such stars as we have considered would be 2 inches, and the value would
+therefore be halved, so that measures of twice the accuracy would be
+possible. Since the stellar images and the cross wires are equally
+magnified by the eye-piece, the value of the screw is in no way affected
+by using eye-pieces of different powers.
+
+In practice it is necessary to determine the value of the micrometer
+screw by actual measurement. For this purpose, the wires are separated
+by a known number of revolutions, say twenty, and the micrometer is
+adjusted so that a star of known declination travels exactly between the
+two fixed wires when the telescope remains at rest. With the telescope
+still fixed, the number of seconds required by the image of the star to
+traverse the distance between the separated wires is noted, and knowing
+the angle through which the star must have moved in that interval, the
+angular value of one turn of the screw is at once deduced. For work of
+extreme precision each individual turn of the screw must be separately
+evaluated, and allowances must also be made for changes of temperature.
+
+When measuring the apparent diameter of a planet, the two threads are
+separated until the image just lies between them, and the sum of the
+readings of the two screws multiplied by the angular value of one turn
+gives the diameter in seconds of arc. The distance having been formed by
+other observations, the diameter of the planet in miles can be
+determined in the manner to which reference has already been made (p.
+142).
+
+[Illustration:
+
+  FIG. 54.—_The Micrometer applied to a Binary Star: a b, Fixed Threads;
+    c d, e f, Movable Threads; s s, Components of Binary Star._
+]
+
+One of the most important applications of the micrometer is in the
+measurement of double and binary stars. In this case the fixed threads
+are made to enclose the two stars, and the movable threads are made to
+bisect the star-images. (Fig. 54.)
+
+THE POSITION CIRCLE.—It is frequently necessary to be able to specify a
+direction, as in the case of a planet’s equator, or the line joining the
+components of a double star. Such directions are expressed by “position
+angle,” which may be defined as the angle from the north point, reckoned
+from 0° to 360° through east, south, and west. For these observations, a
+_position circle_ is usually attached to the micrometer. This is a
+circle graduated from 0° to 360°, which can remain fixed in position as
+regards the telescope, while the part containing the wires and
+micrometer screws can be rotated by means of a rack and pinion. A
+vernier attached to the movable frame indicates the required angles.
+
+To adjust the position circle the vernier is set to zero, and the
+telescope directed to a star; the circle and micrometer are then
+together turned round until the diurnal movement of the star, which is
+east and west, makes its image to traverse the space between the fixed
+wires. The movable threads will then lie in a north and south direction.
+The circle remains in this position during subsequent observations,
+while the micrometer is rotated until the movable threads are in the
+required direction, the position angle then being read off on the
+circle.
+
+THE HELIOMETER.—Another means of measuring small angles for astronomical
+purposes is afforded by the instrument called the _heliometer_, which,
+as the name will at once suggest, was invented for measurements of the
+sun. This instrument is a telescope mounted equatorially, but differs
+from the ordinary telescope, inasmuch as the object-glass is cut across
+the centre, and means are provided for separating the two halves by
+moving one or both parts in the direction of the line of bisection, and
+also for measuring the amount of displacement. The cell containing this
+somewhat peculiar object-glass can be rotated so that the line of
+division of the lens may be placed in the same direction as the line
+representing the distance to be measured.
+
+The action of the instrument depends upon the fact that any small part
+of a lens is competent to form a complete image of a celestial body, so
+that when an object-glass is bisected, and the two halves separated
+laterally, two distinct images will be produced, each differing only
+from the image formed by the complete lens in being less bright.
+
+To measure the distance from a star to a planet, let us say, as in
+observations of the parallax of Mars, the lenses are separated to such
+an extent that the image of the star formed by one half, coincides with
+that of the planet formed by the other half, and the amount of
+separation noted. As a check, the measurement is repeated with the
+lenses separated in the opposite direction. The angular value
+corresponding to a known separation of the semi-lenses being determined,
+just as in the case of the micrometer screw, the angle between star and
+planet at once follows. Angles ranging from a few minutes to about two
+degrees can be measured in this way with great accuracy.
+
+In the hands of Dr. Gill, of the Cape Observatory, the heliometer has
+yielded very valuable results in connection with the distances of the
+sun and stars.
+
+OTHER INSTRUMENTS.—There are other instruments which may fairly be
+classed as instruments of precision, but space permits little more than
+a mention of their names.
+
+The _zenith telescope_ is a telescope specially designed for the
+measurement of the angular distances of stars from the zenith, for
+precise determinations of latitude by Talcott’s method.
+
+The _prime vertical instrument_ is nothing more than a transit
+instrument, so arranged that the observing telescope swings in a
+vertical plane which is perpendicular to the plane of the meridian. From
+the observed times at which a star passes the prime vertical on the
+eastern and western sides, the latitude of the place of observation can
+be ascertained with great accuracy.
+
+It is perhaps at sea that the labours of astronomers are of most direct
+value in everyday affairs, and it is precisely here that the instruments
+of high precision cannot be employed, in consequence of the absence of
+firm supports. Nevertheless, there is one instrument—_the sextant_—which
+yields results that satisfy all requirements when carefully constructed
+and placed in good hands. A graduated arc extending over about 60° (from
+which the name is derived) is supported by a light framework, and
+pivoted truly on the centre of the arc is the radius bar, or index arm,
+which carries a vernier for reading off the angles to be measured. A
+plane mirror is fixed to the index arm, over the centre of movement, and
+another, of which only half is silvered, is fixed to the frame near its
+outer edge. A small telescope parallel to the surface of the frame is
+directed towards the fixed mirror, so that the continuation of its axis
+is in line with the boundary between the silvered and clear part of the
+glass. Thus, while one object may be seen by direct observation through
+the clear glass, another, in quite a different direction, may be seen
+after reflection from the surfaces of the two mirrors.
+
+The sextant is chiefly used for measuring the altitude of the sun, about
+noon for the determination of latitude, and in the morning or evening
+for the correction of chronometers. In such observations, the sextant is
+held in the right hand, with its plane vertical, and the sea horizon is
+sighted directly with the telescope; the index arm is then moved until
+the reflected image of the sun is brought into coincidence with the
+horizon. The reading is then taken, and if the adjustment is such that
+zero is indicated when the reflected and direct images of the same
+object are observed, it will give the altitude. The actual angle
+recorded by the sextant is only half that between the objects observed,
+but by numbering half degrees as whole ones, the true angles are read
+off directly. For observations of the sun the instrument is provided
+with coloured glasses of different shades, attached so that they can
+readily be interposed to reduce the intensity of the light.
+
+
+
+
+                             CHAPTER XVII.
+                       ASTROPHYSICAL INSTRUMENTS.
+
+
+So far we have been concerned with instruments which enable us to
+ascertain the positions, dimensions, and appearances of the various
+orders of heavenly bodies; but we can go further than this, and learn
+something of the physical and chemical constitutions of the glittering
+orbs by which we are surrounded. We can, for instance, bring
+instrumental aid to bear upon the determination of the brightnesses of
+the heavenly bodies, and by means of that powerful appliance of modern
+astronomy—the spectroscope—we can study the chemistry of all those
+bodies which shine by light of their own, and which are not so feebly
+luminous as to be out of our range.
+
+PHOTOMETRY.—The naked eye was alone employed in observations of stellar
+brightness until quite recently. Each step in the advance of
+astronomical research, as in most other branches of science, however,
+depends upon the greater precision of observation which can be
+introduced, and so we now find the eye to be assisted in these inquiries
+by a _photometer_ of some kind or other. The general purpose of
+photometry will be familiar to all in connection with such practical
+matters as the determination of the illuminating power of coal gas. The
+methods here employed, however, are not directly applicable to the
+comparatively feeble light-sources which have usually to be dealt with
+in astronomical photometry.
+
+As will be more fully explained in another part of this work, the stars
+visible to the naked eye are divided into six grades of magnitude. The
+brightest of them are classed as first magnitude, while those only just
+visible to the naked eye are of the sixth magnitude. Now that telescopes
+are used, this division of stars into magnitudes must be continued in
+some form or other, so as to include telescopic stars. From photometric
+comparisons it has been ascertained that the average star of the first
+magnitude may conveniently be reckoned 100 times as bright as a sixth
+magnitude star. Hence, the light-ratio corresponding to a difference of
+a single magnitude is 2·5. Thus, a star which is 2½ times less bright
+than one of the sixth magnitude ranks as seventh magnitude, and so on.
+Fractions of magnitudes are also necessary to express the results which
+can now be obtained.
+
+LIMITING APERTURES.—For the reason that a large telescope enables us to
+see stars which are too dim to be visible in a smaller one, the
+brightnesses of stars may be compared with more or less satisfactory
+results by reducing the aperture of a telescope until the star in
+question ceases to be visible. This is called the method of _limiting
+apertures_, and in practice a telescope intended for this work would be
+provided with a series of diaphragms, or other arrangement for
+conveniently reducing the effective area of its object-glass. A
+telescope which has an object-glass 10 inches in diameter should just
+show stars of the fourteenth magnitude under favourable conditions; a
+star which could just be seen when this aperture was reduced to an inch
+would be of the ninth magnitude, and so on.
+
+There are numerous reasons why this method fails to give satisfactory
+results, but one of the most important is that the image of a star
+becomes more diffuse with each reduction in the aperture of the
+telescope. At best it must evidently fail for a comparison of stars
+which are visible to the naked eye.
+
+WEDGE PHOTOMETER.—One of the simplest and best methods of estimating
+star magnitudes is afforded by the _wedge photometer_. This is a strip
+of neutral-tinted glass about six inches in length, and a quarter to
+half an inch deep, tapering from one end to the other, so as to present
+a gradual reduction in depth of tint from the thick to the thin end. A
+similar wedge of clear glass, tapering the opposite way, is cemented to
+this, in order to get rid of prismatic action. Compensated in this way,
+and mounted in a suitable frame, the wedge is placed in front of the
+eye-piece of a telescope, and is pushed along until the star under
+examination is just extinguished. A scale is then read off, and from the
+results of a previous evaluation of the wedge in the laboratory, the
+corresponding star magnitude is easily deduced.
+
+In order to eliminate the effects of differences in the state of the
+sky, the position of the wedge at which a standard star, such as
+Polaris, ceases to be visible, is determined, and then it is the
+difference of wedge readings upon which the final calculation is based.
+
+The great value of the wedge in stellar photometry was demonstrated by
+the labours of the late Prof. Pritchard, to whom we owe the catalogue of
+the magnitudes of naked eye stars in the northern hemisphere known to
+the astronomical world as the “Uranometria Nova Oxoniensis.”
+
+OTHER PHOTOMETERS.—Some photometers depend for their action upon
+comparisons with terrestrial sources of light. In some cases, an
+artificial star, consisting of a pinhole illuminated by a standard lamp,
+is brought into the same field of view as the star to be compared, and
+then, by polarising apparatus, the brightnesses of the two images are
+equalised. The amount of reduction of either of the stars is determined
+by a scale which measures the rotation of the polariscope, and in this
+way all the stars are compared with an artificial star of known
+brightness.
+
+One of the most notable achievements in this field of astronomical work
+is that of Professor Pickering of the Harvard College Observatory, who
+invented and made splendid use of the so-called _meridian photometer_.
+Here the telescope has two object-glasses of equal aperture side by
+side, and in front of each is a silvered flat mirror inclined at an
+angle of 45° to the optic axes. The telescope is supported in an east
+and west direction, so that one mirror reflects the Pole Star into its
+object-glass, while the other can be rotated so as to reflect any other
+star which is on the meridian into the second object-glass. Again, by a
+polariscope at the eye end of the telescope the images of the two stars
+are made of equal brightness, and the readings give the data for
+calculating the required magnitude.
+
+Photographs of the stars are also largely employed for the estimation of
+magnitudes, stars of different magnitudes being represented on the
+photographs by spurious discs of different sizes. If all stars gave out
+light of the same quality, the photographic method would be very
+trustworthy, but as the colours of the stars vary, the photographic and
+visual magnitudes are not invariably in agreement A bright, reddish
+star, such as Betelgeuse, would photographically be only equivalent to a
+white star which was much less bright to the naked eye.
+
+THE PRISMATIC SPECTROSCOPE.—Reference has already been made in these
+pages to the wonderful field of astronomical research which has been
+opened up by the discovery of the action of a triangular glass prism
+upon rays of light, and the subsequent improvements in the method of
+utilising this effect.
+
+A prismatic spectroscope may be regarded as an arrangement which will
+enable us to get a pure spectrum, and to observe it to the best
+advantage. The light to be analysed is admitted through a narrow
+aperture called the _slit_, which is placed at the focus of a double
+convex lens. Emerging from this _collimator_, as a parallel beam, the
+rays pass through the prism, and after deviation and dispersion they
+fall upon another double convex lens, which brings them to a focus in
+the form of a spectrum. An eye-piece may then be employed to view the
+spectrum, or a sensitive plate may be placed at the focus to photograph
+it.
+
+In a simple form of spectroscope the prism is supported at the centre of
+a graduated circular plate, to which the collimator is firmly fixed,
+while the observing telescope is attached to an arm pivoted at the
+centre of the plate. A vernier moving with the telescope indicates the
+position, on a scale of degrees, of any colour brought to the centre of
+the field of view.
+
+The best results are obtained when the rays of light emerge from the
+prism at the same angle at which they enter it, in which case the prism
+is said to be at _minimum deviation_, for the reason that the deflection
+of the rays from their original path is then the least possible. As
+lights of different colours are refracted unequally, it is clear that
+the prism can only be at minimum deviation for rays of one particular
+colour at any instant. Frequently, however, there is an automatic
+arrangement by which, as the observing telescope is moved so as to bring
+different colours into the field of view, the prism is turned so as to
+be at minimum deviation for the colour actually under observation.
+
+The appearances observed in the spectroscope are a series of images of
+the aperture through which the light is admitted. If the source of light
+be yellow, such as that of a spirit lamp flame when common salt is
+introduced, a yellow image of the aperture will be seen, and so on for
+other monochromatic radiations. When a white light is observed, images
+of every gradation of colour are formed, and in such a “continuous
+spectrum” the separate images cannot be recognised. The form of aperture
+most widely adopted is a narrow straight slit with parallel sides. In
+this case there is the least possible confusion, because the several
+images of the slit appear as so many spectrum “lines.”
+
+For observations of the sun, where the light is so intense, a great
+number of prisms, each drawing out the spectrum into a longer band, may
+be employed, so that the lines of the spectrum may be widely separated,
+and the peculiarities of each more closely investigated. For the fainter
+bodies, however, the instrument must generally be one of comparatively
+small dispersion, so that the light may not be spread out into
+invisibility. It will be evident that the longer the spectrum the
+greater will be the chances of accurate measurements.
+
+Another way of obtaining great dispersion is to use prisms of the new
+dense Jena glass, one of which is equal to three or four of the flint
+glass prisms in general use.
+
+There are various forms of the prismatic spectroscope. In some of them
+reflecting prisms are introduced to turn the rays back through the
+dispersive train, so as to get increased dispersion without increasing
+the number of prisms. In the so-called _direct vision spectroscope_,
+prisms of different kinds of glass are combined so that the rays of
+light leave them in nearly the same direction that they enter. Here the
+collimator and observing telescope are in the same straight line, and
+this is a great convenience in certain classes of observation.
+
+THE GRATING SPECTROSCOPE.—Sometimes, especially in instruments designed
+for solar observations, the prisms are replaced by what is called a
+diffraction grating. Usually this consists of a piece of highly polished
+speculum metal, upon which is ruled a great number of equidistant
+parallel scratches or lines. A portion of the light falling upon the
+grating is simply reflected, while the remainder is spread out into two
+series of beautiful spectra, one on each side of the directly reflected
+beam. The two nearest to the directly reflected beam are called spectra
+of the first order, while following these are spectra of the second,
+third, and fourth orders; the length of spectrum increasing in each
+case, and all being available for observation if the light dealt with be
+sufficiently bright. The production of these spectra is due to the
+interference of light waves.
+
+All gratings produce exactly similar spectra, so that the distances
+between identical lines as seen with one grating are always strictly
+proportional to their distances as seen with any other. With prisms, the
+relative separation of colours is by no means constant; a prism made of
+one kind of glass may, for example, separate the green and yellow more
+than another prism made from different material, while the separation of
+yellow and red might be the same in both cases. The grating spectrum
+accordingly affords a constant standard of reference, and what is called
+the “normal solar spectrum” is the spectrum of the sun mapped with the
+various dark lines in the relative positions shown by a grating
+spectroscope.
+
+Prof. Rowland, of John Hopkins University, has introduced a form of
+grating spectroscope, in which the grating is ruled on a concave
+spherical surface of speculum metal. After passing through the slit the
+rays of light fall directly upon this concave surface, and are brought
+to a focus after reflection, so that no lens except the eye-piece used
+for visual observations is required. Several of these gratings, having
+mostly a radius of curvature of about 21 feet, and a ruled surface of
+about 5½ inches x 2 inches, with 20,000 lines to the inch, are in use at
+the present time. Some idea of the difficulties to be faced in making
+these magnificent aids to research maybe gathered from the following
+remarks of Mr. J. S. Ames:—“It takes months to make a perfect screw for
+the ruling engine, but a year may easily be spent in search of a
+suitable diamond point.... When all goes well it takes five days and
+nights to rule a 6 inch grating having 20,000 lines to the inch.
+Comparatively no difficulty is found in ruling 14,000 lines to the
+inch.”
+
+With the aid of these wonderful gratings, the solar spectrum can be
+photographed with perfect definition, and extending over a total length
+of several yards. Thousands of the tell-tale Fraunhofer lines are
+rendered visible in this way.
+
+MEASUREMENT OF SPECTRA.—The spectra of many substances, including
+hydrogen and iron, are so characteristic as to be recognisable at a
+glance by an experienced observer, but one must as a rule resort to
+measurement for the identification of lines, or for the purpose of
+locating unknown lines for future reference. One of the simplest methods
+of measurement is that of reading the position of the observing
+telescope upon a graduated circle, when the line is seen at the centre
+of the field. If supplemented by a micrometer eye-piece, for
+differential measures with regard to known spectra, this method is
+extremely convenient. As recorded on arbitrary scales of this character,
+the position of the same line would be represented by a number which
+would be different for every instrument, and it is therefore necessary
+to reduce all measurements to a common scale; that now universally
+adopted is the natural one of wave-lengths. The position of a line in
+the spectrum depends upon the length of the waves constituting the rays
+of light which produce it, so that a measure of wave-length completely
+specifies the situation of a line whatever spectroscope maybe employed.
+Light waves are excessively minute, but by the use of the diffraction
+grating they can be measured with great accuracy. So small are they,
+that the most convenient unit of wave-length is the ten-millionth part
+of a millimetre[5]—or tenth metre, as it is technically named. Expressed
+in this way, the wave-length of the glorious red line seen in the
+spectrum of hydrogen is 6563·07, while that of the blue line
+characteristic of the same gas is 4861·51.
+
+When the positions of a certain number of lines of known wave-length
+have been read off on the scale of any spectroscope, the required
+wave-lengths of other lines are ascertained by a graphical
+interpolation, or by calculation. Elaborate tables of the wave-lengths
+of the lines in the spectra of the sun and chemical elements have been
+prepared by various investigators, and these are in constant demand by
+all workers in the field of astrophysics.
+
+THE TELESPECTROSCOPE.—For the examination of the spectra of the heavenly
+bodies, a spectroscope is attached to the eye end of a telescope from
+which the eye-piece has been removed, such a combination forming a
+_telespectroscope_. The slit is placed at the principal focus of the
+object-glass of the main telescope, and an image of the object to be
+observed is thus produced upon it. If the sun be under observation, any
+special part of it, such as a sun-spot or the chromosphere, may be
+separately investigated by bringing the corresponding part of the image
+upon the slit.
+
+In the case of the sun, moon, comets, planets, or nebulæ, the image is
+one of sensible size and the spectrum lines have a perceptible length.
+With a star, however, the image is only an illuminated dot upon the
+slit, and the spectrum would have no appreciable breadth, so that all
+but the strongest lines would in general fail to show themselves.
+Accordingly, when observing star spectra, a cylindrical lens is placed
+in front of the slit, so that the stellar image is drawn out into a
+bright line, and the necessary breadth of spectrum and length of the
+spectrum lines are secured.
+
+For photographing the spectra of the heavenly bodies it is simply
+necessary to replace the eye-piece by a small camera, and to expose a
+sensitive plate for a length of time dependent on the brightness of the
+spectrum. The spectrum of a terrestrial substance, such as hydrogen or
+iron, photographed in juxtaposition, is always a great convenience, and
+is essential for the investigation of stellar movements by the
+displacement of spectrum lines.
+
+THE LICK STAR SPECTROSCOPE.—Among the most complete and perfect
+spectroscopes adapted for use with the telescope is that designed by
+Prof. Keeler for the great refractor of the Lick Observatory. It is
+illustrated in Fig. 55, and it will be at once evident that there are
+ample means for keeping the instrument under control. Towards the upper
+part of the diagram, on the left, is the eye end of the telescope,
+without the eye-piece. Two stout brass rods 3 inches in diameter and 6
+feet long are attached by clamps to a revolving jacket which surrounds
+the end of the telescope tube, and on these the spectroscope is
+supported by clamps which allow of it being moved inwards or outwards
+from the focus of the telescope. The collimator of the spectroscope lies
+midway between the rods, and in order to facilitate the focussing of the
+image upon the slit, it has a small longitudinal movement independently
+of that of the whole spectroscope. The observing telescope is seen on
+the left of the diagram, while the grating rests on the circular
+graduated plate over which the observing telescope can be moved. The
+grating has 14,438 lines to the inch.
+
+Three prisms can also be used with the spectroscope, two of them being
+single prisms of 30° and 60° refracting angles respectively, and the
+third a compound prism giving a very high dispersion. Two observing
+telescopes are provided, one being of extra power for use with the
+grating in solar spectroscopy
+
+The instrument is generously supplied with the small refinements which
+contribute so largely to easy and successful manipulation. Among these
+are a diagonal eye-piece for viewing the image of the object on the slit
+plate, electrical illumination of the graduated scale and wires of the
+micrometer eye-piece, and an automatic arrangement for keeping the
+prisms at minimum deviation.
+
+There is a small totally-reflecting prism covering half of the slit, by
+which the light from an electric spark, or other source of luminosity,
+can be made to pass through the spectroscope so as to produce a series
+of known reference lines which serve as so many mile-posts for the
+measurement of the spectrum of the celestial body under observation. The
+induction coil, seen to the right of the diagram, is for the purpose of
+producing these electrical sparks.
+
+In mounting the spectroscope, which weighs no less than 200 pounds, the
+eye end of the great telescope tube is first supported by a prop, and
+the long rods are inserted. The spectroscope is then placed on the rods,
+and balancing weights equivalent to the weight of the spectroscope are
+removed from the lower part of the telescope tube.
+
+[Illustration:
+
+  FIG. 55.—_The Spectroscope adapted to the Eye End of the Lick
+    Telescope._
+]
+
+THE OBJECTIVE PRISM.—It is a very remarkable fact that many of the
+recent advances in our knowledge of the spectra of stars have followed
+from the revival of a method first employed by Fraunhofer in 1814, in
+which the slit and collimating lens, forming part of an ordinary
+spectroscope, are dispensed with. The rays coming from a star being
+already parallel, and the star itself being a virtual slit without
+length, a large prism placed in front of the object-glass of a telescope
+makes a complete stellar spectroscope. A prism employed in this way is
+known as an _objective prism_.
+
+In place of the image of a star, which would be seen in the absence of
+the prism, a spectrum without appreciable width appears at the focus of
+the telescope, and the spectrum lines will be represented by mere dots.
+To turn these dots into lines so that they may be better visible, a
+cylindrical lens must be employed in conjunction with the eye-piece.
+
+It is to the application of photography, however, that we owe so much,
+and in this case the cylindrical lens is removed, while a small camera
+replaces the eye-piece of the telescope. In this form the instrument is
+often called a _prismatic camera_.
+
+The prism is so arranged that the spectrum lies along the meridian
+passing through the star, and it is then only necessary to allow the
+driving clock to be slightly in error in order that the spectrum may
+trail a short distance perpendicular to its own length, and in this way
+broaden the photographed spectrum. On the proper regulation of the clock
+rate, and consequent “trail” of the spectrum across the plate parallel
+to itself, depends very largely the success of the photograph obtained.
+The spectrum of a bright star must obviously be made to travel more
+quickly than that of a fainter one, and a short exposure suffices. For
+the same clock rate, and in the same time, a star near the Pole will
+give a shorter trail than one nearer the Equator, and declination must
+therefore be taken into account in adjusting the clock error for this
+method of photography.
+
+One great advantage of the objective prism in the photography of stellar
+spectra depends upon the fact that all the light passing through the
+object-glass is utilised in the production of the spectrum, whereas in
+an ordinary telespectroscope a large percentage of the light is lost in
+the jaws of the slit. The large focal length of the telescope also
+enables a long spectrum to be obtained even with a single prism of small
+angle.
+
+When the dispersion is only small, the spectra of stars as faint as the
+tenth or eleventh magnitude can be photographed by this method, so that
+sometimes as many as 200 spectra are registered with a single exposure.
+Here, again, the objective prism has an immense advantage over the
+telespectroscope.
+
+Professor Pickering, of Harvard College, was among the first to
+recognise the value of the objective prism for the photography of
+stellar spectra, and the munificent endowment of this research, by Mrs.
+Draper, as a memorial to Dr. Henry Draper, has enabled him to produce
+the Draper catalogue of stellar spectra, giving the chief
+characteristics of the spectra of over 10,000 stars.
+
+Professor Norman Lockyer, at South Kensington, has also been
+conspicuously successful in this department of astrophysical research.
+The chief instrument he employs is a photographic telescope of only six
+inches aperture, with an objective prism of 45° refracting angle. The
+spectra thus obtained show hundreds of lines in such stars as Arcturus,
+with very fine definition, so that they bear almost unlimited
+enlargement.
+
+An objective prism of twenty-four inches aperture will form one of the
+accessories of the fine telescope which is now being erected at the
+expense of Dr. Frank McClean, for the Cape Observatory, and there can be
+no doubt that the use of this gigantic prism will add greatly to our
+knowledge of the chemistry of the fainter stars.
+
+As yet there is no very practicable method of employing the objective
+prism for determining the velocities of stars in the line of sight from
+the displacement of spectrum lines, and herein lies its one great
+disadvantage as compared with the telespectroscope. The difficulty is to
+ensure that the spectrum always falls absolutely in the same position
+with respect to the terrestrial spectrum, which must be photographed
+alongside for purposes of measurements. It is true that the spectrum of
+an approaching star is somewhat shorter, and of a receding star slightly
+longer than that of one at rest relatively to the observer, but these
+changes are so small as to little more than indicate the direction of
+movement even when a large instrument is employed.
+
+Under the direction of Professor Norman Lockyer, the objective prism was
+very successfully used for photographing the spectra of the solar
+surroundings during the total eclipses of 1893 and 1896. In place of the
+picture of the solar corona, which would appear in the absence of the
+prism, the prismatic camera shows a spectrum consisting of bright rings.
+If, for instance, the corona were wholly composed of hydrogen, there
+would be a picture of it in red, blue-green, blue, and violet,
+corresponding to the lines ordinarily seen in the spectrum of that gas.
+These rings thus indicate the chemical nature of the corona, and at the
+same time show, by their differing forms, the distribution of different
+gases throughout its extent. The spectra of the solar prominences and
+chromosphere are also depicted during the brief time of their
+visibility, during an eclipse, with such distinctness that a series of
+“snap shots” is all that is required to give a lasting record.
+
+THE SPECTROHELIOGRAPH.—A special form of spectroscope—called the
+_spectroheliograph_—has been devised by Prof. Hale, of Chicago, for
+photographing the sun in monochromatic light. It consists of a
+spectroscope, arranged for photography, in which the slit can be made to
+travel by clock-work across the sun’s image, which is projected upon it
+by the telescope to which the instrument is attached. In front of the
+photographic plate there is a secondary slit, so that only a very
+restricted part of the spectrum reaches the sensitive film. The
+secondary slit is connected by mechanism with the primary one, so that
+as the latter traverses the sun’s image, the former exposes different
+parts of the photographic plate to the light which passes through it,
+and in this way builds up an image of the sun in monochromatic light,
+matters being so arranged that light of the same wave-length always
+falls upon the secondary slit. By utilising the brightest lines which
+appear in the spectrum of the solar prominences, monochromatic images of
+those interesting appendages to our luminary have been successfully
+photographed without waiting for a total solar eclipse.
+
+THE BOLOMETER.—Besides the luminous effects of the spectrum, there are
+heating effects which can be measured by the _bolometer_, an instrument
+invented by Prof. Langley. A very thin strip of metal is connected with
+a delicate galvanometer, and is arranged so that it can be passed a long
+the whole spectrum. The electrical resistance of the strip varies
+according to its temperature, and the galvanometer at once signals any
+fluctuations which may occur. If, for instance, the strip comes to the
+place occupied by a dark line, there will be a notable fall of
+temperature. In this way, the bolometer is used to map the solar
+spectrum in the “infra-red” region—a part of the spectrum invisible to
+the eye, and of which we should otherwise have remained in ignorance.
+
+
+
+
+                               ASTRONOMY
+
+
+[Illustration:
+
+  DONATI’S COMET, OCTOBER 9, 1858. (FROM LANGLEY’S “NEW ASTRONOMY”.)
+]
+
+
+
+
+                     SECTION III.—THE SOLAR SYSTEM.
+
+                          BY AGNES M. CLERKE.
+
+
+
+
+                               CHAPTER I.
+                      THE SOLAR SYSTEM AS A WHOLE.
+
+
+The solar system consists of one supereminent body, with a train of
+miscellaneous attendants. By its immense gravitative power, their
+movements are so governed that they not only revolve round it as a
+common centre, but accompany its march through space; they are, in
+various degrees, warmed and enlightened by its copious emissions of heat
+and light; they are linked with it by origin and destiny. Some, indeed,
+much more closely than others. Planets, satellites, and asteroids belong
+to the immediate family of the sun; periodical comets and revolving
+meteoric rings have been adopted into it. The planets are eight in
+number; the six nearest the sun—Mercury, Venus, the Earth, Mars,
+Jupiter, and Saturn—have been known immemorially; Uranus and Neptune
+were discovered respectively in 1781 and 1846. Mercury, Venus, and Mars
+form, with the Earth, a group of “terrestrial planets,” so-called
+because they differ not very greatly in scale from our globe, and are
+constructed on nearly the same lines. The outer quartette of planets are
+giants by comparison, and show obvious symptoms of being in a very
+different physical condition. And it is noteworthy that the zone of
+asteroids, lying between Mars and Jupiter, divides the planetary
+classes.
+
+The asteroids are sometimes designated minor planets; but the former
+term is preferable, as accentuating their distinctive character. For
+they are not simply diminutive planets. A planet revolves in solitary
+state within its own broad domain. The asteroids traverse intercrossing
+and entangled paths, indefinitely numerous, ranging widely in celestial
+latitude, and covering with their network nearly the entire chasm of
+space between Mars and Jupiter. The small bodies moving in them have
+doubtless been formed in a manner totally different from that by which
+the single body they seem to replace would have taken shape.
+
+Satellites bear in many respects the same relation to planets that
+planets bear to the sun. They are united with them into secondary
+systems, one of which is particularly well known to us, since it is
+constituted by the earth and the moon. The existence of twenty-one
+satellites has been ascertained, and many more possibly remain to be
+detected. Their apportionment is singularly unequal. Only three of the
+twenty one belong to the four small interior planets, while eighteen are
+attached to the four exterior giants. Moreover, both Mercury and Venus
+are solitary; so that the solar neighbourhood appears to be a region
+unpropitious to the development of subordinate systems.
+
+Seventeen comets certainly, and many more probably, are domiciled in the
+solar kingdom. And even these preserve traces of an alien origin. They
+revolve round the sun in closed orbits, and are hence periodical in
+their apparitions; but their periodicity has to be qualified by a saving
+clause. They come up to time _barring accidents_. For their orbits, not
+being adjusted to stability, are liable to violent changes through the
+influence of the powerful masses, the tracks of which they intersect. In
+running up to, or back from perihelion, comets have to cross many
+railroads, so to speak, and do not always escape disturbing or
+destructive encounters with passing trains. Thus, many are entered in
+our astronomical visitor’s book as lost or strayed. Halley’s is the only
+well-secured cometary prisoner of the sun of imposing magnitude; the
+rest are of little spectacular, although of very high theoretic,
+interest. Comets are the only self-luminous members of the solar system.
+
+Meteorites, besides being intrinsically obscure, reflect, owing to their
+minuteness, so little sunlight that they remain invisible until ignited
+in our atmosphere. They travel round the sun in annular systems, each
+mote-like component of which pursues its way, independently of the
+others, under the strict regimen of gravitational law. The number of
+these meteoric rings must be prodigious. Some hundreds have been brought
+to our acquaintance, which can only include such as cut the earth’s
+orbit; and these must be an insignificant fraction of the whole. The
+innumerable closely-related orbits grouped into each ring are
+ill-regulated for the safety of the bodies moving in them, since they
+conform in no way to the rules of planetary circulation. Hence the
+numerous encounters with the earth announced by the luminous trails of
+shooting stars.
+
+Our system, as at present known, is 5,585 millions of miles in diameter.
+It is limited by the orbit of Neptune. But no less than three
+trans-Neptunian planets have been, on some show of evidence, alleged to
+exist. One of them, held by Professor Todd of Amherst College, U.S., to
+be responsible for some outstanding perturbations of Uranus, was placed
+by him in 1877 at a distance from the sun fifty-two times that of the
+earth (the radius of Neptune’s orbit being measured by thirty of the
+same units); the two others, called into existence by Professor Forbes
+of Edinburgh in 1880, to account for the formation of two groups of
+comets with aphelia respectively at one hundred, and three hundred
+astronomical units, were believed to occupy those enormously remote
+positions. Although none of the three, in spite of telescopic and
+photographic search, has yet been found, the possibility is not excluded
+that the appearance on a long-exposed sensitive plate of a line in lieu
+of a dot as the representative of a seeming star, may in the future
+announce the annexation by the sun of a further immense slice of
+territory out in the depths of space. The boundaries of our system are
+thus only provisionally fixed.
+
+Intra-Mercurian planets have proved equally recalcitrant to
+prediction; and it may safely be said that no globe of the superficial
+dimensions of an English county lies concealed in the comparatively
+narrow space available for its circulation. The necessity for the
+presence of “Vulcan” was deduced by Leverrier from an unexplained
+displacement of Mercury’s perihelion, and a transit of the required
+body, supposed to have been observed March 26, 1859, was thereupon, in
+all good faith, brought forward by Dr. Lescarbault of Orgères. Another
+pseudo-discovery—this time of a pair of Vulcans—was made during the
+total eclipse of July 29, 1878; but neither on nor off the sun has the
+body needed to satisfy the French mathematician’s theory been
+genuinely seen, and few believe that it will ever be forthcoming.
+
+Professor Titius of Wittenberg pointed out in 1772 that the relative
+distances of the planets from the sun could be expressed by adding 4 to
+the series 0, 3, 6, 12, 24, 48, etc. Thus, if the distance of Mercury
+were called 4, those of Venus, the Earth, Mars, and so on, would
+severally be 7, 10, 16. The validity of this relation—known as “Bode’s
+Law”—was strengthened by the conformity to it of Uranus and Ceres,
+neither of which had been discovered when it was enunciated; Neptune,
+however, proved to be much nearer to the sun than he should have been,
+and the formula hence ranks as an empirical one, not grounded in the
+nature of things.
+
+Yet the grand outlines of the solar system are traced on a visibly
+symmetrical plan. The larger bodies composing it move nearly in the same
+plane, in orbits nearly circular, and at regulated intervals, augmenting
+rapidly outward. All revolve from west to east, or “counter clockwise,”
+and this fundamental current of motion carries with it, besides the
+asteroids, all the periodical comets, save Halley’s. Among secondary
+systems only the Uranian and Neptunian escape from its sway; there being
+a visible tendency towards deviations from rule towards the confines of
+the solar domain. These deviations, however, are not of a subversive
+character.
+
+The planetary machine may continue working forever without a hitch. Such
+irregularities as would be likely to throw it out of gear are found only
+in parts of almost evanescent mass and negligeable influence. Two modes
+of action which should, in the long run, bring about a collapse, are
+non-existent or insensible. These destructive agencies are a resisting
+medium, and the progressive transmission of gravity. The presence of
+either should prove fatal in the same ultimate fashion. Along slowly
+narrowing tracks, the planets would descend, one after the other, into
+the ample lap of the sun. Their circulation is, however, to the best of
+our present knowledge, unimpeded and undeflected; the disturbances
+affecting it are self-compensatory.
+
+But while the mechanical stability of the system is assured, its
+physical state is continually changing. And the change is always in the
+same direction. A degradation of energy steadily progresses. The sun is,
+in fact, spending his capital, and even with a millionaire of his stamp
+this cannot last. The time must come, if science is to be believed, when
+his radiative powers will have become exhausted. Five millions of years
+hence they will, in all probability, be much less efficacious than they
+are now. Within twice or thrice that interval they may have become
+almost extinct.
+
+Planetary globes, too, grow old through the wasting of their internal
+heat. The moon seems in a measure to prefigure the future condition of
+all, should their decay not be arrested. Possibly the lunar stage is not
+the last. Death may, in the long ages to come, be succeeded by
+disintegration, when a ring of rubbish will be substituted for our
+“wan-faced” companion. To what purpose, then, our readers will ask, the
+mechanical perfections of a system destined eventually to be involved in
+darkness and destruction? To what purpose its exquisite balance, the
+nicely-adjusted relations of its members, its self-righting faculty, its
+compensatory springs? We can reply only by recalling that the extreme
+conclusions of science are invariably pessimistic, because they are
+reached without taking any account of the intelligent control
+perpetually, though insensibly, overruling the workings of blind forces.
+If, in one sense, heaven and earth pass away, we still know that, in
+good time, “a new heaven and a new earth” shall inscrutably arise. Not
+“faintly,” then, but boldly and ardently, we “trust the larger hope”
+that renovation will succeed, or anticipate subversion.
+
+Whatever _can_ have an end _must_ have had a beginning, and the origins
+of things have an especial fascination for our minds. As regards the
+history of the planetary world, we are not altogether in the dark. The
+problem of the maintenance of the sun’s heat was satisfactorily solved
+by Helmholtz in 1854. Its radiative supplies, as he showed all but
+conclusively, are derived from gravitative power. As they are diffused
+into space, the cooled particles from which they proceed, clash
+together, and their arrested motion is converted into a fresh thermal
+stock. This implies a steady diminution, although to a surprisingly
+slight extent, in the bulk of the solar globe. It has been computed that
+a shortening of the sun’s diameter by 380 feet yearly would suffice to
+keep this grand heat-producing machine in full working order; and at
+least ten thousand years should elapse before the contraction became
+measurable by any instrumental means at our command. Its progress
+should, nevertheless, eventually reduce our glowing luminary to an
+obscure, inert mass.
+
+Now, evidently, its shining in the past was sustained in the same way as
+at present. The globe that blazes in our summer skies is, accordingly,
+but the shrunken remnant of what it once was. It is shrunken in
+proportion to the vast quantity of its former emissions. Hence, the
+farther we look back into the ages, the more voluminous its dimensions.
+And, sounding the utmost profundities of time, we arrive at an epoch
+when all the planets were swallowed up in a sphere girdled by the
+present orbit of Neptune.
+
+The tenuity of this distended body was unimaginable. At ninety miles of
+altitude, our air is one hundred million times rarer than it is at
+sea-level; yet the primitive solar “nebula” was considerably more
+attenuated still. This aerial mass had, doubtless, been in some way
+impressed with a slow movement of rotation, which, by mechanical
+necessity, quickened as condensation progressed. The planets represent a
+few fragments detached during the process; nearly the whole of its
+substance being compacted into the sun. How the fragments came to be
+detached is the crux of cosmogonists. According to Laplace’s famous
+hypothesis, equatorial rings of matter separated successively from the
+parent nebula at certain critical epochs when gravity was overcome by
+the gaining centrifugal tendency due to accelerating rotation. These
+rings drew together into planets, from which satellites were generated
+by a repetition of their own birth-process. Many incongruities are,
+however, involved in this _modus operandi_. Only two need here be
+mentioned. Reason and experience teach us that globes of small interior
+consistence easily break up into rings, while cosmic rings show not the
+slightest tendency to collect into globes. Again, Laplace supposed that
+the production of each planet relieved a long antecedent strain. But
+nebulous stuff is almost absolutely incoherent. Hence it _cannot be
+stretched or strained_. As the nebula condensed and whirled, it would,
+accordingly, have left behind innumerable disaggregated particles, but
+no massive rings.
+
+M. Faye of the French Academy has attempted to remedy these defects. The
+planets, he considers, were not abandoned, but formed at centres of
+condensation within the nebular matrix. The order of their formation
+would thus have been quite different from that assigned by Laplace, in
+whose theory the exterior globes were necessarily the earliest to take
+shape. M. Faye, on the contrary, argues Uranus and Neptune, from their
+retrograde rotation, to be the _youngest_ instead of the _oldest_
+members of the solar system, while the terrestrial group belong to the
+first era of planetary development.
+
+Astronomers are now virtually agreed that “The world was once a fluid
+haze of light,” but by what precise means, in what succession, under
+what compulsion, its constituent bodies were set wheeling in the void,
+they are less ready to pronounce than were their predecessors, who,
+dazzled with the analytical triumphs of the eighteenth century, accepted
+unquestioningly the plan of creation it complacently transmitted to
+them. The complexities of world-making have, besides, been instructively
+illustrated by Professor G. H. Darwin’s discovery that tidal friction
+was essentially concerned in the process. By an able mathematical
+investigation, he showed, in 1879, that it was particularly effective in
+modelling the earth-moon system, owing to the fact that our satellite,
+comparatively to its primary, is by far the largest in the solar system.
+
+Tidal friction may be regarded under a two-fold aspect. Its effect in
+grinding down the speed of rotation has been explained in Section II.
+(page 166). The energy, however, thus apparently destroyed is only
+transformed. The rotational momentum subtracted from the earth is added
+to the orbital momentum of the moon, which thus travels (setting aside
+other causes of change) along continually widening spires. This retreat
+from the earth is even now going on, although with elusive slowness,
+amid the rise and fall of secular change. Its effects in past ages,
+nevertheless, coupled with those due to the slackening of rotation by
+the friction of the tidal wave—the two forming, as it were, the obverse
+and reverse of one medal—must have been of overruling importance. Laying
+hold of the clue they offer, Professor Darwin succeeded in tracing back
+the history of the moon through a “corridor of time” nearly a hundred
+million years long. It was then spinning at a vertiginous rate, round,
+and nearly in contact with the earth, which must have been fluid or
+plastic, while of about its present size. The _month_ of that epoch was
+three or four hours in duration; the _day_ was shorter still. The actual
+existence of the moon convinces us of this latter fact. Otherwise, the
+huge tidal wave raised by the moon upon the earth should have lagged,
+however slightly. Its attraction would have pulled the moon backwards at
+the decisive moment of its emergence into separate being, and led
+infallibly to its re-engulfment.
+
+The origin of the moon has been, by Professor Darwin’s analysis, made
+clearer than that of any other heavenly body. Certainty regarding such
+remote events is unattainable; but it is highly probable that our globe,
+at a late stage of its development, gave birth, amid the throes of
+disruption, to its solitary offspring. But the case is unique. The
+terrestrial system presents conditions not repeated elsewhere.
+Generalisations founded upon them are sure to be misleading. We have
+indeed gained, from all recent inquiries into cosmogony, the profound
+conviction that no single scheme will account for everything; that the
+utmost variety prevailed in the circumstances under which the heavenly
+bodies attained their present status; and that a rigidly constructed
+hypothesis can only misrepresent the boundless diversity of nature.
+
+
+
+
+                              CHAPTER II.
+                                THE SUN.
+
+
+The sun is an immense reservoir of radiant energy. For our daily uses we
+have no other store worth mentioning to draw upon, our fuel being the
+embalmed sun-heat of former ages; and all the physical and vital
+operations carried on over the whole globe derive their motive power
+from the same copious source. Yet only 1/2,128,000,000th part of the sum
+total of solar radiations strike its comparatively diminutive surface;
+while all the planets combined intercept no more than 1/234,000,000th of
+that inconceivable effluence.
+
+The sun gives as much light as 600,000 full moons, or two and a half
+billions of the most powerful electric lights, or as 1,575 billions of
+billions of standard candles. And since his disc is the projection of a
+hemisphere, and is thus equivalent only to one-fourth the globular
+surface, these vast numbers must be quadrupled to represent the whole
+luminous emissions of this surpassing body. Their amazing profusion is
+the combined result of immensity of shining area, and vivid intrinsic
+brilliancy. Each square inch of the sun’s surface has been estimated to
+integrate the lustre of twenty-five electric arcs,[6] and Professor
+Langley, by direct experiment, proved it to be 5,300 times brighter, and
+87 times hotter, area for area, than the white-hot “pour” from a
+Bessemer converter; notwithstanding that the circumstances of the
+comparison were exceedingly “unfair to the sun.”[7]
+
+Radiant heat and light do not indeed differ in themselves, but only in
+their effects. The sun sends out into space ethereal waves of various
+lengths, but all of the same kind, subject to the same laws, and
+travelling with the same velocity of 186,000 miles a second. They
+appear, however, under diverse forms of energy according to the
+qualities of the substances upon which they impinge. Thus a small
+section of this long range of undulations affects our eyes as light, the
+human retina being so fashioned as to be able to _see_ with their help.
+There is nothing in the nature of the rays themselves to make them
+visible, and it is in fact more than probable that other living
+creatures perceive vibrations to which we are blind. Our eyes are
+sensitive over nearly two octaves; from waves measuring about 760
+millionths of a millimetre, to those of less than 400 millionths. In the
+solar spectrum the limits are roughly marked at one end by a great dark
+band in the deep red—Fraunhofer’s “A,”—and at the other by “H,” in the
+extreme violet. Beyond H extend undulations so short as to be visually
+imperceptible, while photographically active. This means that certain
+salts of silver are capable of taking up the energy they bring from the
+sun, and of using it to break their chemical bonds; while on differently
+prepared plates similar effects can be produced by rays in all parts of
+the spectrum, even in the ultra-red, where the undulations, too long to
+be sensible as light, are mainly felt as heat. Here, as Professor
+Langley has shown by “bolometric”[8] explorations, reside three-fourths
+of the energy distributed throughout the solar spectrum; nor is it
+impossible that this great stretch of heat waves may merge, without
+interruption, into electrical _rollers_, measured, not by millionths of
+a millimetre, but by metres, or even by kilometres. The important point
+to be borne in mind, however, is that the solar energy is diffused
+abroad by means of ethereal vibrations of a single type, but immensely
+varied size and frequency, and hence susceptible of dispersion into a
+spectrum.
+
+The “solar constant” expresses the quantity of heat received by the
+earth from the sun. Its value, according to the most trustworthy
+determinations, is three calories per square centimetre per minute. This
+means that a vertical sun pours down upon each square centimetre of the
+globe heat enough (supposing the atmosphere out of the way) to raise the
+temperature of three grams of water by one degree centigrade in a
+minute. Putting it otherwise, the energy imparted would suffice to keep
+an engine of three-horse power continually at work on every square yard
+of the terrestrial surface. Or, if the heat were distributed uniformly
+in all latitudes, it would annually melt a complete ice-jacket one
+hundred and seventy feet thick.
+
+The temperature of the body lavishing heat at this tremendous rate must
+obviously be very high; but enquiries on the point are necessarily
+limited to the actual emitting shell, or “photosphere.” Their success is
+testified to by a noteworthy reduction of late in the range of
+uncertainty. The difficulty attending them consists mainly in our
+ignorance of any systematic relation between temperature and radiation.
+Excessively hot bodies lose heat much more rapidly, under the same
+conditions, than moderately hot ones; and empirical “laws of radiation”
+have been, over and over again, arrived at as the upshot of long series
+of laboratory experiments. But such laws are only too apt to turn
+traitors if trusted without control; and since the thermal power of the
+sun vastly exceeds that of any terrestrial source, they are precarious
+guides in this particular research. Nevertheless, as the outcome of
+various improvements and refinements, it has, within the last few years,
+been prosecuted with excellent results. That obtained in 1894 by Messrs.
+Wilson and Gray deserves particular confidence. The _effective_
+temperature of the sun was by them fixed at 8,000°, or allowing for
+absorption in the solar atmosphere (measured by Wilson and Rambaud), at
+8,800° centigrade. This estimate, which makes the sun’s surface more
+than twice as hot as the carbons of the electric arc, is unlikely to be
+widely erroneous. The word “effective” signifies the condition that the
+photosphere is equivalent in radiative power to a stratum of lampblack;
+if it fall short of this standard, as appears probable, then the
+temperature must be raised by a corresponding amount.
+
+The solar atmosphere, of which the absorptive effects have just been
+alluded to, is a shallow envelope, stopping predominantly the shorter
+wave-lengths of the light transmitted through it. Hence, if it were
+removed, the sun would appear, not only much brighter, but also much
+_bluer_ than it does at present. The general darkening of the limb due
+to its action is apparent to visual, and conspicuous in photographic,
+observations. By its aid, “faculæ”—brilliant and elevated portions of
+the photosphere—were early detected. Invisible on or near the middle of
+the disc, they stand out in relief against its dusky edges as they are
+brought round, and carried off again by the sun’s rotation.
+
+The magnitude of this astonishing luminary fairly baffles our
+conceptions. Its mass is 745 times that of all the planets taken
+together. Its volume is such, that if Jupiter were located centrally
+within it, two of his Galilean moons, besides the lately discovered
+inner satellite, would have “ample room and verge enough” to revolve
+round him, keeping well inside the photosphere. The entire Uranian
+system could be easily accommodated in the same way; while Neptune and
+his satellite, and the earth and moon, could very nearly perform their
+evolutions side by side in the sun’s excavated interior.
+
+The sun is 865,000 miles in diameter, and in figure is sensibly
+spherical. Its surface is 12,000 times, its volume 1,300,000 times that
+of the earth. In mass it is equal to 332,000 earths. Its mean density,
+then, is only one-quarter that of the earth, or 1·4 times that of water.
+In other words, the terrestrial globe, if equally bulky, would contain
+four times the quantity of matter contained in the solar globe. Yet we
+know that it is largely made up of iron and still heavier metals; while
+gravity at its surface is 27·6 more powerful than it is here. Thus, the
+sun’s materials are weighed down by an inconceivable pressure, and would
+be of a density utterly transcending our experience but for the
+counteracting agency of heat. The comparative insubstantiality of such a
+globe gives us some faint notion of the violent molecular agitation
+affecting every particle of its mass. Contrasted with the fires raging
+within, the surface temperature of 8,000° or 9,000° might perhaps be
+deemed moderate or cool. There is much evidence that it is throughout
+gaseous, although of a consistence approaching more nearly that of pitch
+or treacle than can easily be reconciled with established ideas as to
+the qualities proper to an aerial substance. Yet the laws governing the
+gaseous state are plainly those obeyed in the sun.
+
+Its function, as a great thermal engine, is to produce and diffuse heat
+For these purposes it is essential that the interior stores should be
+brought rapidly to the surface; and this is accomplished, not, as in
+solids, by conduction, but by actual transport, or “convection.” Only
+the enormous elasticity of highly compressed gases could render this
+process swift enough to sustain the incessant outpourings of heat from
+the photosphere. It may be accompanied by an actual rise in temperature.
+If the sun be truly gaseous throughout, it _must_ be so accompanied. The
+reason of this seeming anomaly is that a sphere of radiating and
+contracting gas develops by shrinkage more heat than it can dispose of
+by radiation. Whether or no the sun comes within the scope of this
+principle, known as “Lane’s Law,” cannot at present be decided. It is,
+in other words, an open question whether the sun is growing hotter or
+colder. Help towards answering it might have been expected from the
+study of geological climates; but their variations have evidently been
+due to a complexity of causes. At any rate, the sun’s decline, if the
+inevitable turning-point has already been reached, is going on with
+extreme slowness.
+
+The visible structure of the photosphere, or lustrous envelope of the
+solar globe, is, in itself, suggestive of the vertical circulation by
+which the indispensable communications between its interior and exterior
+are kept up. It is composed of brilliant granules and dusky interstices,
+the former representing, it is supposed, the vividly incandescent
+summits of uprushing currents, the latter the cooled, descending
+return-flows. It may be safely described as the limiting surface of
+thermal interchange, and is often spoken of as a cloud-sphere, or level
+of condensation, where the ascending vapours, like mounting volumes of
+water-gas in our atmosphere, are chilled into liquid droplets. To the
+brilliant luminosity of these incandescent droplets, the blaze of the
+solar emissions is ascribed. Or the droplets might equally well be solid
+particles on the model of the ice-spicules collected to form the
+delicate fields of cirrus in our upper air. The cloud theory of the
+photosphere is, however, hampered by the difficulty of finding a
+substance capable of liquefying or solidifying at a temperature of
+8,000° C. Carbon has generally been selected as the material of the
+solar “granules,” but carbon evaporates at about 4,000°, and although
+its boiling point might be raised by enormous pressure, there are no
+signs that the requisite conditions exist in the sun. Hence, some
+speculators turn towards electricity as the exciting agent of the
+photospheric radiance; but it would be waste of time to attempt, at
+present, to discuss the vague possibilities connected with an hypothesis
+which offers no holding ground for distinct reasoning.
+
+[Illustration:
+
+  FIG. 1.—_Photograph of a Sun-spot._ (From _Knowledge_, February,
+    1890.)
+]
+
+The photospheric texture is often rent and perforated. This ragged
+condition (well exemplified in Fig. 1 from a photograph taken by Dr.
+Janssen at Meudon) is accompanied or caused by a violent disturbance of
+the sun’s bodily circulation. A typical sun-spot consists of a dark
+opening, or “umbra,” within which a still darker “nucleus” can often be
+discerned. The umbra is garnished all round with a semi-luminous
+“penumbra,” composed of elongated shining bodies placed side by side,
+and all, when undisturbed, pointing radially inwards towards the centre
+of the spot. The effect has been compared to that of “straw-thatching,”
+although the solar “straws” are, at times, thrown somewhat wildly about.
+Where they hang over the _eaves_ of the spot they are always brightest,
+because set most closely together. The penumbra may be called a modified
+extension of the ordinary mottled surface of the photosphere, the
+lustrous grains being drawn out into filaments, the “pores” into obscure
+interspaces.
+
+Spots commonly occur in groups (as in our Figure) belonging to a single
+area of disturbance marked by the brightening, and probably by an
+elevation of the photosphere. The members of such families show curious
+and unexplained mutual relations. The size of these extraordinary
+formations is on the gigantic scale of all solar phenomena. They are
+often visible, individually or collectively, to the naked eye, and
+attracted notice accordingly in pre-telescopic times. In 1858, a spot
+opened to the extent of 144,000 miles, so that sixteen earths, side by
+side, might have been engulfed in it. A still more remarkable outbreak
+took place in February, 1892. Three thousand three hundred and sixty
+million square miles of the photosphere were riddled as if by some
+tremendous bombardment, the extreme dimensions of the affected district
+being 150,000 by 75,000 miles. This spot, the largest ever photographed
+at Greenwich, attained its acme on February 13th, when a magnetic storm
+and widely diffused auroral display attested the sympathy of the earth
+with commotions in the sun. Five times brought back to view by the sun’s
+rotation, its history was followed from November until March; but this
+duration is not an extreme case, a spot having been known to survive
+throughout eighteen rotations. Although the group of February, 1892,
+covered ¹⁄₇₀₀th of the sun’s entire surface, its proportions were
+outdone by those of a spot and its immediate attendants, without
+counting outliers, measured by Sir John Herschel at the Cape, March
+29th, 1837.
+
+Spots are always associated with faculæ. The two are correlated
+phenomena. There is no certainty as to their order of precedence, if any
+fixed order there be, but faculæ both survive spots and develop apart
+from them. Not infrequently the faculæ garlanding a spot throw a
+“bridge” right across it (see Fig. 1). In stereoscopic views these
+brilliant projections show as veritable _suspension bridges_. They float
+almost palpably at a high altitude above the black gulf they span.
+
+The distribution of spots is easily perceived to depend immediately upon
+the sun’s rotation. Two zones of its surface, parallel to the solar
+equator, are alone infested by them. These may be defined as lying
+between 6° and 35° of north and south latitude; but the prohibition of
+spot-development is much more absolute in the polar than in the
+equatorial direction. One solitary macula has been observed in 50° north
+latitude.
+
+The periodicity of sun-spots was first recognised by Schwabe at Dessau
+in 1851. Since abundantly confirmed, it constitutes one of the
+fundamental data of solar physics. Once in about eleven years a
+“maximum” is attained; for months together the photosphere is never calm
+and unbroken; its agitated condition betrays the turmoil of the
+interior. The superabundance of spots is succeeded, after some years, by
+a scarcity, or “minimum,” when the perturbing agencies appear to have
+sunk into repose, preparatory to another outburst of activity. In this
+highly irregular, although well-marked, cycle, the ascent is almost
+always much more rapid than the descent; the upspringing of the
+disturbance occupies, as a rule, not much more than half the time
+allotted to its quieting down. Nor is its intensity by any means
+uniform. High and low maxima alternate with, or succeed each other, with
+no obvious regularity. Sometimes we have a divided or double maximum, as
+in 1882–4, followed by an unusually swift ebb of agitation. The minimum
+of 1889 was premature and brief; for spots were again numerous in 1891,
+and developed prodigiously throughout the years 1892 and 1893. Only in
+January, 1894, a slight falling off became apparent, and the
+tranquillity which set in with 1895 may very probably reign with only
+temporary interruption for some time. The cause of these vicissitudes is
+completely unknown; but they so closely resemble, in character, the
+changes of variable stars, that it seems impossible to exclude the sun
+from that category, spot-maxima corresponding with stellar light-maxima
+and _vice versâ_.
+
+[Illustration:
+
+  FIG. 2.—_Sun-spots and Magnetic Variations._ (From Langley’s “New
+    Astronomy.”)
+]
+
+Solar disturbances, however originating, are a sort of universal
+pulse-beat, with which the earth, and doubtless every other member of
+the solar cortège, throb in unison. The accompanying diagram (Fig. 2)
+shows how closely the magnetic needle sympathises with the variations in
+the state of the sun. The amplitude of its daily oscillations is
+represented by the dotted curve, while the smooth curve is constructed
+from the relative numbers of spots. The striking conformity in point of
+time-development, between two effects so disparate in their nature,
+extends to minute details. Violent commotions on the sun seldom fail to
+be reflected in magnetic storms and auroral manifestations on the earth;
+and exact correspondences have sometimes been observed; yet it does not
+seem possible to trace these simultaneous effects to the immediate
+magnetic action of the sun.
+
+No meteorological cycle corresponding with the spot-cycle has yet been
+satisfactorily made out. The direct diminution of heat and light through
+the obscuration of a small part of the sun’s photosphere amounts, at the
+utmost, to ¹⁄₁₀₀₀th of the whole. The spots are far from being totally
+dark or cool. Their blackest nuclei are really no less brilliant than
+limelight; while about half as much heat is derived from them as from
+the surrounding disc when they are centrally situated, and 80 per cent.
+when they are near the limb.[9] Their dimming and cooling effects then
+are insignificant; they are probably more than compensated by the
+quickening of the sun’s circulatory processes, and consequent increase
+of emission, through the disturbance of internal equilibrium of which
+outbreaks of spots are among the consequences.
+
+The spot-zones do not always occupy the same positions. They shift with
+the progress of the eleven-year cycle. This curious circumstance,
+discovered by R. C. Carrington in 1856, illustrates, in his words, “the
+regular irregularity, and irregular regularity,” distinguishing solar
+periodicity. At maxima, the mean latitude of the zones in question is
+about 16°; but they close down towards the equator as each wave of
+agitation dies out, its few latest products appearing in quite low
+latitudes. Then, when minimum is passed, a fresh start is made with the
+opening of a few small spots in 30° or 35° north or south latitude; and
+this newly-organised disturbance begins to descend as before, gaining
+strength as it proceeds. Thus, each impulse acts independently of the
+succeeding one.
+
+The most cursory observation of sun-spots suffices to show that the
+shining body marked by them rotates on an axis from west to east, in the
+same direction as the planetary revolutions. True, they emerge to sight
+on its eastern, and vanish at its western limb; but this is because we
+are located at its _backside_, and see their courses inverted. Attempts,
+however, to fix the sun’s period of rotation were long baffled; for the
+spots, instead of being carried round as if attached to a rigid surface,
+gave signs of possessing “proper motions” of uncertain and inconstant
+amount. The subject was first thoroughly investigated by Carrington; and
+he reached the unexpected conclusion that the sun has no uniform period,
+but gyrates in a composite fashion, quickest at the equator, and
+gradually slower towards the poles. From less than twenty-five days, he
+found the time of circuit to lengthen steadily to twenty-seven and a
+half in 50° of latitude. The axis round which this remarkably
+conditioned movement is performed makes an angle of 7° 15′ with the pole
+of the ecliptic; it inclines towards the earth’s northern hemisphere
+from June to December, when the spots describe, in crossing the disc,
+paths curved downwards (to the eye of a northern observer); but the
+conditions being reversed between December and June, their paths are
+then curved upwards; while on June 3rd and December 5th, they pursue
+straight tracks, the earth being on those two days in the line of
+intersection between the sun’s equatorial plane and that of the
+ecliptic.
+
+Only a rough approximation, however, to the laws of solar rotation can
+be derived from spots. For they do not simply drift with the
+photospheric currents, but are subject to accelerations and retardations
+connected with their internal economy, as well as to mutual attractions
+and repulsions depending, it is supposed, upon their electrical
+condition. Fortunately, however, a method has been perfected by which
+these complications are abolished. Something has already been said as to
+spectroscopic determinations of motion in the line of sight. They are
+evidently applicable to the sun’s axial movement. For, through its
+effect, his eastern limb is always advancing uniformly towards us, while
+the western limb is retreating at the same rate. Thus, the whole
+Fraunhofer spectrum is shifted slightly upward, or towards the blue, at
+the left-hand edge of the solar disc, and as much towards the red at the
+right-hand edge. The same lines of solar absorption, in fact, taken from
+opposite sides of the solar equator, and placed end to end, appear
+evidently notched, and can be distinguished at a glance from terrestrial
+absorption lines, which, having nothing to do with the sun’s rotation,
+show no break at the junction of their sections. They in this way
+“virtually map” themselves, as Professor Langley proved experimentally
+in 1877.
+
+In 1887–9, M. Dunér, of Upsala, succeeded in extending these delicate
+measurements to within fifteen degrees of the sun’s poles, where the
+movement is so slow that it can only, by incredible refinements, be
+dealt with successfully. The upshot was to emphasise the law of
+slackening _angular_ speed detected by Carrington and confirmed by
+Spoerer. From 25½ days at the Equator, the sun’s period of rotation was
+found to become protracted to 38½ days at the seventy-fifth parallel of
+latitude. Its investigation from photographs of faculæ has been lately
+carried out by M. Stratonoff at Taschkent in Russia. The results of the
+three methods are collected in the following little table.[10]
+
+                          THE SUN’S ROTATION.
+ ┌────────────────┬────────────────┬────────────────┬────────────────┐
+ │                │  Period from   │  Period from   │  Period from   │
+ │   Mean Solar   │    Faculæ.     │     Spots.     │ Spectroscopic  │
+ │   Latitude.    │ (Stratonoff.)  │   (Spoerer.)   │   Measures.    │
+ │                │                │                │    (Dunér.)    │
+ ├────────────────┼────────────────┼────────────────┼────────────────┤
+ │       0°       │    24^d·66     │    25^d·09     │    25^d·46     │
+ │      15°       │    25  ·26     │    25  ·44     │    26  ·35     │
+ │      30°       │    25  ·48     │    26  ·53     │    27  ·57     │
+ └────────────────┴────────────────┴────────────────┴────────────────┘
+
+These facts, although so various, are not necessarily discordant. They
+apply to different parts of the great solar machine, each one of which
+may rotate with a certain independence. The spots drift, more or less
+passively, _with_ the photosphere. The faculæ are elevated above it, and
+appear to be everywhere accelerated relatively to its systematic
+currents. The strata originating the Fraunhofer lines, to which alone
+the spectroscope is applied, display, on the contrary, effects of
+retardation. “This peculiar law of the sun’s rotation,” Professor Holden
+remarks, “shows conclusively that it is not a rigid body, in which case,
+every one of its layers in every latitude must necessarily rotate in the
+same time. It is more like a vast whirlpool where the velocities of
+rotation depend on the situation of the rotating masses, not only as to
+latitude, but also as to depth beneath the exterior surface.”
+
+Solar chemistry progresses by successive interpretations; and the
+characters to be read are so multitudinous and so similar as to require
+very delicate discrimination. The work, carried on simultaneously in the
+sun and laboratory, becomes more arduous as it advances, and is still
+far from complete. Indeed, the difficulties attending detailed
+comparisons between the Fraunhofer lines and the innumerable components
+of terrestrial spectra, would be insuperable but for the aid of
+photography, here, as elsewhere, the versatile handmaiden of physical
+astronomy.
+
+Here is a list of 36 solar elements published by Professor Rowland of
+Baltimore in 1891, and arranged according to the number of their
+representative lines in the solar spectrum.
+
+                           Iron (2000 +)
+                           Nickel
+                           Titanium
+                           Manganese
+                           Chromium
+                           Cobalt
+                           Carbon (200 +)
+                           Vanadium
+                           Zirconium
+                           Cerium
+                           Calcium (75 +)
+                           Scandium
+                           Neodymium
+                           Lanthanum
+                           Yttrium
+                           Niobium
+                           Molybdenum
+                           Palladium
+                           Magnesium (20 + )
+                           Sodium (11 + )
+                           Silicon
+                           Hydrogen
+                           Strontium
+                           Barium
+                           Aluminium (4)
+                           Cadmium
+                           Rhodium
+                           Erbium
+                           Zinc
+                           Copper (2)
+                           Silver (2)
+                           Glucinium (2)
+                           Germanium
+                           Tin
+                           Lead (1)
+                           Potassium (1)
+
+Only two of these substances, carbon and silicon, are non-metallic,
+hydrogen ranking as a gaseous metal. Neither oxygen, nitrogen, nor
+argon, have yet spoken their “Adsum,” but it is not impossible that they
+may do so in the future. Negative evidence, at any rate, is, in
+spectroscopic inquiries, absolutely inconclusive.
+
+The spectra of sun-spots are, as might have been expected, characterised
+by a great increase of absorption. There is a general darkening which
+extends far up in the ultra-violet, and is modified, in the green and
+blue, into remarkable dusky gratings made up of closely-set fine rays;
+and some of the ordinary Fraunhofer lines are besides thickened and
+blackened. The formation in spots of oxides is thought by Dr. Scheiner
+to be possibly indicated by these symptoms; “if so,” he adds, “the
+presence of oxygen in the sun would thus be indirectly suggested.”[11]
+Bright lines, too, flash out in the immediate neighbourhood of
+sun-spots, especially the “great twin brethren,” “H” and “K,” due to
+calcium, which stand in imposing breadth and strength at the violet end
+of the Fraunhofer spectrum, and are of corresponding importance as
+indexes to solar phenomena. With this pair, brilliant hydrogen rays are
+often associated, besides other “reversals,” by which, upon the
+customary dark lines, flaming rays of identical wave-lengths are
+superposed. But these signs of incandescence evidently belong to the
+facular stratum high up above the spot-umbra.
+
+So long ago as 1769, the observations of Dr. Wilson of Glasgow were
+believed to have established, once for all, that spots are funnel-shaped
+depressions in the photosphere. But the perspective effects from which
+he argued are certainly not always, perhaps not very often, present. Mr.
+Howlett, after thirty-five years—1859 to 1895—devoted to testing the
+truth of the traditional conviction, has at last succeeded in shaking,
+if not in overthrowing, it. Most solar observers now admit that spots
+are of extremely various and extremely variable construction, so that
+the obscure umbra, at times a sort of pit or crater, in which vapours,
+cooled by expansion, well up from below, may, at another stage in the
+life-history even of the same spot, represent an actual accumulation of
+absorbent material above the brilliant solar cloud envelope. In any
+case, a spotted area appears to be an area of elevation. This might be
+due to a wide-spreading relief of pressure, or an accession of internal
+heat. The fact emerged clearly from a series of measurements of the
+sun’s diameter executed by M. Sykora at Charkow, Russia, in 1895.[12]
+
+The intensity of the agitations connected with sun-spots can be most
+fully appreciated from spectroscopic observations. Lines torn,
+displaced, and _branching_, testify to velocities in the line of sight
+of the matter surrounding or overlaying them up to three or four hundred
+miles a second! These tumultuous uprushes and downrushes are not of a
+systematic nature; they afford no insight, consequently, into the
+formative laws of spots. Of these we are indeed far more ignorant than
+Sir William Herschel supposed himself to be. Recent work on the sun has
+provided a grand store of facts ascertained with surprising skill and
+ingenuity. But they want _colligating_. No framework has yet been
+constructed that will hold them, each in its proper place. It has been
+truly said: “Considering the rapid progress which has been made in the
+observational or practical side of solar physics, it must be confessed
+that the theoretical side has been very imperfectly developed. Almost
+every student of solar physics has his own theory, and usually he
+himself is the only one who believes in it.”
+
+Since Sir John Herschel propounded his “cyclonic theory” of sun-spots in
+1847, there has been a marked tendency to assimilate solar to
+terrestrial phenomena. But the circumstances of the two bodies are so
+utterly unlike that such attempts can only prove misleading. The earth
+is a solid globe warmed from without, hence, with hot tropical and
+frigid polar regions. This disparity is the prime motor in the
+circulation of its atmosphere and oceans; a circulation, essentially in
+latitude, directed towards the equalisation of temperature. The sun, on
+the contrary, is heated from within; there is no appreciable difference
+of temperature between its poles and equator; and its circulation is of
+the bodily kind belonging to fluid masses, and is carried on by vertical
+currents effecting exchanges of heat between the surface and the
+profundities beneath. Were these to stop, or even notably to slacken,
+the sun would promptly cease to shine, and lapse into the condition of a
+“dark star.” It is not then surprising that the drifting movements of
+the photosphere are _along_, not _across_, parallels of latitude. Solar
+meteorology, in a word, has almost nothing in common with terrestrial
+meteorology; and explanatory schemes, based upon an analogy which does
+not exist, must sooner or later be consigned to the limbo of vanities.
+
+
+
+
+                              CHAPTER III.
+                        THE SUN’S SURROUNDINGS.
+
+
+“What we ordinarily call the sun,” wrote the late Mr. Ranyard, “is only
+the bright spherical nucleus of a nebulous body.”[13] But it is only
+when the interposing moon cuts off the dazzling rays of the nucleus that
+we see directly anything of its nebular surroundings. Partial or annular
+eclipses are of little or no use for this purpose; the revelation
+belongs exclusively to the sombre, yet splendid moments of totality. No
+sooner has the last glint of sunshine vanished than the corona starts
+into view, encompassing the black lunar globe with a sort of “glory” of
+silvery streamers. Its radiated shape suggests vacillation of form and a
+flickering radiance; yet its immobility is absolute. The awe and wonder
+of the sight tend, for the moment, to supersede scientific curiosity,
+and they are enhanced by the perception, at the base of the corona, of
+the serrated scarlet “chromosphere” fringing the moon’s circumference,
+while the towering “prominences” that are usually seen to spring from it
+produce the startling effect of a conflagration.
+
+These marvellous appendages received no adequate notice until their
+disclosure during the total eclipse of July 8, 1842. Even the
+uninstructed crowds in the streets of Milan and Pavia shouted with
+amazement at what they saw; while by solar students the recurrence of
+similar opportunities has ever since been eagerly anticipated and
+diligently turned to account. The question that first pressed for
+solution related to the local habitation of prominences; for some
+unwisely persisted in attaching them to the moon. A decisive answer was
+given by photography at its first _effective_ application to eclipses on
+July 18, 1860. From a comparison of negatives exposed at the beginning
+and end of totality, it became clearly apparent that the moon had, in
+the interval, moved _over_ the prominences, uncovering, to a small
+extent, those on the west side and concealing those on the east.
+
+Their solar connexion having thus been established by the camera, the
+spectroscope was called upon to determine their physical and chemical
+nature. An admirable opportunity for taking this further step was
+presented by the Indian eclipse of August 18, 1868. The result was
+decisive. The light of a huge spire of flame, 89,000 miles high, had no
+sooner passed through a prism than its gaseous origin declared itself.
+The spectrum consisted of several hydrogen lines, and one unknown line
+in the yellow, slightly more refrangible than the sodium-pair D_{1}, and
+D_{2}, and hence called D_{3}. “Je verrai ces lignes-là en dehors des
+éclipses!” M. Janssen exclaimed, as they caught his eye; and on the
+following morning, at Guntoor in the Neilgherries, he actually started
+daylight spectroscopic work at the edge of the sun. He owed his success
+to a perfectly simple principle. The ordinary invisibility of
+prominences is due to the drowning of their light in reflected sunshine.
+But sunshine, because it is continuous—that is, made up of beams of all
+refrangibilities—can be weakened to almost any extent by dispersion,
+while the detached prominence-rays lose nothing by being separated.
+Hence, the result of passing the mixed light from near the solar limb
+through a train of prisms is that the tell-tale bright lines stand out
+distinctly from an _emaciated_ prismatic background. The method was
+independently discovered by Mr. Norman Lockyer in England, and his and
+Janssen’s communications on the subject were laid before the French
+Academy of Sciences on the same day of October, 1868. It has proved of
+inestimable value, and was further improved in 1869 by Dr. Huggins’s
+device for viewing these objects in their proper shapes through an open
+slit, instead of building them up in narrow sections by successive
+observations through a narrow one. This was made possible by the
+intensity of their light. They can be observed in variously coloured
+images corresponding to the different rays they emit; but the least
+refrangible of the hydrogen series—the blood-red C (alias Hκ)—is
+generally chosen as being the most brilliant and best defined.
+
+The unrecognised substance giving the yellow prominence-line was named
+by Dr. Frankland “helium.” It evidently existed near the sun in enormous
+quantities, and in close companionship with hydrogen. Yet no dark line
+corresponding to its absorption was to be found in the Fraunhofer
+spectrum, although it now and then emerged in spot-spectra. Conjectures
+were rife as to its nature and relations. It was generally believed to
+be specifically lighter than hydrogen, and some held it a product of its
+dissociation, and so of a different elemental standing. Everything about
+it, however, remained doubtful until, in March, 1895, Professor Ramsay
+produced a sample for inspection close at hand, extracted by heat from
+the rare mineral “clevite.” The recognition-mark was its emission, when
+electrically excited, of the solar D_{3}, with which were associated
+several other chromospheric rays previously registered as of unknown
+origin, but now linked together as vibrations of the same molecules. A
+sudden and entirely unlooked-for advance was thus made in the chemistry
+of the sun’s surroundings.
+
+Helium is a colourless gas of about twice the density of hydrogen. Its
+peculiar qualities are shared only by argon, the new constituent of the
+earth’s atmosphere. Both have unusual thermal relations; both are
+chemically inert. They refuse to combine with any other element, and
+thus stand apart from the round of multiform change involving the whole
+material world. Helium is nevertheless distributed freely throughout the
+universe. Hydrogen itself is scarcely more ubiquitous.
+
+A considerable mass of information regarding the solar prominences was
+rapidly collected by means of the Janssen-Lockyer invention. They were
+at once divided into two classes. The “quiescent” kind occur in all
+solar latitudes; they change their shapes very gradually; they have no
+immediate relationship with spots. In form they resemble _pillared
+clouds_ resting in banks like heavy cumuli, or floating, like expanses
+of thin cirrus, high above the chromosphere with which they are
+ordinarily connected by slender supports or conduit-pipes. But these are
+at times invisible or non-existent. Father Secchi occasionally watched
+isolated cloudlets form and grow spontaneously as if by condensation
+from saturated air; and on October 13, 1880, Professor Young made a
+confirmatory observation. About 11 A.M. he noticed a detached fiery mass
+at an elevation of 67,500 miles above the limb. “It grew rapidly,
+without any sensible rising or falling, and in an hour developed into a
+large stratiform cloud, irregular on the upper surface, but nearly flat
+beneath. From this lower surface pendent filaments grew out, and by the
+middle of the afternoon the object had become one of the ordinary
+stemmed prominences.”[14] The size of these formations is enormous. They
+vary in height from about 10,000 to 100,000 miles; and ranges of them
+450,000 miles in extent have been photographed during total eclipses.
+
+[Illustration:
+
+  FIG. 3.—_Curves showing the development of Sun-spots and Prominences
+    during the period 1880 to 1891._ (Sidgreaves.)
+]
+
+The second class of prominences, known as “eruptive,” are obviously
+manifestations of intense energy. In some of their forms they suggest
+geyser-like spoutings of incandescent vapours. They represent swords and
+scimetars, palms with twisted trunks composed of mounting flames,
+igneous vegetation of sundry types. Their chemistry is much more complex
+than that of the quiescent sort. Not only hydrogen and helium, but iron,
+magnesium, sodium, and a number of other metals enter into their
+composition. Belonging to the same order of disturbance with spots, they
+are closely conjoined with them, both in time and space. They conform to
+the sun-spot cycle, as well as to the “law of zones,” showing that
+photospheric and chromospheric disturbances spring from a common cause.
+Fig. 3 (from the _Observatory_ for March, 1893) embodies a comparison
+between the “spotted area” as determined at Greenwich 1880–1891, and the
+“profile area” of prominences (without distinction of kind) observed
+spectroscopically at Stonyhurst during the years 1880–1892. The
+agreement between the two curves is very striking; but the minimum of
+solar activity in 1889 is decidedly better represented by the
+prominence-tracing. Father Sidgreaves, director of the Stonyhurst
+Observatory, adds the important remark that wide-spreading elevations of
+the chromosphere attend spot-maxima, while depressions of equal extent
+occur at minima.
+
+The chromosphere is a solar envelope, but not a solar atmosphere. It
+completely surrounds the sun to the depth of about 4,000 miles with a
+close tissue of scarlet flames, their filamentous or tufted summits
+swaying and intercrossing as if under the gusty sweep of fiery winds.
+Any of these summits which attain an unwonted height become
+“prominences,” but it is a mere matter of convention when the change of
+nomenclature should take place. The chemical composition of the
+chromosphere does not differ essentially from that of prominences. Its
+permanent constituents were found by Professor Young to be hydrogen,
+helium, “coronium,” and calcium, the last represented _only_ by “H” and
+“K.” But disturbances never failed to be indicated by the blaze of
+metallic lines, of which 273 in all have been determined by the same
+authority. Their appearance signified, without doubt, the injection from
+below of the corresponding vapours, chiefly those of iron, titanium,
+sodium, magnesium, strontium, barium, and manganese. At moments the
+reinforcement of the spectrum with bright rays was so extensive that it
+seemed as if the entire “reversing layer” had been uplifted bodily into
+the chromosphere.
+
+The reversing layer lies quite close to the photosphere. It is scarcely
+more than 300 miles deep, and is hence invisible except during about a
+second at the beginning and end of total eclipses. Young was the first
+to be favoured with a sight of it, on December 22, 1870. No sooner was
+the direct solar spectrum intercepted by the moon, than “all at once, as
+suddenly as a bursting rocket shoots out its stars, the whole field of
+view was filled with bright lines, more numerous than one could count.
+The phenomenon was so sudden, so unexpected, and so wonderfully
+beautiful, as to force an involuntary exclamation.”[15] It was
+afterwards frequently observed, and at last satisfactorily photographed
+by Mr. Shackleton, a member of Sir George Baden-Powell’s expedition to
+Novaya Zemlya, for the purpose of observing the total solar eclipse of
+August 9, 1896. The permanent record then secured was of peculiar
+importance as affording the means of confronting in detail the
+components of the vario-tinted flash at the eclipsed sun’s limb with the
+dusky legion of the Fraunhofer lines. The correspondence is striking,
+and leaves no doubt that Young’s stratum is the actual locality where
+the characteristic solar spectrum is produced. It may be described as an
+universal solar ocean of glowing metallic vapours, the rays emanating
+from which, although vivid when seen _off_ the sun, are thrown out in
+dark relief by projection upon the white-hot photosphere. The existence
+of just such a heterogeneous absorbing layer had been predicted, on
+theoretical grounds, some years before it came into view.
+
+The movements taking place in eruptive prominences are often of
+portentous speed. They are betrayed, so far as they coincide with the
+visual ray, by spectroscopic line-displacements; so far as they are
+directed _across_ the visual ray, by immediate observation of the
+spectroscopic images. Thus, the up-and-downrushes of flaming hydrogen
+above spots on the disc reach velocities of 320 miles a second; and
+solar tornadoes (detected by Mr. Lockyer more than a quarter of a
+century ago) are often observed to whirl at rates which would be
+incredible were they less well authenticated. Vertical explosions at the
+limb, on the other hand, of still more unruly violence are rendered
+manifest by displacements, not of the emitted lines, but of the
+radiating substances themselves.
+
+On September 19th and 20th, 1893, Father Fényi, director of the Kalocsa
+Observatory in Hungary, witnessed the development and dissolution of a
+pair of objects perhaps the most extraordinary in the astonishing record
+of solar phenomena.[16] They broke out within nineteen hours of each
+other, showed a close similarity of shape and structure, underwent
+analogous changes, and, strangest of all, were situated at almost
+diametrically opposite points of the solar limb. The first was already,
+when first viewed at 2 P.M., 168,000 miles high; within half an hour, it
+had sprung up to 224,000 miles (8′ 18″), and again subsided into a
+commonplace flame of the modest dimension of 13,650 miles (30″). The
+rate of ascent, directly measured (always necessarily through the medium
+of the spectroscope), was 132 miles a second. This vast, though
+transient construction, seemed to be formed of a multitude of distinct
+fiery tongues, each leaping and flaring independently. As a whole, it
+was also tongue-shaped, and “stood erect nearly in the direction of the
+sun’s radius,” travelling, meanwhile, towards the earth at an average
+rate of 186 miles a second.
+
+The companion-prominence began to show at nine next morning, and, rising
+with a velocity of 300 miles per second, attained in twelve minutes to a
+height of 220,000 miles. This tremendous apparition was of the same
+“ragged” texture as its predecessor, and shone, even in its loftiest
+fragments, with the same intense glow. As might have been expected from
+its opposite position, its radial movement was _from_ the earth. A
+prominence measured by the same observer, July 15, 1895, was diminishing
+its distance from the earth with the extraordinary velocity of 533 miles
+a second; and on September 30 of the same year, a colossal object
+resembling the bent and riven trunk of a great tree, was in the course
+of half an hour flung upwards to a minimum altitude of 313,000 miles,
+and had again faded out of sight. “The appearance,” Father Fényi wrote,
+“of all the numerous great eruptions which I have observed has been such
+as would be produced by a kind of explosion over a spotted region,
+which, seizing upon a prominence already developed, hurls it upward from
+the surface, tears it to pieces, and brings it to a speedy end.” The
+matter thus acted upon is of enormous volume, but negligeable mass.
+
+Photographs of prominence-spectra, obtained by Dr. Schuster during the
+eclipse of May 17, 1882, brought out the remarkable predominance in
+their light of the “H” and “K” emissions of calcium. It was
+re-discovered by means of spectrographs of those objects, taken in 1891
+without an eclipse, by Professor Hale at Chicago, and by M. Deslandres
+in Paris. Both investigators promptly seized upon the advantage it
+offered for their chemical delineation in full daylight. The lines in
+question are dark and abnormally wide in the sun itself, bright and
+sharp in prominences. Thus, at these particular parts of the spectrum,
+the obliterating effects of scattered sunlight are non-existent. Just
+here, too, photographic sensitiveness is at its maximum. Hence, by
+working with either of these lines (K is preferable) nothing could be
+easier than to get impressions of the brilliant forms of prominences
+relieved against the background of solar absorption. (See Figures 4 and
+5.) The thin, bright line is _sheltered_ from daylight glare by the
+dusky, broad one. By the use of a “double slit,” the method was
+completed. This, again, was simultaneously invented by Hale and
+Deslandres, although they had, without suspecting it, been anticipated
+by Janssen in 1869. The second slit is adjusted so as to exclude all but
+a single ray of the spectrum formed by dispersing the light admitted
+through the first. An unlimited power of selection is in this way
+afforded as to the quality of light to be employed; but for general
+purposes, K is not likely to be superseded.
+
+In the Chicago spectroheliograph, two moveable slits, together with a
+powerful diffraction spectroscope, are attached to a twelve-inch
+refractor. With this instrument, monochromatic impressions of the sun
+with its spots, faculæ, and flame-garland are obtained without
+difficulty. To begin with, the solar disc is covered with a metal
+diaphragm, then the first slit is caused to traverse the artificially
+eclipsed image, the second following at such a rate that the K line
+alone always falls upon the sensitive plate. The result is a complete
+photographic record of the chromosphere and prominences. The diaphragm
+having been then removed, the return journey of the slits is very
+quickly made, so as to guard against the formidable actinic strength of
+even that small element of direct sunlight contained in the K line. The
+object of the second transit is to _insert_ an autographic print of the
+sun itself into the space previously left blank to receive it. The
+entire operation occupies less than one minute. Portrayed thus in
+calcium light, the solar disc has a strange effect. It is entirely
+overspread with a reticulation of irregular bright markings, greatly
+emphasized over the spot-zones, and corresponding in general with the
+positions of faculæ. According to Professor Hale, these masses and
+wreathings of calcium vapour _are_ faculæ. M. Deslandres regards them
+rather as gaseous formations connected with faculæ. Their extension and
+intensity are at times so great that M. Deslandres has actually
+succeeded, through the prevalence of their light, in photographing the
+sun as a “bright-line star.” The double-slit method also affords the
+means of studying the distribution of each element of the reversing
+layer in the leisure of ordinary daylight, as M. Deslandres has shown by
+some preliminary experiments.[17]
+
+To this extent astronomers have made themselves independent of eclipses.
+These momentous occurrences are, fortunately, not needed for researches
+concerned with distinct coloured rays separable by dispersion from
+diffuse sunshine. But with the corona it is different. For here we have
+a white glory to deal with. Coronal light is derived from three sources:
+from the original incandescence of solid or liquid particles, from
+sunshine reflected by them, and from gaseous emissions. The most
+conspicuous of these is a green ray of unknown chemical meaning. It
+proceeds from every part of the corona, even from the dark rifts
+separating its brilliant streamers, and the inconceivably tenuous
+substance to which it owes its origin has, accordingly, received the
+name of “coronium.” The coronal spectrum includes many other bright
+lines, especially in the ultra-violet, photographed during eclipses; but
+the hydrogen, helium, and calcium lines which accompany them probably
+represent scattered chromospheric light.
+
+[Illustration:
+
+  FIG. 4.—_Eruptive Prominence photographed by Professor Hale at the
+    Kenwood Observatory, March 24, 1895, at 22h. 40m. Chicago mean
+    time._ (_The photosphere is covered with a metallic disc._)
+]
+
+[Illustration:
+
+  FIG. 5.—_The same, 18m. later._
+
+  (From the _Astrophysical Journal_, May, 1896.)
+]
+
+The green coronal ray is much too faint to be isolated with the
+spectroscope; but the continuous coronal spectrum has maxima of
+intensity compared with ordinary daylight, which suggested to Dr.
+Huggins, in 1882, a differential method of photographing the entire
+structure apart from eclipses. It has however, as yet come to nothing,
+and Hale and Deslandres have been equally unsuccessful with their
+“double slit” apparatus. Hence, it is only by favour of the moon that
+this wonderful appendage can be investigated, and the available moments
+have not been allowed to pass in vain.
+
+[Illustration:
+
+  FIG. 6.—_The Eclipsed Sun, photographed at Sohag in Egypt, May 17,
+    1882. A Comet is almost involved in the Corona._ (From
+    “Philosophical Transactions,” vol. clxxv.)
+]
+
+One result fully ascertained is that it changes in form concurrently
+with the progress of the sun-spot period. The maximum coronal type is
+entirely different from the minimum type, and reappears in unmistakable
+connexion with vehement solar disturbance. This cyclical relation was
+first pointed out by Mr. Ranyard. On July 29, 1878, a totality of 165
+seconds was observed, under splendid conditions of weather, in the
+Western States of North America. No prominences worthy of note were
+visible, but the corona wore a most surprising aspect. A pair of
+enormous equatorial streamers stretched east and west of the sun to a
+distance of at least ten millions of miles. Indeed, they came to no
+definite end. They were best seen with the naked eye, and made no show
+on sensitive plates, but the application of low telescopic powers
+disclosed, near the base of the effusions, a mass of delicate and
+complex detail. The solar poles were as distinctively, although not so
+strikingly, garnished as the solar equator. Each was the centre from
+which diverged a dense brush of straight, electrical-looking rays. The
+sun was at the time in a state of profound tranquillity; and it was
+recalled that, at the previous minimum, in 1867, Grosch had delineated,
+at Santiago, just the same equatorial extensions, and just the same
+polar brushes. The connexion was emphasised during the maximum of
+1882–4, by the substitution, when the moon covered the sun on May 17,
+1882, and May 6, 1883, of a dazzling stellate formation for the winged
+corona of 1878. In Fig. 6 is reproduced a photograph by Dr. Schuster of
+the Sohag, or Egyptian corona, with the added embellishment of a comet
+hurrying up to perihelion, conspicuous to the eye at the time, but never
+seen again.
+
+In 1889 the minimum type of corona reasserted itself. A drawing made by
+Miss M. L. Todd during the eclipse of January 1, gave the characteristic
+equatorial “fish-tails,” reaching out on the west to four solar
+diameters.[18] And although the camera, owing to special difficulties,
+has not yet been able to pursue them so far, Professor Barnard’s
+exquisite picture (Fig. 7), taken at Bartlett’s Springs, California,
+with an exposure of 4½ seconds, portrays the type to perfection, with
+its suggested indefinite expansions, “the soft feathery details of the
+inner corona, and the delicate fan-structures at the poles.” Two minute
+notches mark the points where a couple of prominences have, by the
+intensity of their actinic power, _eaten into_ the black circumference
+of the lunar image.
+
+[Illustration:
+
+  FIG. 7.—_The Corona of January 1, 1889, photographed by Professor E.
+    E. Barnard._
+]
+
+Nine negatives were secured by the artist, but at a considerable
+personal sacrifice. “So impressive,” he wrote, “was the magnificent
+spectacle upon the crowd that had gathered just outside our enclosure,
+that not a murmur was heard. The frightened, half-whining bark of a dog,
+and the click-click of the driving clock, alone were audible. When the
+sun suddenly burst forth, an almost instantaneous and highly-surprised
+cackling of the chickens, that had hastily sought their roosts at the
+beginning of totality, would have been amusing could one have shaken off
+the dazed feeling at the unexpectedly rapid termination of the
+semi-darkness. My own feelings were those of excessive disappointment
+and depression. So intent was I in watching the cameras and making the
+exposures, that I did not look up to the sun during totality, and
+therefore saw nothing of the corona.”
+
+On April 16, 1893, at the height of the last sun-spot maximum, a
+shadow-track crossed South America and Central Africa. Once more the
+coronal type had changed. Not a trace remained of the equatorial
+“wings”; not a trace of the polar “fans.” Instead, the “compass-card”
+aureole of 1882 and 1883, shaped regardless of heliographic latitude,
+reemerged from beneath the veil of daylight. That the sun’s filmy
+“crown” follows, after its own inexplicable fashion, the general round
+of solar vicissitudes, no longer admitted of a doubt. The fact is thus
+stated by M. Deslandres, who observed the eclipse at Fundium, in the
+Senegal district.
+
+“The form of the corona,” he says, “undergoes periodical variations,
+which follow the simultaneous periodical variations already ascertained
+for spots, faculæ, prominences, auroræ, and terrestrial magnetism. This
+important relation, indicated by preceding eclipses, is strongly
+confirmed by the eclipse of 1893.”[19]
+
+Professor Schaeberle’s photographs, taken on the same occasion at Mina
+Bronces in Chili, marked a decided advance in coronal portraiture. The
+sun’s disc measured four inches on his plates, exposed with a
+photoheliograph forty feet in length; and the details of inner coronal
+construction came out accordingly with unprecedented perfection. The
+corona of August 9, 1896, reproduced the most striking features of the
+corona observed August 29, 1886; and both corresponded to an
+intermediate epoch of the spot-cycle. The polar brushes were present
+without the equatorial extensions, while in both a protruding ray made
+an angle of some thirty or forty degrees with the solar axis. This
+distinctive trait imprinted itself with surprising emphasis on some of
+Sir George Baden-Powell’s Novaya Zemlya photographs.
+
+Researches, prosecuted under cover of eighteen eclipses, have greatly
+strengthened the visible analogy between coronal streamers, auroral
+coruscations, and comets’ tails. The persuasion that electrical
+discharges in high vacua are concerned in all these phenomena is not
+easily resisted. Repulsive forces such as are at work in Crookes’ tubes
+perhaps come into play, on the vast solar scale, to produce the strange
+and beautiful luminous forms revealed during eclipses. Their tenuity is
+certainly extreme. They probably contain very much less matter, volume
+for volume, than the incredibly exhausted tubes of modern physicists.
+The unresisted passage of comets through the corona demands this
+supposition, which is in complete accord with the fineness of the
+Fraunhofer lines. The corona shows no increase of density downwards, and
+the chromosphere very little. Hence neither can be a true solar
+atmosphere, weighing freely upon the sun’s surface. For, under the
+immense power of solar gravity, the accumulated pressure of the
+superincumbent layers, even if there were only one hundred miles’
+thickness of them, could not be intelligibly conveyed in figures; how
+much less when the piling-up of the aerial strata is reckoned by
+thousands of miles!
+
+To recapitulate. Starting from the photosphere, we meet first an
+envelope producing the _general_ absorption, by which sunlight is
+enfeebled and reddened as if by the interposition of a slightly rufous
+shade. Next comes the reversing layer composed of mixed incandescent
+vapours, giving rise, by their _selective_ absorption, to the Fraunhofer
+lines. No alterations in correspondence with the spot-cycle have yet
+been determined in either of these couches, which, close as they lie to
+the photosphere, remain, nevertheless, apparently indifferent to its
+agitations. They are overspread by the chromosphere and prominences;
+while above and beyond shines the mysterious corona; both chromosphere
+and corona strictly conforming, by manifest changes, to the sun’s
+periodicity. One other solar appendage remains to be noticed.
+
+After sunset in spring, and before sunrise in autumn, a mass of soft
+luminosity, often brighter than the Milky Way, may be seen tapering
+upward from the horizon along an axis approximating to the line of the
+ecliptic. Its more conspicuous visibility at those times just reverses
+the case of the harvest moon. As a rule, the apex of the cone barely
+reaches the Pleiades; but it does not really end here. Thrice during the
+present century, by Brorsen, Backhouse, and Barnard, the zodiacal
+“counterglow” has been independently discovered and studied. This is a
+hazy, luminous patch, ten to fifteen degrees across, and exactly 180°
+from the sun. It represents the _opposition aspect_ of the Zodiacal
+Light, hence proved to be a formation in planetary space, extending
+considerably beyond the earth’s orbit. Two plausible hypotheses as to
+its nature have been proposed. Professor Searle[20] holds it to
+represent the reflection of sunlight from “an infinite number of small
+asteroids.” Professor Bigelow[21] considers it as an amassment in the
+plane of the sun’s equator—“a place of zero potential”—of the particles
+electrically expelled from the poles. The Light is then, if this view be
+correct, an extension of the corona—a sort of “pocket or receptacle,
+wherein the coronal matter is accumulated and retained as a solar
+accompaniment.” A continuous spectrum is derived from it; no element of
+original emission can be detected; so that the spectroscope “holds the
+balance even” between the two theories. If, however, the latter were
+true, the Zodiacal Light should spread out from the sun’s equator; if
+the former, then its medial plane should deviate very slightly from that
+of the ecliptic, to which the fundamental, or “invariable” plane of the
+solar system is inclined only one and a half degrees. M. Marchand’s
+observations from the Pic du Midi[22] appear to be decisive on the
+point. During three years, he mapped down the limits assigned by his
+observations night after night, to an emanation which, in that pure air,
+was seen to compass the entire sphere. The eventual comparison of his
+collected data showed its axis to be a great circle sensibly coincident
+with the sun’s equator. All reasonable doubt as to the nature of the
+Zodiacal Light has thus been removed. It is a reservoir for the sun’s
+waste matter—the sink, into which are daily flung the particles rejected
+through the agency of the aigrettes and streamers composing the
+wonderful eclipse-vision of the corona.
+
+
+
+
+                              CHAPTER IV.
+                         THE INTERIOR PLANETS.
+
+
+The Interior Planets are those which revolve within the earth’s orbit.
+They are two in number—Mercury and Venus. Mercury, with a diameter of
+three thousand miles, is the smallest of the eight principal planets. It
+pursues a track, too, more eccentric and more highly inclined to the
+ecliptic than any other planetary orbit. The zodiac had of old to be
+made 16° wide in order to afford room for its excursions. These
+irregularities are, however, quite innocuous as regards the stability of
+the system, for the reason that they belong to a body of insignificant
+mass. The successive approaches to it of Encke’s comet have afforded a
+means of ascertaining its gravitative power; and, according to the
+latest report from this filmy messenger, it is even less than had been
+supposed. Mercury, it appears, weighs little more than one-ten-millionth
+of the sun, or one-thirtieth of the earth. And since its volume is about
+one-nineteenth the terrestrial, the matter of which it is composed must
+be less dense in the proportion of 30 to 19. So that the planet would
+turn the balance against one equal globe of granite, or three and a half
+of water. We can hence easily calculate that gravity, at Mercury’s
+surface, possesses less than one-fourth its power at the earth’s
+surface. A man of sixteen stone transported thither, would find himself
+relieved of fully three-quarters of his habitual burthen.
+
+The plane of Mercury’s orbit makes an angle of 7° with the ecliptic, and
+he traverses it with a speed varying from 23 to 35 miles a second. The
+corresponding distances from the sun are 43½ and 28½ million miles,
+while the mean distance, or semi-major axis of the ellipse, measures
+just 36 millions. Independently then of what we call seasons, Mercury is
+subject, in the course of its year of 88 days, to considerable
+vicissitudes of temperature. At perihelion it receives nine times, at
+aphelion only four times, more heat than is imparted by the sun to an
+equal area of the earth.
+
+The crucial point as regards the physical condition of a planet is the
+presence or absence of an atmosphere. And there is decisive evidence
+that Mercury is in this respect poorly provided. Certain luminous
+phenomena, often observed during its transits across the sun, appear to
+be of purely optical production, since they are less conspicuous with
+good than with indifferent telescopes; while, on the other hand, genuine
+refractive effects are absent. A corresponding indication is afforded by
+the low “albedo,” that is, the slight reflective power of this planet.
+Of the light flooding its surface only 17 per cent.[23] is returned; 83
+per cent. is absorbed. Now the albedo of clouds is about 72; a
+cloud-wrapt globe is little less brilliant than if it were covered with
+fresh-fallen snow. Hence a high albedo accompanies a dense, vapour-laden
+atmosphere; a low albedo indicates a transparent one. And since Mercury,
+which sends back only about as much light as if it were made of grey
+granite, has the lowest albedo of any of the principal planets, it may
+be safely concluded to possess the thinnest aerial covering. Yet it is
+not, apparently, a totally airless globe. Spots upon its surface have
+been seen to become effaced as if by atmospheric veilings; and the
+spectroscope hints (although doubtfully) at aqueous absorption.
+
+Mercury is “new” when nearest to the earth, and “full” when most remote
+from it. At both these periods, moreover, its position with regard to
+the sun renders it ordinarily invisible; so that it is usually seen as
+either gibbous or crescent shaped. The study of its phases has brought
+out a noteworthy circumstance. It is easy to understand that geometrical
+light changes will not proceed by the same gradations upon a smooth and
+upon a rugged globe, where they are complicated by irregular shadows and
+illuminations. The laws of variation are quite different in each case,
+and their respective prevalence can be distinguished by steady
+observation. There seems no reason to doubt that the latter are obeyed
+by Mercury. After several years’ watching of its phases, Professor G.
+Müller[24] of Potsdam concludes them to be such as characterise a broken
+and uneven surface.
+
+[Illustration:
+
+  FIG. 8.—_Map of Mercury, by Schiaparelli._ (From _Astronomische
+    Nachrichten_, No. 2944.)
+]
+
+Little or nothing was known about the rotation of Mercury when
+Schiaparelli of Milan undertook its determination in 1882. His
+observations were made in full daylight, in order to reduce atmospheric
+disturbances to a minimum; and he executed, in the course of a few
+months, a series of 150 Mercurian delineations upon which is founded the
+planisphere exhibited in Fig. 8. The surface of the planet, coloured
+light rose with a coppery tinge, was seen to be diversified by
+brownish-red markings which became effaced towards the limb as if
+through atmospheric absorption. Although evidently of a permanent
+nature, their outlines escaped precise definition. The most remarkable
+circumstance about them was that they showed no effects of rotation.
+During several consecutive hours of watching, they remained sensibly
+fixed in their places. The conclusion was finally arrived at that
+Mercury rotates on a nearly upright axis in the same time that it
+revolves round the sun. Its day, no less than its year, is equal to 88
+of our days. Consequently it turns at all times substantially the same
+face towards the sun; and the “terminator,” that is, the dividing-line
+between darkness and light, only “librates,” without travelling right
+round the globe. The librations of Mercury are, however, extensive in
+proportion to the eccentricity of its orbit; hence, five-eighths of its
+surface come in for some share of illumination during the Mercurian
+year. Over the remaining three-eighths darkness reigns supreme.
+
+                 “There is no light in earth or heaven,
+                   But the cold light of stars.”
+
+Satisfactory confirmation of this curious result was obtained by Mr.
+Percival Lowell at the Flagstaff Observatory in Arizona during the
+autumn of 1896.[25] In Schiaparelli’s map, the axis of rotation lies in
+the plane of the paper, and the centre of the projected sphere thus
+represents the point on Mercury’s surface where the sun is vertical at
+perihelion and aphelion; A and B, 23° 41′ to the east and west of it
+respectively, marking the places where the sun is vertical at the
+libration-limits. That formidable luminary oscillates from the zenith of
+A to the zenith of B and back in 88 days, occupying, in consequence of
+the planet’s unequal motion, 51 in describing the arc from east to west
+(left to right), but only 37 in retracing it from west to east.[26]
+
+The effects of these arrangements upon climate must be exceedingly
+peculiar. They cannot readily be traced in detail; but, thin as the
+Mercurian atmosphere is, it must be to some extent operative in
+modifying the contrast in temperature between the two hemispheres.
+Except in a few favoured localities, the existence of liquid water must
+be impossible in either. Mercurian oceans, could they ever have been
+formed, should long ago have been boiled off from the hot side, and
+condensed in “thick-ribbed ice” on the cold side.
+
+Mercury is then, according to our ideas, totally unfitted to be the
+abode of organic life. Nor can it at any time have been more favourably
+circumstanced than at present. We need not hesitate to assert that its
+rotation was reduced to its actual minimum rate by the power of tidal
+friction. The brake was, moreover, applied by the sun. The attainment of
+rapid gyration was prevented by the resistance of solar tides raised on
+a plastic mass. Disruption was accordingly rendered impossible. The
+planet was, by anticipation, deprived of satellites, and remained
+undivided and solitary.
+
+Venus, the earths nearest planetary neighbour, might be called its twin.
+Its diameter being 7,700 miles, it is of nearly the same size; it is not
+greatly inferior in mean density; gravity at its surface is of more than
+four-fifths its terrestrial strength, and it is supplied with an
+extensive atmosphere. Its movements are placid and well-regulated. In a
+period of 225 days it revolves at the rate of 22 miles per second in an
+almost circular track, deviating but slightly from the plane of the
+ecliptic. Its distance from the sun is 67,200,000 miles; hence it
+receives just twice as much heat and light as the earth. Moreover, it
+reflects at least 65 per cent. of the light incident upon it. Viewed in
+the same telescopic field with Mercury during a close conjunction in
+1878, it shone, James Nasmyth reported, like burnished silver, while
+Mercury appeared as dull as zinc or lead. Yet Mercury is illuminated, on
+an average, three and a half times more intensely than its neighbour.
+
+Atmospheric effects are conspicuous on Venus. At the beginning and end
+of transits, the part of the little black disc off the sun, has
+constantly been seen silver-edged through refraction; and when the
+planet, at inferior conjunction, passes above or below the sun, its
+whole circumference is not unfrequently bordered with a halo of solar
+rays, bent inwards as if by the action of a lens. Just in the same way,
+the _geometrical_ rising of the heavenly bodies is _visually_
+anticipated, and their setting delayed on the earth, by the curvature of
+the beams refracted in passing through its atmosphere—or rather, through
+half of it; while we, as spectators of Venus from the outside, perceive
+the entire effect. Made on equal terms, the comparison is greatly to the
+disadvantage of the earth. Refraction, as directly measured on Venus,
+considerably exceeds its terrestrial amount; and the measurable
+refraction is only that produced in the higher part of the air
+surmounting the shell of clouds which constitutes the planet’s visible
+surface. Thus, at the cloud-level a barometer would, by the lowest
+estimate, stand at 35 inches, while at the same altitude of, say, two
+miles, the column of mercury would, on the earth, drop to 21 inches. It
+is, indeed, very likely that the aerial envelope of Venus weighs twice
+as much as our own.
+
+The occasional visibility of the dark side of Venus is still
+unexplained. The appearance is indistinguishable except in scale from
+that of the “old moon in the new moon’s arms”; but illumination by
+earthshine, which is fully competent to produce the lunar effect,
+practically vanishes at the distance of Venus. The “ashen light,” as it
+is called, ordinarily shows only when the planet figures as a narrow
+crescent; but M. Brenner of the Manora Observatory, who has a knack of
+being unprecedented, saw it in June, 1895,[27] during the gibbous phase.
+The appearances of this pale gleam follow no traceable law. They occur
+unsought; and are recalcitrant to vigilant expectation. Their closest
+analogy is with our auroræ. The “phosphorescence” of the dark side of
+Venus may quite reasonably be set down as of an electrical nature. But
+it does not seem, like terrestrial auroræ, to follow the lines of a
+magnetic system.
+
+Distinct spectroscopic indications of aqueous absorption in the
+atmosphere of Venus were perceived, during the transits of 1874 and
+1882, by Tacchini, Riccò, and Young. They accord well with the
+“snow-caps,” which are one of the many puzzling Cytherean features.
+Since these can be resolved into groups of brilliant points, they
+represent, in the opinion of the late M. Trouvelot, mountainous
+formations penetrating the reflective stratum, and shining, lustrous
+with snow, in the clear upper air. They might almost equally well be
+cloud-like condensations of a permanent kind, called into existence by
+topographical peculiarities, and hence, after a fashion, _rooted in the
+soil_. On the other hand, Mr. Lowell questions their reality in any
+form; and his drawings represent extraordinarily sure seeing.
+
+[Illustration:
+
+  FIG. 9.—_Venus, from a drawing by Mascari._ (_Nature_, February 20,
+    1896.)
+]
+
+The only point regarding the planet’s rotation upon which astronomers
+are agreed is that its axis is nearly perpendicular to the place of its
+orbit. As to its period, the divergence is enormous. It reaches all the
+way from 24 hours to 225 days. Bad as is the telescopic holding-ground
+on Mercury, that afforded by Venus is worse still. The disc falls off
+rapidly in brightness from the limb towards the terminator, and is
+sometimes diversified by filmy and indefinite markings, obviously of
+atmospheric origin (in Fig. 9 the shadings are much too pronounced).
+Attempts to use them as fiducial points are foredoomed to failure. The
+period, accordingly, of 23^h 21^m arrived at by forcing into artificial
+agreement the observations of Cassini at Bologna, of Bianchini and De
+Vico in Rome, obtained small credit. The subject lay, as it were,
+dormant until Schiaparelli made, in 1890, the provisional announcement
+that Venus rotates on the same plan as Mercury. A clamour of
+contradiction was immediately raised, and a large amount of evidence on
+both sides of the question has since been collected. It is curious to
+notice that, setting aside the opposite conclusions of Terby and
+Brenner, the Alps mark a dividing-line between the pros and the cons.
+Schiaparelli’s period of 224·7 days (ratified by himself in 1895) is
+supported by Perrotin’s observations both at Nice and Mont Mounier; by
+Tacchini’s at Rome, Cerulli’s at Teramo, and Mascari’s at the
+complementary establishments of Catania and Mount Etna; while Niesten,
+Trouvelot, Villiger, Stanley Williams, and Flammarion, all under some
+disadvantage as regards climate, aver that the debated gyration is
+performed in “about” 24 hours. Now, in the first place, a period of 24
+hours is in itself open to suspicion, since all delicate observations
+are liable to be affected by diurnal atmospheric variations; in the
+second, it is mainly, if not entirely, based upon supposed changes in
+almost evanescent shadings, while the long period of 224·7 days has been
+derived fundamentally, from the immobility relative to the terminator,
+of definite and permanent topographical features. The perfect roundness
+of the disc of Venus affords independent proof of extremely slow
+rotation.
+
+Spectroscopic evidence may before long become available. The quantity to
+be measured by the exquisite method of line-displacements is, indeed, at
+the most extremely small. The equatorial velocity of Venus would, with
+the 24-hour period, but slightly exceed a quarter of a mile a second;
+but this effect being doubled by reflexion from the planet, and doubled
+again by juxtaposition of light from its east and west limbs, could
+probably be made distinctly perceptible. In the negative case, the value
+of the support lent to the long-period hypothesis can only be appraised
+by the degree of refinement attained in the research.
+
+The “long-period hypothesis” has, however, almost ceased to need such
+support. Schiaparelli’s facts are inconsistent with any other; and they
+are scarcely controvertible. They have besides, as in the case of
+Mercury, been verified by Mr. Lowell’s recent observations. Assuming,
+then, its truth, we may consider what it implies. Since the rotation and
+revolution of Venus synchronise, she always looks inwards toward the
+sun, perpetual day reigning on one hemisphere, perpetual night on the
+other. And these regulations are much more strictly conformed to than on
+Mercury. For the orbital motion of Venus is so nearly uniform that
+libratory effects count for very little. The equatorial breadth of the
+libration-zones, where light alternates with darkness, is only
+thirty-three miles. On the other hand, the atmospheric diffusion of
+sunshine is a powerful illuminating agency. The meteorology of the
+planet presents great difficulties. Its conditions are so remote from
+our experience that we can barely sketch out their results. The most
+obvious of these is the vehement aerial circulation which must proceed
+without ceasing between the hemisphere upon which the sun never rises
+and the hemisphere upon which the sun never sets. We should expect it to
+be accompanied by agitated conflicts of winds, and surgings of the
+atmosphere from its lowest to its highest strata, betrayed by rendings
+of the brilliant condensation-canopy, by the rapid transport of torn
+scuds, and wheeling vortices of clouds. But nothing of all this is
+telescopically visible. The aspect of the morning star suggests serenity
+rather than interior tumult.
+
+One of the most remarkable instances of persistent optical illusion
+refers to a supposed satellite of Venus. It was first seen by Fontana at
+Naples in 1645; it was last seen by Horrebow at Copenhagen in 1768; and
+the intermediate observations were numerous, usually careful, and
+apparently authentic. Yet the body, of which they affirmed the
+existence, was purely fictitious; and it is a suggestive circumstance
+that it never ventured into the field of view of an achromatic lens.
+
+Comparing the two planets nearest to the sun, the first spontaneous
+impression is of astonishment at their unlikeness. One travels in an
+almost circular, the other in a highly eccentric orbit. One possesses a
+dense and extensive atmosphere; the other is barely gauze-clad, and is
+hence exposed to almost unmitigated extremes of temperature, while the
+conformation of its solid surface is left open to telescopic scrutiny,
+impeded only by the inconvenient glare of the sun. That surface is of a
+reddish hue, and absorbs more than four-fifths of the light with which
+it is flooded; the disc of Venus being, on the contrary, of a dazzling
+whiteness, and little less reflective than a summer cloud. Yet these two
+globes, so dissimilar individually, have apparently had the same destiny
+prepared for them. Deprived of all but a remnant of their rotation by
+the frictional resistance of sun-raised tides, they were debarred from
+the production of satellites, and subjected to what we, in our
+ignorance, might be apt to call fantastic climatal conditions. With due
+reserve it may be added that they have thus apparently been rendered
+unfit to be the abodes of highly developed organisms. Why this has been
+so ordained we are unable to conjecture; we must wait to know.
+
+
+
+
+                               CHAPTER V.
+                          THE EARTH AND MOON.
+
+
+The earth occupies a critical position in the solar system. Its greater
+distance from the sun preserved it from the fate of Mercury and Venus.
+The influence of solar tidal friction fell short of predominance over
+the terrestrial future. All that it could do was to defer to the latest
+possible moment (so to speak) the separation of the moon, the
+comparatively large size of which was doubtless due to this
+postponement. For a viscous body, such as the earth must then have been,
+can bear much more rotational strain than a less coherent mass; but when
+the strain comes to be relieved, the needful sacrifice of material is
+proportionally greater. The process of fission, instead of being a mere
+incident, becomes a catastrophe. The most violent explosions are
+precisely those which are longest delayed.
+
+Had the earth then been situated a few millions of miles nearer to the
+sun there would have been, so far as we can see, no moon; and the
+terrestrial day and year would have been of equal length. This
+equalisation was rendered impossible by lunar influence.[28] We are
+indebted to our satellite for the alternations of day and night which
+make life possible. How this came about is quite clear upon some brief
+consideration. Lunar tides are now about three times more effective than
+solar tides, and at their origin the disproportion was enormous. Their
+power might be called exclusive. Now, how was that power exercised?
+Primarily, in compelling an agreement between the duration of the month
+and day—that duration, to begin with, being of only a few hours. The day
+might, and in the long run did, fall short, but it could not possibly
+get ahead of the month. Hence the earth’s rotation was for ages
+protected against the destructive agency of solar tidal friction. By the
+time that the moon left it, as it were, to take care of itself, the
+plastic stage, during which alone rapid change could take place, had
+passed, and the earth was solid and secure.
+
+Thus, the axial rotation of our planet in twenty-four sidereal hours is
+the outcome of a delicate balance of relations established in the “deep
+backward and abysm of time.” Its shape matches, or has accommodated
+itself to the period, which has perhaps not varied much since the epoch
+when interior fires were first banked in by the formation of a rigid
+crust. The compression of rotating globes is so connected with the
+quickness of their spinning that one can be calculated from the other;
+and the earth’s theoretical compression, or ellipticity, is found to be
+practically identical with its measured ellipticity of about ¹⁄₂₉₃. Its
+mean diameter is 7,927 miles; the equatorial is 26 miles longer than the
+polar diameter; so that the globe is belted with a protuberance, 13
+miles high, corresponding to the excess of centrifugal force at the
+Equator.
+
+The heat by which it was originally maintained in a liquid condition is
+still in process of dissipation. A small part escapes year by year, but
+enough remains to keep the earth _alive_ for ages to come. Were the
+supply exhausted, the oxygen of our air, and the water forming our
+oceans, would be rapidly absorbed, chemically and mechanically, and with
+them, vitality should disappear. Volcanic action, in some of its many
+forms, is accordingly a condition of existence. One unmistakable symptom
+of central fires still glowing is the increase of subterranean
+temperature. It averages one degree Fahrenheit for fifty-five feet of
+descent. Below two miles then, water can only remain liquid through the
+compulsion of the overlying strata, the slightest relaxation of which
+occasions it to flash explosively into steam; the devastating power of
+“super-heated” water being one of the chief causes of volcanic
+outbreaks. The growth of temperature downward cannot be supposed to
+proceed indefinitely; otherwise, a fabulous thermal state would be
+reached long before we got near the core of the globe; but the region of
+maximum heat depends upon an unknown quantity—that is, the lapse of time
+since the antique lava-globe began to crust over. Assuming it to be
+fifty million years, Lord Kelvin showed that the limiting temperature of
+about 5,400° F. is located not more than fifty miles from the surface.
+But 5,400° approaches the temperature of the electric arc, at which
+there is an all but universal vaporisation of material substances, and
+rocks liquefy while comparatively cool. Diabase, for instance, a typical
+basalt, is completely fluid at 2,200° F. On the other hand, the pressure
+at 50 miles beneath the earth’s surface is of inconceivable power; and
+it is employed in resisting the expansive tendency of heat. The
+condition of matter subjected to these opposing and potent influences we
+are unable to divine, and have no means of ascertaining. We do, however,
+know from the results of various astronomical lines of enquiry that the
+earth is effectively as rigid as steel. Its mean density is about five
+and a half times that of water, the entire globe being more than twice
+as heavy as if made of the ordinary surface rocks. This, however, is not
+surprising, since oxygen enters largely into the composition of the
+exterior strata, while the subjacent materials are likely to be in large
+measure metallic.
+
+The epoch of the earth’s superficial solidification has again, quite
+lately, been under discussion. “The subject,” Lord Kelvin wrote, “is
+intensely interesting. I would rather know the date of the
+_Consistentior Status_ than of the Norman Conquest; but it can bring no
+comfort in respect to the demand for time in palæontological geology.
+Helmholtz, Newcomb, and another (Kelvin) are inexorable in refusing
+sunlight for more than a score, or a very few scores of millions of
+years.”[29]
+
+Improved data having been substituted, the problem was solved anew, with
+the result of very notably diminishing the “age of the earth.” It is for
+the present fixed at twenty-four million years, and upon such strong
+evidence as to “throw the burden of proof upon those who hold to the
+vaguely vast age derived from sedimentary geology.”[30]
+
+The earth is the largest of the terrestrial planets; and it is
+specifically the heaviest of all the planets. Its compactness is more
+likely to be a consequence of a particular relation between internal
+temperature and pressure, than of a difference in chemical constitution.
+
+The mass of its atmosphere can be directly determined. We have only to
+look at a barometer in order to gain the information that our “cloud of
+all-sustaining air” weighs as much as a universal ocean of mercury
+thirty inches in depth. The corresponding depth of air, were it of the
+same density throughout, would be nearly five miles. But it is _not_ of
+the same density throughout. With each three and a half miles of ascent,
+atmospheric pressure is halved; and the interval is lessened by making
+due allowance for decrease of temperature upwards. To the succession of
+these tenuous strata, no definite end can be assigned. The duration of
+twilight shows that, above forty-five miles, they cease to reflect
+light; yet meteors can be set ablaze at heights up to 120 miles, through
+the resistance offered to their motion by air reduced to
+1/250,000,000,000th its density at sea-level!
+
+The cloud-bearing capability of the atmosphere has only of late been
+fully recognised. Ordinary cirrus float about five miles high. On
+December 4, 1894, an aeronaut, Dr. A. Berson, passed right through a
+bank of them at an altitude of five and a half miles, and was able to
+verify by actual contact their composition out of snow-flakelets.[31]
+But since 1885, a still more delicate kind of floating formation has
+come within our acquaintanceship. “Luminous night-clouds” were first
+noticed by Ceraski; they have been systematically studied by O. Jesse of
+Berlin.[32] They appear long after sunset, between May and July, and
+derive their silvery radiance from the sun-rays which their elevated
+situation enables them to intercept, while all below is wrapt in
+darkness. Their height has been determined, from the comparison of
+photographs taken simultaneously at different places, to average
+fifty-one miles, and to range from fifty to fifty-four miles. They are
+an entirely new order of phenomenon.
+
+This globe upon which we dwell is a great magnet. Its directive action
+upon the compass sufficiently proves the fact. But it is a magnet
+probably only by virtue of the electric currents which course round it.
+And since these currents originate from diverse interacting causes, the
+laws of terrestrial magnetism are necessarily complex. They are
+conditioned, yet not prescribed by the earth’s rotation. The magnetic
+and geographical systems of co-ordinates approximate, but by no means
+coincide. The former is, indeed, both complex and variable.[33] The
+inclination, or “dip,” of the needle does not vary in the same way as
+the declination, or horizontal position. There are two points on the
+earth’s surface, called “poles of verticity,” where a magnetic needle,
+freely swung, points vertically downward. One is situated in the arctic
+peninsula Boothia, the other on the antarctic continent within a few
+hundred miles of Mount Erebus. An intermediate line where the needle
+poises itself horizontally, corresponds roughly with the geographical
+equator. Each hemisphere contains besides two centres of maximum force,
+by the joint action of which magnetic deviations from true north and
+south are determined. Their mutual relations are highly intricate. The
+North American focus is stationary, the Siberian focus oscillates. Their
+relative and absolute intensity is probably also subject to
+fluctuations. Hence the inconstancy of magnetic directive influences.
+The variation of the compass varies.
+
+It varies hour by hour, as well as year by year. The needle performs a
+diurnal oscillation, reaching an eastward maximum about eight A.M., and
+a corresponding westward maximum towards four P.M. Moreover, the range
+of this vibration increases concordantly with the growth of spotted area
+upon the sun, and falls off again as spots diminish (see Fig. 2). The
+cosmical relations of terrestrial magnetism are emphasised by the
+obvious connexion between a disturbed state of the sun and the
+occurrence of “magnetic storms.” During these crises, the smooth
+progression and regression of the needle are superseded by violent and
+irregular movements. The photographic tracing in which they are recorded
+presents only a series of lawless zigzags; earth-currents are set up;
+telegraph-wires transmit messages without batteries; and the skies are
+at night draped with auroral streamers.
+
+Auroræ are possibly a survival of our planet’s original self-luminosity.
+If so, their dependence upon the terrestrial magnetic system is highly
+significant. They obey the magnetic period, they accompany magnetic
+disturbances, they illuminate magnetic lines of force. That they are
+immediately caused by electrical discharges in the high vacua of our
+upper air is no longer doubtful. In these latitudes, the auroral arch
+and crown are formed at a height of ninety to one hundred miles, in
+(about) 1/1,000,000,000th of an atmosphere; but in the polar regions
+they approach much nearer to the earth. There, indeed, they more usually
+assume the form of a curtain, undulating in luminous folds, and
+traversed by vertical electric currents. That they are so traversed is
+demonstrated by the behaviour of the magnetic needle, the deviations of
+which change their sign as the auroral drapery crosses the zenith.[34]
+Auroræ seem to be confined to two zones of the earth, which, like the
+sun-spot zones, approach the equator as the solar cycle advances. Their
+frequency in temperate regions corresponds, accordingly, to a scarcity
+in high latitudes. The auroral spectrum consists of a number of bright
+rays, one of which is invariably present, and seems to be essential and
+fundamental. Its origin is unexplained.
+
+The velocity of the earth in its orbit exceeds more than sixty times
+that of a cannon ball just leaving the muzzle of an eighty-ton gun. In
+other terms, the third planet from the sun travels at an average rate of
+18½ miles per second. Its albedo has been estimated—probably
+under-estimated—at 0·30. This would leave 70 per cent. of the solar
+emanations striking the upper surface of its atmosphere available for
+interior consumption. Most of this supply is absorbed or scattered in
+the atmosphere. The proportion sent back to space after reflection from
+the actual terrestrial surface must be extremely small. Very little
+topographical detail could be made out by telescopic scrutiny from the
+moon or Venus. At the most, the trend of some great mountain ranges,
+such as the Andes and Himalayas, and a dozen snow-clad peaks, could be
+visible. No sign of the teeming organic life brought forth by mother
+earth could be detected from without.
+
+The more we know of the moon, the less inviting, from our point of view
+as animated beings, it appears. It is a harsh and inhospitable world,
+from which vital possibilities, if they were ever present, have plainly
+long ago departed. The diameter of our satellite is 2,162 miles. Its
+disc, so far as the most exact measurements tell, is perfectly round.
+This in itself indicates a slow rotation; and even casual observations
+suffice to show that they relate to only one lunar hemisphere. Rotation
+and revolution here again synchronise. In 27 days 8 hours (nearly), the
+moon executes one circuit of the earth, and one gyration on its axis.
+The coincidence was brought about in remote ages by the power of
+terrestrial tidal friction. The averted hemisphere does not, however,
+remain wholly invisible. Two-elevenths of it are, by the effect of
+librations, both in longitude and latitude, brought piecemeal into view.
+But the additional “lunes,” thus thrown open to glimpses round the
+corner, are greatly foreshortened.
+
+The area of the moon is somewhat less than one-thirteenth that of the
+earth. Yet room could be found there for the entire British Empire, with
+six million square miles to spare. Its volume is ¹⁄₄₉th, its mass
+¹⁄₈₂th, the volume and mass of the earth. Hence the lunar materials are
+less dense than the terrestrial in the proportion of about three to
+five. But this may be because they are under comparatively slight
+pressure.
+
+At the moon’s surface, gravity possesses only one-sixth its power here,
+so that a stone thrown upward with equal force would reach a six-fold
+height. Further, a projectile shot straight from our satellite with a
+velocity of one and a half miles a second would never return, while a
+speed of seven miles a second is just controllable by the earth, to say
+nothing of the immense efficacy of her dense atmosphere in hindering
+escape from her precincts. No terrestrial bomb, it may therefore be
+safely asserted, has ever been hurled into space, although volcanic
+ejecta may very well, in past ages, have made their way hither from the
+moon.
+
+But lunar volcanoes are no longer active. Only their remains stand as
+records of a fiery past. In guiding a telescope across the scarred face
+of our satellite we seem to traverse a volcanic charnel-house. The
+evidence of ancient seismic action on the moon is overwhelming. Its
+surface is pitted all over with cones and craters. Nearly 33,000 are
+marked on Schmidt’s map, and the list is very far from being exhaustive.
+The resulting chiaroscuro is obvious to the naked eye. Dante tried to
+explain it in the “Divina Commedia”; Galileo detected its cause and
+manner of composition. The chief facts about it are these.
+
+[Illustration:
+
+  FIG. 10.—_Map of the Moon._ (From Fowler’s “Telescopic Astronomy.”)
+]
+
+   1. Furnerius
+
+   2. Petavius
+
+   3. Langrenus
+
+   4. Macrobius
+
+   5. Cleomedes
+
+   6. Endymion
+
+   7. Altas
+
+   8. Hercules
+
+   9. Römer
+
+  10. Posidonius
+
+  11. Fracastorius
+
+  12. Theophilus
+
+  13. Piccolomini
+
+  14. Albategnius
+
+  15. Hipparchus
+
+  16. Manilius
+
+  17. Eudoxus
+
+  18. Aristotle
+
+  19. Cassini
+
+  20. Aristillus
+
+  21. Plato
+
+  22. Archimedes
+
+  23. Eratosthenes
+
+  24. Copernicus
+
+  25. Ptolemy
+
+  26. Alphonsus
+
+  27. Arzachel
+
+  28. Walter
+
+  29. Clavius
+
+  30. Tycho
+
+  31. Bullialdus
+
+  32. Schiller
+
+  33. Schickard
+
+  34. Gassendi
+
+  35. Kepler
+
+  36. Grimaldi
+
+  37. Aristarchus
+
+   A. Mare Crisum
+
+   B.  Mare Fecunditatis
+
+   C.  Mare Nectaris
+
+   D.  Mare Tranquilitatis
+
+   E.  Mare Serenitatis
+
+   F.  Mare Imbrium
+
+   G. Sinus Iridum
+
+   H. Oceanus Procellarum
+
+   I. Mare Humorum
+
+   K.  Mare Nubium
+
+   V. Altai Mountains
+
+   W. Mare Vaporum
+
+   X. Apennine Mountains
+
+   Y. Caucasus Mountains
+
+   Z. Alps
+
+The general albedo of the lunar surface is 0·17; but portions of the
+disc are as obscure as basalt or obsidian, while isolated spots glitter
+like snow-peaks. The former are usually admitted to be the oldest of
+conspicuous lunar formations, the latter to be comparatively recent. The
+dusky spaces too, are dead levels, if not depressions; they were
+formerly taken for seas, and retain the name of “Maria.” One “ocean,”
+extending over two million square miles, is included amongst them. This
+is the “Oceanus Procellarum” (see Fig. 10), which is five times larger
+than its nearest rival, the “Mare Nubium.” The late Mr. Gwyn Elger
+regarded the lunar “seas” as lava outflows, by which certain earlier
+formations were all but obliterated. M. Suess explains them as areas
+where the primitive thin “slag-crust” re-melted. To the same category
+belong the vast “bulwark plains,” the ramparts enclosing which are of so
+wide a sweep as to be, not merely “hull-down,” but completely invisible
+to an imaginary spectator placed at their centres. Yet Pelions by the
+dozen are tumbled upon Ossas for their construction, with here and there
+an Olympus flung on the top. Typical examples are Ptolemæus, 115 miles
+across; and Plato (near the Northern Pole), “sixty miles in diameter,
+with its bright border and dark steel-grey floor.”[35]
+
+The bottoms of lunar craters and “circuses” are nearly always
+depressed—sometimes thousands of feet—below the general level. Thus, the
+central peak of the great crater Copernicus towers to 11,300 feet above
+the depressed plain from which it rises, but surmounts by only 2,600
+feet the average level of the moon.
+
+Successive stages of activity have left ineffaceable marks upon this now
+stereotyped page. Groups of immense craters mutually encroach, and seem
+to have been scooped out of each other’s flanks, like Kilauea from Mauna
+Loa; craters occur within craters, as Vesuvius inside the broken rampart
+of Somma; and the most recent are invariably the deepest and steepest.
+Cup-shaped depressions or “crater-pits” are innumerable; they result,
+according to Suess’s theory,[36] each from a single explosion, the
+bursting of a “big bubble” of gas in a cooling lava-field. Mountain
+ranges are profusely strewn with them. These lunar Alps and Apennines
+appear to be as unmistakably igneous in their origin as Tycho or
+Aristarchus. They are colossal slag-walls. There are apparently no
+sedimentary deposits upon the moon. Aqueous action had no concern with
+its geological history. Yet on the earth water is essential to the
+production of volcanic phenomena. If they are to be developed without
+it, M. Angelot concludes, it must be by explosive escapes from
+solidifying materials, of gases absorbed by them when in a state of
+fusion.
+
+The mountains of the moon are much higher, proportionally, than the
+summits of the Hindu-Kush, or of the Himalayas. Mount Everest, reduced
+to the lunar scale, would be a modest elevation of 8,200 feet; while
+pinnacles in the lunar Apennines spring up to 22,000 feet, and
+crater-peaks of eighteen or twenty thousand abound. The disparity is
+scarcely surprising when it is remembered that there the convulsive
+throes of cooling were restrained by gravity reduced to one-sixth the
+power it exerts here.
+
+Among the puzzles of selenography are the objects termed respectively
+“rills” and “rays” The former are very numerous. Considerably more than
+a thousand of them have been mapped or photographed. They resemble the
+cañons of Colorado. Some few run to 150 miles; most are a couple of
+miles wide, and above a quarter of a mile deep. Their volcanic origin
+cannot be doubted. The “rays” diverge in extensive systems from such
+huge ring-craters as Tycho and Copernicus. They cast no shadows, and
+come out best at full moon, circumstances suggestive of their being
+immemorial lava-streams bleached by the chemical action of fumes from
+the interior. The whiteness of Aristarchus has been similarly explained;
+but accumulations of pumice and snow-like volcanic ashes perhaps enhance
+the effect. The flashing back by this wonderful peak, of earthshine at
+determinate angles of illumination, has often counterfeited the vivid
+glow of actual eruptions. Their possibility, however, belongs to the
+past. Nor have any of the rumoured alterations in lunar topography,
+which from time to time excited interest and raised controversy, made
+good their footing as solid facts. Agencies of change are certainly
+there, in tidal strains and alternations of temperature, but they work
+very slowly. There is no erosion by air or water; no grinding by ice; no
+transport of materials. Repose reigns apparently undisturbed. Lunar
+landscapes exhibit abrupt transitions from the blinding glare of crude
+sunlight to the blackness of absolute shadow. Their aspect excludes any
+but the thinnest possible atmospheric remnant To all intents and
+purposes, the moon is an airless globe. Occultations of stars afford a
+very refined test of this condition; and their instantaneousness alone
+suffices to demonstrate its reality. Spectroscopic evidence is to the
+same effect. Dr. Huggins watched, January 4, 1865, a _prismatic_
+occultation of the small star, ε Piscium. Had there been the slightest
+inequality of dispersion or absorption at the moon’s limb, it could not
+have failed to be perceived. There was none. The spectrum remained
+unaffected, and vanished abruptly, all the colours together. And
+moonlight, analysed by the most powerful apparatus, varies not an iota
+from sunlight. It is reflected without the smallest selective change.
+
+The absence of water is equally well attested. There are no river-beds
+to be seen, no rounded surfaces, no alluvial plains. A mosquito could
+not find a moist corner to lay its eggs in. There is nothing to show
+that this was otherwise in any past age, although it is not improbable
+that the lunar rocks contain large volumes of oxygen once free. As
+regards the earth, we can entertain no doubt that a goodly proportion of
+its original atmosphere and oceans is now permanently lodged in its
+bedded crust. But the geological histories of the earth and moon
+probably diverged from the first.
+
+Indeed water, as such, could probably not exist upon the moon’s surface.
+It would promptly take the form of ice. Professor Langley has shown that
+the temperature prevailing there, under vertical sunshine, is about that
+of frost; while it sinks, during the moon’s long night of fourteen days,
+almost to absolute zero. This frigid state is due to the absence of
+atmospheric protection, leaving heat free to depart into space as fast
+as it is received. Thus, of the small quantity of heat contained in
+moonlight, nearly the entire comes to us by mere superficial reflection;
+a minute residuum only is absorbed previously to being emitted. The
+distinction is brought into view by comparing the solar and lunar
+heat-spectra, when moonlight is found to contain longer invisible heat
+waves than can be detected in sunlight Moreover, Professor Frank Very,
+through his experimental demonstration that the equatorial are slightly
+hotter than the polar regions, has established the fact of a slight
+retention of heat by the moon’s substance. How slight the retention is,
+has been proved by Dr. Boeddicker’s observations with the Rosse
+three-foot speculum, showing that, during total eclipses, moon-heat
+vanishes almost completely. Less than 1 per cent, survives. The thermal
+phases are not, however, identical with the luminous phases.
+
+The eclipsed moon, on June 10, 1816, is said to have been utterly lost
+to sight; but, as a rule, with very few exceptions, our satellite
+traverses visibly the densest part of the earth’s shadow. Even during
+“black eclipses,” such as that of October 4, 1884, a dusky spot remains
+as an index to its locality; while in “red eclipses,” the great craters
+and bulwark plains can be easily distinguished with an opera-glass.
+Occasionally, the moon seems turned to blood, and the people cry out in
+the streets with fear. Such a phenomenon was witnessed by the writer at
+Florence, February 27, 1877. Its explanation is not difficult The
+refractive power of the earth’s atmosphere suffices to bring
+illumination to the lunar disc at the very middle of the shadow-cone. It
+is shut off from direct solar rays, not from those that are bent into
+convergence by the lens of our air. That they must be reddened by the
+process, sunset-effects on the earth tell plainly enough. But when the
+air is vapour, or dust-laden, and consequently opaque, little light is
+transmitted, and a scarcely mitigated eclipse ensues. That of 1884 is
+believed to have been darkened by the outpourings from Krakatoa. A
+photograph by Professor Barnard, of the totally eclipsed moon, September
+3, 1895, is reproduced in Fig. 11. It was one of a _search-series_ for a
+lunar satellite. None was found: but the question of its possible
+existence was set at rest.
+
+De la Rue’s and Rutherfurd’s plan of photographing the moon as a whole
+is no longer followed. Bit by bit photography, on a large scale, has
+superseded it. Splendid pictures of individual formations and separate
+regions have in this way been obtained, both at the Paris and the Lick
+Observatories; and their microscopic study has given some interesting
+results; yet it is undeniable that the “chemical retina” cannot here
+claim its usual superiority. “The best photograph of the moon ever
+taken,” Professor W. H. Pickering avers,[37] “will not show what can be
+seen with a six-inch telescope, under favourable atmospheric conditions.
+For general outlines, for completeness of the coarser detail, and for
+purposes of future testimony, the photograph evidently stands without a
+rival; but as regards that which is really most interesting upon the
+moon—the finer detail and more delicate features—the photograph does not
+even hint at their existence.” One of the most successful specimens of
+lunar photography forms the frontispiece to this volume. It was taken by
+MM. Loewy and Puiseux, with the large Coudé equatorial, February 14th,
+1894, at 7^h 27^m Paris time, and cannot easily be surpassed in
+pictorial effect.
+
+[Illustration:
+
+  FIG. 11.—_Photograph of the Totally Eclipsed Moon. By Professor
+    Barnard. Exposure, 3 Minutes._
+]
+
+Atmospheric agitations are one cause of imperfection in lunar
+photographs. The eye can seize the instant of exquisite definition; the
+camera must take what comes. Then the disparities of actinic intensity
+in the various lunar formations are so wide that, in order to get an
+ideal picture, a different length of exposure should be given to each.
+What is enough for a plain—to take an example—is too much for the crater
+rising from it, or for the rampart enclosing it. Minute irregularities
+in the following motion of the telescope during the few seconds of
+exposure occasion further difficulties. A momentary shifting, by half a
+millimetre, of the image upon the sensitive plate, would suffice to blur
+the negative seriously, if not fatally. For this, as for several other
+lines of work, the instrument of the future may be of a type with which
+the equatorial has little in common. Professor Pickering considers it
+probable that “a horizontal telescope of three or four hundred feet
+focus, and twelve to fifteen inches aperture, would give the most
+satisfactory results. In such a case, it might be found best that the
+mirror should remain fixed during the exposure, while the plate was
+given an uniform motion by clock-work.”
+
+The suggestion is one among many signs that a revolution in the mounting
+of telescopes is at hand.
+
+
+
+
+                              CHAPTER VI.
+                            THE PLANET MARS.
+
+
+The furthest terrestrial planet from the sun is Mars, the “star of
+strength.” No other heavenly body, except the moon, is so well placed
+for observation from our position in space. As a superior planet, it
+does not merely, like Mercury and Venus, oscillate about the sun, but is
+best seen when in opposition. It is then “full”; it crosses the meridian
+at midnight, and is at its least distance from the earth. These
+occasions recur every 780 days; but they are not all equally favourable.
+The opposition distance of the planet varies, owing to the eccentricity
+of its orbit, from thirty-five to sixty-one million miles; so that the
+area of the disc is three times larger when a perihelion than when an
+aphelion passage coincides with a midnight culmination. Under the best
+circumstances it is of the apparent dimensions of a half-sovereign 2,000
+yards from the spectator.
+
+The diameter of Mars is 4,200 miles; its surface is equal to
+two-sevenths, its volume to one-seventh those of the earth. But, in
+consequence of its inferior mean density, nine such spheres would go to
+make up the mass of our world. The superficial force of gravity on Mars,
+compared with its terrestrial value, is as thirty-eight to a hundred. A
+man could leap there a wall eight feet four inches in height with no
+more effort than it would cost him here to spring over a two-foot fence.
+
+The planet’s rotation is performed in 24 hours 37 minutes on an axis
+deviating from the vertical by 240° 50′. Hence its seasons resemble our
+own, except in being nearly twice as long, for the Martian year is of
+687 days. They are modified, too, by the considerable elongation of the
+ellipse traversed by Mars, causing a difference of 26½ millions of miles
+in its greatest and least distances from the sun. These are respectively
+155 and 128½ millions of miles, the mean distance being 141½ millions. A
+polar compression of ¹⁄₂₂₀ is just what should be expected from its
+rotatory speed. When at quadrature, it is plainly gibbous; but our
+interior position with regard to it makes it impossible that it should
+ever take the crescent form. Its albedo, according to Zöllner, is 0·26—a
+figure intimating that sunlight is reflected from no cloud-canopy, but
+by the soil itself. This atmospheric transparency leaves the door open
+for researches into the condition of a very curious little world.
+
+The disc of Mars is diversified with three shades of colour—reddish, or
+dull orange, dark greyish-green, and pure white. The last shows mainly
+in two diametrically opposite patches. Each pole is surrounded by a
+brilliant cap, suggesting the deposition of ice or snow over the chilly
+spaces corresponding to our arctic and antarctic regions. Nor is this
+all. Each of the polar hoods shrinks to a mere remnant as the local
+summer advances, but regains its original size when wintry influences
+are again in the ascendant. Here, and nowhere else in the planetary
+system, we meet evidence of seasonal change; and seasonal change is
+associated with vital possibilities. Again, a globe upon which snow
+visibly melts must contain water; hence the green markings cannot but
+image to our minds seas and inlets sub-dividing continents, the blond
+complexion of which may be caused by some native peculiarity of the
+soil. It is in no way connected with vegetation, since it neither fades
+nor flushes with the advent of spring; and an atmospheric origin is
+excluded by the circumstance that it becomes effaced by a whitish haze
+near the limb, just where the densest atmospheric strata are traversed
+by the line of sight.
+
+The spots on Mars are by no means so sharply defined as lunar craters
+and _maria_; yet they are fundamentally permanent. Some can be
+recognised from drawings made over two hundred years ago; and these
+antique records have served modern astronomers to determine with minute
+accuracy the rotation-period of the planet. There is accordingly no
+doubt that “areography” has assured facts to deal with, although the
+facts are not quite as “hard” as they might be. Continents are somewhat
+vaguely outlined. Great tracts of them are of an uncertain and variable
+hue, as if subject to inundations. This peculiarity, thoroughly
+certified during the favourable opposition of 1892, makes a strong
+distinction between Mars and the Earth. Terrestrial oceans keep within
+the limits assigned to them. On the neighbouring planet—as M. Faye
+observed in 1892—“Water seems to march about at its ease,” flooding,
+from time to time, regions as wide as France. The imperfect separation
+of the two elements recalls the conditions prevailing during the
+terrestrial carboniferous era.
+
+[Illustration:
+
+  FIG. 12.—_Chart of Mars on Mercator’s Projection._
+
+  (From Proctor’s “Old and New Astronomy.”)
+]
+
+The main part of the land of Mars is situated in the northern
+hemisphere. It covers two-thirds of the entire globular surface. Rather
+than land, indeed, it should be called a network of land and water. Fig.
+12, from a chart by Schiaparelli, illustrates the remarkable fashion of
+their intermixture. The great continental block—so its orange tint
+declares it to be—is cut up in all possible directions by an intricate
+system of what appear to be waterways, running in perfectly straight
+lines—that is, along great circles of the globe—for distances varying
+from 350 to upwards of 4,000 miles. They are frequently seen in
+duplicate, strictly parallel companions developing thirty to three
+hundred miles apart from the original formations. This mysterious
+phenomenon is evanescent, or rather periodical. Canal-duplication is a
+recurrent change, depending upon the Martian seasons, and becoming
+obvious, according to Schiaparelli, chiefly near the equinoxes.
+
+The canals invariably connect two bodies of water; hence they need no
+locks or hydraulic machinery; their course is on a dead level. The
+broadest of them are comparable with the Adriatic; those at the limit of
+visibility, stretching like the finest spider-threads across the disc,
+have a width of eighteen miles. “The canals,” Schiaparelli says, “may
+intersect among themselves at all possible angles, but by preference
+they converge towards the small spots to which we have given the name of
+lakes. For example, seven are seen to converge in Lacus Phoenicis, eight
+in Trivium Charontis, six in Lunae Lacus, and six in Ismenius
+Lacus.”[38]
+
+These “lakes” evidently form an integral part of the canal system. They
+resemble huge railway-junctions; and the largest of them—the “Eye of
+Mars” (Schiaparelli’s Lacus Solis)—seems, in Mr. Lowell’s phrase, like
+the hub of a five-spoked wheel. It is depicted in Fig. 13 from a drawing
+made by Professor Barnard with the great Lick refractor, September 3,
+1894. Mr. W. H. Pickering in 1892, and Mr. Percival Lowell in 1894, were
+amazed at their extraordinary abundance.
+
+“Scattered over the orange-ochre groundwork of the continental regions
+of the planet,” the latter wrote, “are any number of dark, round spots.
+How many there may be it is not possible to state, as the better the
+seeing, the more of them there seem to be. In spite, however, of their
+great number, there is no instance of one occurring unconnected with a
+canal. What is more, there is apparently none which does not lie at the
+junction of several canals. Reversely, all the junctions appear to be
+provided with spots.”
+
+[Illustration:
+
+  FIG. 13.—_The “Eye of Mars,” drawn by Prof. Barnard with the great
+    Lick Refractor. The southern snow-cap is visible much shrunken by
+    melting._
+]
+
+Most of these foci are about 120 miles in diameter, and appear most
+precisely circular when most clearly seen. “Plotted upon a globe,” Mr.
+Lowell continues, “they and their connecting canals make a most curious
+network over all the orange-ochre equatorial parts of the planet, a mass
+of lines and knots, the one marking being as omnipresent as the other.
+Indeed, the spots are as peculiar and distinctive a feature of Mars as
+the canals themselves.”
+
+Like the canals, too, they emerge periodically, and in the same but a
+retarded succession. They “are therefore, in the first place, seasonal
+phenomena, and, in the second place, phenomena that depend for their
+existence upon the prior existence of the canals.”[39]
+
+Mr. Lowell terms them “oases” (see Fig. 14), and does not shrink from
+the full implication of the term.
+
+The most important result of the numerous observations of Mars, made
+during the oppositions of 1892 and 1894, was the recognition of a
+regular course of change dependent upon the succession of its seasons.
+Schiaparelli had long anticipated this result; he is commonly in advance
+of his time. These changes, moreover, when closely watched, are really
+self-explanatory. The alternate melting of the northern and southern
+snow-caps initiates, and to some extent determines them. As summer
+advances in either hemisphere, the wasting of the corresponding white
+calotte can be followed in every minute particular. “The snowy regions
+are then seen to be successively notched at their edges; black holes and
+huge fissures are formed in their interiors; great isolated fragments
+many miles in extent stand out from the principal mass, dissolve, and
+disappear a little later. In short, the same divisions and movements of
+these icy fields present themselves to us at a glance that occur during
+the summer of our own arctic regions.”[40]
+
+Indeed, glaciation on Mars is much less durable than on the earth. In
+1894, the southern snow-cap vanished to the last speck 59 days after the
+solstice; and the remnant usually left looks scarcely enough to make a
+comfortable cap for Ben Nevis. An immense quantity of water is thus set
+free. The polar seas overflow; gigantic inundations reinforced,
+doubtless, from other sources, spread to the tropics; Syrtis regions of
+marsh or bog deepen in hue, and become distinctly aqueous; canals dawn
+on the sight, and grow into undeniable realities. We seem driven to
+believe that they discharge the function of flood-emissaries.
+
+Mr. Lowell does not hesitate to pronounce them of artificial formation,
+and, on that large assumption, the purpose of their connexion with his
+“oases” becomes transparently clear. They bring to these Tadmors in the
+wilderness the water supply by which they are made to “blossom as the
+rose.” The junction-spots, we are told, do not enlarge when the vernal
+freshet reaches them; they only darken through the sudden development of
+vegetation. These circular “districts, artificially fertilised by the
+canal system,” are strewn broadcast over vast desert areas, the
+orange-ochreous sections of Mars, covering the greater part of its
+surface, but deep buried in the millennial dust of disintegrated red
+sandstone strata.
+
+“Here, then,” Mr. Lowell remarks,[41] “we have an end and reason for the
+existence of canals, and the most natural conceivable—namely, that the
+canals are constructed for the express purpose of fertilising the oases.
+When we consider the amazing system of the canal lines, we are carried
+to this conclusion as forth-right as is the water itself; what we see
+being not the canal itself, indeed, but the vegetation along its banks.”
+
+[Illustration:
+
+  FIG. 14.—_The Oases of Mars. Drawn by Percival Lowell._
+
+  (From “Popular Astronomy,” April, 1895.)
+]
+
+The idea that we see the water only by its effects along the shores of
+these prodigious troughs, originated with Professor W. H. Pickering. It
+is strikingly illustrated by the aspect of rivers from a balloon. Thus
+the Rhine, as M. Flammarion attests,[42] seen from a perpendicular
+altitude of 8,000 feet, shows like a green thread drawn in the midst of
+a ribbon of meadow. The Martian canals, it is suggested, correspond to
+the “ribbon of meadow.”
+
+The hypothesis is seductive, but should not be hastily adopted. It gives
+no account of the doubling of the canals, yet the process takes place on
+a grand scale, at determinate epochs, and under fairly well ascertained
+conditions. It undoubtedly belongs to the series of vernal changes going
+forward upon the planet, and is accomplished with amazing rapidity. A
+single canal may be transformed into a double canal within twenty-four
+hours, and that simultaneously along its whole course. The two stripes,
+so curiously substituted for one, “run straight and equal with the exact
+geometrical precision of the two rails of a railroad.”[43] The tendency
+is shared by the lakes or “oases.” “One of these,” we learn from the
+same authority, “is often seen transformed into two short, broad dark
+lines parallel to one another, and traversed by a yellow line.”
+
+This singular principle of subdivision offers at present no hold for
+profitable speculation. Schiaparelli trusts to the “courtesy of nature”
+for some ray of light by which, in the future, to penetrate the mystery;
+but wisely deprecates recourse being had to the intervention of
+intelligent beings. Such arbitrary modes of dealing with perplexing
+problems constitute, as he says, a grave obstacle to the acquisition of
+just notions concerning them. They raise prepossessions by which the
+progress of genuine research is impeded.
+
+The proportion of water to land is much smaller on Mars than on the
+earth. Only two-sevenths of the disc are covered by the dusky areas, and
+of late the aqueous nature of some, if not all of these, has been
+seriously called in question. Professor Pickering was convinced by his
+observations, in 1892 and 1894, “that the permanent water area upon
+Mars, if it exist at all, is extremely limited in its dimensions.”[44]
+He estimated it at about half the size of the Mediterranean. Professor
+Schaeberle is similarly incredulous. If the dark markings are seas, he
+asks, how explain the irregular gradations of shade in them?[45] How,
+above all, explain their apparent intersection by well-marked canals?
+Professor Barnard, observing with the Lick thirty-six inch in 1894,
+discerned on the Martian surface an astonishing wealth of detail, “so
+intricate, small, and abundant, that it baffled all attempts to properly
+delineate it.”[46] It was embarrassing to find these minute features
+belonging more characteristically to the “seas” than to the
+“continents.” Under the best conditions, the dark regions lost all trace
+of uniformity. Their appearance resembled that of a mountainous country,
+broken by cañon, rift, and ridge, seen from a great elevation. These
+effects were especially marked in the “ocean” area of the hour-glass
+sea.
+
+Evidently the relations of solid and liquid in that remote orb are
+abnormal; they cannot be completely explained by terrestrial analogies.
+Yet a series of well-attested phenomena are intelligible only on the
+supposition that Mars is, in some real sense, a terraqueous globe. Where
+snows melt there must be water; and the origin of the Rhone from a great
+glacier is scarcely more evident to our senses than the dissolution of
+Martian ice-caps into pools and streams.
+
+The testimony of the spectroscope is to the same effect. Dr. Huggins
+found, in 1867, the spectrum of Mars impressed with distinct traces of
+aqueous absorption, and the fact, although called in question by
+Professor Campbell of Lick, in 1894, has been re-affirmed both at Tulse
+Hill and at Potsdam. That clouds form and mists rise in the thin Martian
+air, admits of no doubt. During the latter half of October, 1894, an
+area much larger than Europe remained densely obscured. Whether or no
+actual rain was at that time falling over the Maraldi Sea and the
+adjacent continent, it would be useless to conjecture. We only know that
+with the low barometric pressure at the surface of Mars, the boiling
+point of water must be proportionately depressed (Flammarion puts it at
+115° Fahrenheit), which implies that it evaporates rapidly, and can be
+transported easily.
+
+If the Martian atmosphere be of the same proportionate mass as that of
+our earth, it can possess no more than one-seventh its superficial
+density. That is to say, it is more than twice as tenuous as the air at
+the summits of the Himalayas.[47] The corresponding height of a
+terrestrial barometer would be four and a half inches. Owing, however,
+to the reduced strength of gravity on Mars, this slender envelope is
+exceedingly extensive. In the pure sky scarcely veiled by it, the sun,
+diminished to less than half his size at our horizons, probably exhibits
+his coronal streamers and prominences as a regular part of his noontide
+glory; atmospheric circulation proceeds so tranquilly as not to trouble
+the repose of a land “In which it seemeth always afternoon”; no cyclones
+traverse its surface, only mild trade-winds flow towards the equator to
+supply for the volumes of air gently lifted by the power of the sun, to
+carry reinforcements of water-vapour north and south. Aerial movements
+are, in fact, by a very strong presumption, of the terrestrial type, but
+executed with greatly abated vigour.
+
+Brilliant projections above the terminator of Mars were first distinctly
+perceived at the Lick Observatory in 1890. They have been re-observed at
+Nice, Arequipa, and Flagstaff (Mr. Lowell’s Observatory), coming into
+view, as a rule, when circumstances concur to favour their visibility.
+They strictly resemble lunar peaks and craters, catching the first rays
+of the sun, while the ground about them is still immersed in
+darkness;[48] and Professor Campbell[49] connects them with “mountain
+chains lying _across_ the terminator of the planet,” and in some cases
+possibly snow-covered. He calculates their height at about ten thousand
+feet. Their presence was unlooked-for, since a flat expanse is a
+condition _sine quâ non_ for the minute intersection of land by water,
+which seems to prevail on Mars.
+
+Although the sun is less than half as powerful on Mars as it is here,
+the Martian climate, to outward appearance, compares favourably with our
+own. Polar glaciation is less extensive and more evanescent, and little
+snow falls outside the arctic and antarctic regions. Yet the theoretical
+mean temperature is minus 4°C., or 61° of Fahrenheit below freezing.
+This means a tremendous ice-grip. The coldest spot on the earth’s
+surface is considerably warmer than this cruel average. Fortunately, it
+exists only on paper. Some compensatory store of warmth must then be
+possessed by Mars, and it can scarcely be provided by its attenuated
+air. Possibly, internal heat may still be effective, and we see
+exemplified in Mars the geological period when vines and magnolias
+flourished in Greenland, and date-palms ripened their fruit on the coast
+of Hampshire.
+
+The climate of Mars, according to Schiaparelli,[50] “must resemble that
+of a clear day upon a high mountain. By day a very strong solar
+radiation hardly at all mitigated by mist or vapour; by night a copious
+radiation from the soil towards celestial space, and hence a very marked
+refrigeration; consequently, a climate of extremes, and great changes of
+temperature from day to night, and from one season to another. And as on
+the earth, at altitudes of from 17,000 to 20,000 feet, the vapour of the
+atmosphere is condensed only into the solid form, producing those
+whitish masses of suspended crystals which we call cirrus-clouds, so in
+the atmosphere of Mars it would be rarely possible to find collections
+of cloud capable of producing rain of any consequence. The variation of
+temperature from one season to another would be notably increased by
+their long duration, and thus we can understand the great freezing and
+melting of the snow, renewed in turn at the poles at each complete
+revolution of the planet round the sun.”
+
+But the anomalies in the Martian domestic economy cannot thus easily be
+removed, and the only safe conclusion is Flammarion’s, that “the general
+order of things is very different on Mars and on the earth.”
+
+The German astronomer, Mädler, searched in 1830 for a Martian satellite,
+and although his telescope was of less than four inches aperture, he
+satisfied himself that none with a diameter of as much as twenty-three
+miles could be in existence. As it happened, he was right. The pair of
+moons detected by Professor Asaph Hall with the Washington twenty-six
+refractor, August 11 and 17, 1877, are unquestionably below that limit
+of size. Neither of them can well be more than ten miles across. Their
+names, “Deimos” and “Phobos,” are taken from the _Iliad_, where Fear and
+Panic are introduced as attendants upon the God of War. Deimos revolves
+in 30 hours and 18 minutes at a distance of 14,600 miles from the centre
+of Mars. And, since the planet rotates in 24 hours 37 minutes, the
+diurnal motion of the sphere from east to west is so nearly neutralised
+by the orbital circulation of the satellite from west to east that
+nearly 132 hours elapse between its rising and its setting. During the
+interval, it changes four times from new to full, and _vice versâ_.
+Professor Young estimates that Mars receives from it when full only
+¹⁄₁₂₀₀th of full moonlight.
+
+Phobos is more effective in illumination, both because it is larger, and
+because it is less distant. At the Martian equator, its brightness is
+equal to ¹⁄₆₀th that of our moon, but beyond 69° of latitude it is
+permanently shut out from view by the curvature of the globe. This
+exclusion is an effect of its uncommon closeness to its surface, the
+interspace being only 3,700 miles, while its distance from the centre is
+5,800. Moreover, the period of Phobos being only 7 hours 39 minutes, or
+less than ⅓ the time of rotation of its primary, it rises in the west,
+sets in the east, and courses across the heavens in 11 hours, during
+which interval it accomplishes one entire cycle of its phases, and gets
+through half another. This is an unique phenomenon, and points to an
+unique origin for the little moon. No other known satellite revolves
+more quickly than its primary rotates, and the discovery of the fact has
+dealt a fatal blow to Laplace’s method of planetary evolution. Were
+Phobos capable of raising any appreciable tide on Mars, its frictional
+effects would hence be of an opposite character to those of other tidal
+waves; and instead of being pushed outward, it would be drawn inward,
+and finally precipitated upon the planet. But it derives safety, on the
+one hand, from its small mass; on the other, from the insensibility of
+Mars to tidal action. The satellite is incapable of exerting the
+required influence; the planet is not in a state to respond to it, were
+it exerted. For the configuration of land and water upon its surface is
+such as effectually to prevent the flow of tides, were the compulsive
+power a thousand-fold that possessed by its pair of diminutive
+satellites.
+
+
+
+
+                              CHAPTER VII.
+                             THE ASTEROIDS.
+
+
+Between the orbits of Mars and Jupiter is interposed a huge gap. On one
+side of it lie the terrestrial planets; on the other, the “major
+planets”—orbs belonging to a different order, both as to magnitude and
+as to constitution. The hiatus marks a change of front in planetary
+development, and its existence gravely compromises the symmetry of the
+solar system. Its inconsistency with Bode’s law of planetary distances
+long troubled investigators. A member of the series had somehow dropped
+out; it was sought for under the form of a planet, and found,
+apparently, as its disintegrated constituents. The discovery of Uranus
+nearly at the distance indicated for it by the law roused astronomers to
+the necessity for a systematic chase; but before their organisation had
+got into full working order, the missing occupant of the vacant zone
+presented itself spontaneously. This was Ceres, the first asteroid,
+discovered by Piazzi at Palermo, January 1, 1801, the opening day of the
+present century.
+
+A series of surprises followed. While watching its path, Dr. Olbers,
+March 28, 1802, came across an associated body. He named it Pallas, and
+it was at once proved by the calculations of Gauss to revolve
+practically at the same distance from the sun as Ceres. _Both_ occupied
+nearly the position required by Bode’s law. This double fulfilment was
+more than was bargained for; it was unprecedented and perplexing; but
+the anomaly was temporarily removed by Olbers’ daring hypothesis of an
+exploded planet. The prediction based upon it that the acquaintance made
+with two specimen-products of the catastrophe would be followed by an
+introduction to many more, was strikingly verified by Harding’s
+discovery of Juno, September 1, 1804, and by Olbers’ of Vesta, March 29,
+1807. By a further coincidence, both were at the time situated in the
+positions suggested as the most promising for a successful search—that
+is, near the line of intersection which should necessarily be common to
+orbits described by fragments of a single original mass.
+
+The four asteroids received for many years no accession to their
+numbers. They were found to deviate, in several respects, from the
+example set them by the planets, properly so-called. They revolve,
+indeed, from west to east, thus following the current of systemic
+movement; but their paths are considerably eccentric and highly tilted.
+Each one of the quartette transgresses the zodiacal limits; and Pallas
+travels at an angle of no less than thirty-five degrees to the plane of
+the ecliptic.
+
+Vesta, the brightest asteroid, can occasionally be seen with the naked
+eye; but the natural inference that it is the largest has lately been
+disproved. No trustworthy measurements of the real _discs_ of the
+asteroids had been made until Professor Barnard in 1894 successfully
+performed the feat with a power of 1000 on the Lick refractor. The
+upshot has been to substitute Ceres for Vesta as the leading member of
+the group. Its diameter proved to be 485 miles, Pallas coming next with
+304, while those of Vesta and Juno are respectively 243 and 118 miles.
+Now, Professor Edward Pickering, by comparing the brightness of the same
+bodies, and assuming for all indiscriminately an albedo equal to that of
+Mars, had arrived at a diameter for Vesta of 319, for Pallas of 169
+miles. The disparity between his results and Barnard’s can be reconciled
+only on the supposition of marked differences in reflective power. Their
+reality was established by G. Müller’s photometric observations at
+Potsdam.[51] Thus Ceres is large and dull, Vesta comparatively small,
+but exceedingly bright—almost incredibly bright, indeed, since its
+albedo is estimated at 0·72, which represents a lustre midway between
+those of white paper and fresh-fallen snow. Ceres, on the other hand, is
+as obscure as Mercury, while Pallas throws back proportionately somewhat
+less, and Juno considerably more light than Mars.[52] The phases of
+these last two bodies progress besides in such a manner as to show that
+they are superficially uneven, and at quadratures flecked with profound
+shadows.
+
+The facts thus arrived at are disconcerting to the views previously
+entertained. Few expected to meet with so much individuality in the
+asteroids. They were looked upon rather as loaves from the same batch.
+But now we find among them bodies as physically unlike as Venus and the
+moon. Ceres must be composed of rugged and sombre rock, unclothed
+probably by any vestige of air. Vesta displays a brilliant shell of
+clouds. And from Vesta alone among the asteroids, Vogel derived in 1873
+some uncertain indications of atmospheric action upon the sun-rays
+reflected by it. There is, nevertheless, great difficulty in supposing a
+body of no more than one-thousandth the mass of Mars endowed with a
+dense atmosphere. Yet it must be dense and extensive in order to
+maintain the heavy cloud-layer implied, so far as our present knowledge
+goes, by an unusually high albedo. The difficulty is this. All gases
+tend, by their nature, to become indefinitely diffused through space.
+They can be restrained within a sphere of finite radius only through the
+exertion of some force capable of holding their elasticity in check.
+This force is gravity; none other suitable for the purpose is known. It
+acts as a counterpull to the translational velocities of the gaseous
+particles which, according to the dynamical theory of gases, constitute
+their elasticity. But if the confining power be insufficient, the roving
+particles will dart away, each on its own account, and will cease to
+form an atmosphere. This condition was adverted to some years ago by Dr.
+Johnstone Stoney, and he calculated the mass needed to secure to a
+heavenly body the lasting possession of an aerial envelope. It differs
+naturally for different gases; the lightest particles being affected by
+the swiftest movements, and hence being the readiest to escape. The
+earth, on this view, is impotent to retain hydrogen; since the critical
+velocity at its surface is seven miles a second, and hydrogen-molecules
+can, now and again, attain 7·4 miles, so that they would dribble away,
+one after another, until the whole original supply was exhausted. Mars
+(a projectile fired from which, with a speed exceeding three miles a
+second, would depart irrevocably), can but just hold oxygen, nitrogen,
+and water-vapour, all with more massive and sluggish molecules than
+those of hydrogen; while the moon has long ago been forsaken by whatever
+gaseous substances primitively belonged to it. The mass of Vesta,
+however, is only ¹⁄₃₁₂ the lunar mass (supposing their mean densities
+the same); hence, if the relation just described holds good under all
+circumstances, its surface _ought_ to be as bare and dry as any lunar
+volcano. The albedoes of the asteroids raise, then, questions of
+fundamental importance in planetary physics.
+
+Endeavours to add to the asteroidal group, after having been
+relinquished for over a score of years, were resumed, in 1830, by a
+retired Prussian post-master named Hencke. His watch was rewarded with
+the discoveries of Astraea, December 8, 1845, and of Hebe eighteen
+months later. Since then, every year has regularly brought its quota of
+detections. About forty astronomers devoted themselves systematically to
+the search, and some of them reckoned their trophies by the score. No
+less than eighty-five were credited, in 1893, to Palisa of Vienna;
+Peters of Clinton (N.Y.), whose career closed in 1890, owned
+forty-eight; Watson, another American professor, made testamentary
+provision for his twenty-two clients, lest, for lack of computational
+care, they should relapse into their former outcast condition. The task
+is, indeed, a heavy one of keeping guard over some hundreds of minute
+objects threading their way through a maze of orbits, amid throngs of
+stars, from which they are indistinguishable except by continuous
+observation, and the question, _Cui bono?_ has been asked, and has only
+with hesitation been answered. But the business has, up to the present,
+been kept going; the registry and inquiry asteroidal office remains open
+at Berlin, and the almost overwhelming mass of calculations, necessary
+for identification, is punctually dealt with.
+
+The work and responsibilities of this department have, of late, been
+alarmingly augmented. Until five years ago the telescope was the sole
+implement of research in connection with it, but on December 22, 1891,
+Professor Max Wolf of Heidelberg, discovered No. 323, afterwards named
+Brucia, on a sensitive plate exposed with a six-inch portrait lens, of
+thirty inches focus, and a field of seventy square degrees. Before the
+year 1892 had closed, his photographic discoveries of the same kind
+numbered eighteen, and they had, in January, 1897, run up to fifty-six,
+of which five were recorded on the same night. He picked up, besides,
+several “lost” or strayed asteroids. M. Charlois of Nice immediately
+adopted Wolf’s method, and emulated his success. About ninety of these
+objects have already fallen to his share by telescopic and photographic
+means. In either case they are discriminated from stars solely by their
+motion; but on sensitive plates its effects are directly visible, fixed
+objects being represented by round dots, travelling objects by lines,
+the length of which is proportionate to the amount of displacement
+during the hour, or hours, of exposure.
+
+About 440 asteroids are now established members of the solar system. It
+has long been thought that numerical identification is as much as they
+can properly claim; but the old and inconvenient system of mythological
+nomenclature is still pursued. Indeed, the supply of goddesses is
+running out, and has to be reinforced by apotheosis or invention.
+Already, to some extent, as Professor Holden remarks, the asteroidal
+catalogue “reads like the Christian names at a girls’ school.” Needless
+to say that the brightness of the objects annually registered is in
+steady course of decline. Very few of those now drawn to shore in the
+photographic net are likely to exceed twenty miles in diameter. Yet
+although mere planetary shreds, they are probably large compared with
+the grains of planetary dust, numberless as the sands of the seashore,
+which indiscernably revolve round the sun under analogous conditions.
+
+Their aggregate mass is very small. Leverrier assigned for its superior
+limit one-fourth that of the earth, but the limit, we may rest assured,
+is very far from being attained. M. Niesten of Brussels estimated that
+the first 216 asteroids, including all the larger ones, amounted to
+¹⁄₁₀₀₀th the earth’s volume, and we may add, since they are beyond doubt
+specifically lighter, to about ¹⁄₈₀₀₀th the earth’s mass. Mr. Roszl
+finds for the mass of 311 asteroids one-fortieth that of the moon.[53]
+Still later, M. Gustave Ravené has attempted to account for the
+superfluous movement of the perihelion of Mars by the gravitational
+influence of these bodies.[54] He computes the required mass to be
+two-thirds that of the moon. In other words, he assumes the group to be
+fairly represented by 500 globes as large as Juno (124 miles in
+diameter), and of terrestrial density. But he obviously puts some
+constraint on nature in order to secure the desired agreement.
+
+The distribution of these dwarfed globes is not without significant
+features. It is such, at any rate, as absolutely to negative Olbers’s
+hypothesis of their origin through the explosion of an already formed
+planet. They represent, on the contrary, the materials of a planet that
+never was, and never will be formed. They follow paths curiously
+intertwined. D’Arrest noticed forty-five years ago, as a proof of the
+intimate relation subsisting among the members of what was then a small
+group, “that, if their orbits are figured under the form of material
+rings, these rings will be found so entangled that it would be possible,
+by means of one among them taken at hazard, to lift up all the rest.”
+They are not, however, scattered at random over the wide zone
+appropriated to them which, at its extreme limits, measures three times
+the radius of the earth’s orbit. It includes blank spaces which seem as
+if cleared by some expulsive agency. That agency, as Professor Kirkwood
+divined in 1866, is the disturbing power of Jupiter. For the blank
+spaces occur where there would be commensurability of periods, and
+whence, accordingly, revolving particles should be ejected by
+accumulated perturbations. The clearing power was not exerted once for
+all; it is still active. But its effectiveness in modifying distribution
+is now perceived to be less complete than it seemed when our
+acquaintance with the bodies in question was more limited. It has
+produced in general only partial vacancies. M. Parmentier[55] analysed
+in 1895 the arrangement in space of 390 orbits, with the result of
+finding that some of the originally noted gaps had ceased to exist. The
+mean distances, for instance, corresponding to periods two-sevenths and
+three-sevenths the Jovian period, are fairly well frequented; while, on
+the other hand, there is an unmistakable thinning out where five
+revolutions are performed while Jupiter accomplishes two. He found again
+that no asteroid circulates either in half, or in one-third the same
+dangerous period. Yet, even since he wrote, No. 401 has been detected
+occupying the former of these prohibited spaces. But this apparent
+breach of rule may turn out to result from a miscalculation, as in the
+case of Menippe, which has in consequence never been recaptured since
+she first presented herself in 1878, and was erroneously assigned a
+period two-fifths that of Jupiter. There is no doubt that the asteroids
+are collected most densely about the mean distance 2·8 of the earth’s,
+just where conformity to Bode’s law would place them. Nor is it less
+certain that Kirkwood’s “rule of commensurability” has fundamentally
+influenced their distribution.
+
+He further discerned among them groups of two or three moving in
+closely-related orbits. Additional examples of this sort of connexion,
+which is far too close to be casual, have been pointed out by M.
+Tisserand and Mr. Monck, and eighty asteroids are at present known to
+have companions, their actual ties with which indicate, as Kirkwood
+held, original identity. Each group consists of fragments of a primitive
+nebular mass torn asunder by the unequal attraction of Jupiter shortly
+after its detachment from the great parent sphere eventually condensed
+to form the sun. As an example, we may take Juno and its twin Clotho.
+Both revolve at a mean distance from the sun 2·67 times that of the
+earth, in orbits of sensibly the same eccentricity, and of nearly the
+same inclination to the ecliptic, their major axes diverging, however,
+to the extent of ten degrees, obviously through unequal perturbations.
+As surely as corresponding scars on opposite cliffs vouch for their
+antique disruption, do these concurrent paths attest the primitive unity
+of the pair of planetules traversing them. And bodies similarly
+connected occur not in pairs only, but in triplets as well.
+
+From whatever point of view the “planetary cluster” composed by the
+asteroids is regarded, the influence of Jupiter is perceived as dominant
+in the background. The manner of planetary production underwent a marked
+change subsequently to the separation of his mighty mass. No interval of
+repose followed; but a constant shredding off of chips and shavings.
+This may safely be attributed (in accordance with Professor Kirkwood’s
+surmise) to the tide-raising power of Jupiter at close quarters, by
+which strain in the central rotating mass was almost prevented, through
+the facility with which it was relieved. Hence the parent nebula long
+remained incapable of parting with any appreciable portion of its
+substance, and never resumed planet-making on the ancient scale. The
+asteroids then came into existence under Jupiter’s auspices; they were,
+while still in an inchoate state, subdivided, or even pulverised by his
+disruptive influence, and scattered over the zone allotted to them under
+the compulsion of his perturbing power.
+
+
+
+
+                             CHAPTER VIII.
+                          THE PLANET JUPITER.
+
+
+Jupiter is by far the most important member of the solar family. The
+aggregate mass of all the other planets is only two-fifths of his, which
+316 earths would be needed to counter-balance. His size is on a still
+more colossal scale than his weight, since in volume he exceeds our
+globe 1,380 times. His polar and equatorial diameters measure
+respectively 84,570 and 90,190 miles,[56] giving a mean diameter of
+88,250 miles, and a polar compression of ¹⁄₁₆th. The corresponding
+equatorial protuberance rises to 2,000 miles, so that the elliptical
+figure of the planet strikes an observer at the first glance. This at
+once indicates rapid axial movement; and Jupiter’s rotation is
+accordingly performed in nine hours and fifty-five minutes, with an
+uncertainty of a couple of minutes. The cause of this uncertainty will
+presently appear.
+
+The numbers just given imply that this great planet is of somewhat
+slight consistence, and its mean density is in fact, a little less than
+that of the sun. The sun is heavier than an equal bulk of water in the
+proportion 1·4 to 1, Jupiter in the proportion of 1·33 to 1. The earth
+is thus more than four times specifically heavier than the latter globe.
+Three Jupiters would keep in equipoise four equal globes of water, while
+the earth would turn the scale against five and a half aqueous models of
+itself. This low density, an unfailing characteristic of all the giant
+planets, is charged with meaning. It at once gives us to understand
+that, in crossing the zone of asteroids, we enter upon a different
+planetary region from that left behind. The bodies revolving there are
+on an immensely larger scale of magnitude than those on the hither side;
+they are of solar, rather than terrestrial, density; they rotate much
+more rapidly, and are in consequence of a more elliptical shape; they
+display, and most likely possess, no solid surface; they are attended by
+retinues of satellites.
+
+Jupiter circulates round the sun in 11·86 years, in an orbit deviating
+by less than one and a half degrees from the plane of the ecliptic, but
+of thrice the eccentricity of the ellipse traced out by the earth. With
+a mean distance from the sun of 483 millions of miles, it accordingly
+approaches within 462 at perihelion, and withdraws to 504 millions of
+miles at aphelion. And since the heat and light received from the sun
+are inversely as the squares of these numbers, it follows that Jupiter
+is better warmed and illuminated when at the near than when at the far
+extremity of its orbit, in the proportion of 109 to 100. Seasons it has
+none worth mentioning; nor could they be of much effect even if they
+were better marked. At its mean distance of 5·2 “astronomical
+units”—that is, radii of the earth’s orbit—the sun’s potency is reduced
+to ¹⁄₂₇th what it is here; we might accordingly have expected to meet in
+this planet the conditions of a frozen world. But this anticipation has
+been singularly falsified.
+
+Under propitious circumstances Jupiter comes within 369 million miles of
+the earth. These occur when he is in opposition nearly at the epoch of
+his perihelion passage. His maximum opposition distance, on the other
+hand, is 411 million miles. He is then at aphelion. Thus, at the most
+favourable opposition, he is 42 million miles nearer to us than at the
+least favourable. The effect on his brightness is evident to the eye.
+When his midnight culmination takes place in October, he in fact sends
+us one and a half times more light than when the event comes round to
+April. We need only recall the unusual splendour of his appearance in
+September and October, 1892, when his lustre was double that of Sirius.
+His opposition period, as we may call it, is 399 days.
+
+The intrinsic brilliancy of his surface is surprising, especially when
+we consider that it is somewhat deeply tinged with colour. According to
+Müller’s determination (relative to Mars), it actually returns 78 per
+cent. of the incident light. But this would imply self-luminosity, the
+presence of which is negatived by trustworthy evidence. Hence Zöllner’s
+absolute albedo of 0·62 seems preferable. In either case, Jupiter does
+not fall far short of being as reflective as white paper.
+
+The minimum diameter of the visible disc considerably exceeds the
+maximum of that of Mars. The latter never measures more than 25″;
+Jupiter at conjunction, when (in round numbers), 600 million miles
+distant from us, presents a surface 32″ in diameter, widened at a
+favourable opposition to 50″. Even with a low power it thus makes a
+beautiful and interesting telescopic object Its distinctive aspect is
+that of a belted planet, the belts varying greatly in number and
+arrangement. As many as thirty have, on occasions, been counted,
+delicately ruling the disc from pole to pole. They are always parallel
+to the equator, but are otherwise highly changeable, and cannot be too
+closely studied as an index to the planet’s physical constitution. Two
+in particular are remarkable. They are called the north and south
+equatorial belts, and enclose a lustrous equatorial zone. The poles are
+shaded by dusky hoods.
+
+This general scheme of markings, however, when viewed with one of the
+great telescopes of the world, is so overlaid with minor particulars as
+sometimes to be scarcely recognisable. One cannot see the wood for the
+trees. Lovely colour-effects, too, come out under the best circumstances
+of definition and aerial transparency. The tropical belts may be
+summarily described as red; but they are of complex structure, and their
+subordinate features and formations are marked out, under the sway of a
+ternating and tumultuous activities, by strips and patches of vermilion,
+pink, purple, drab and brown. The intermediate space is divided into two
+bands by a line, or narrow riband, pretty nearly coinciding with the
+equator, and rosy, or vivid scarlet in hue. The polar caps are sometimes
+of a delicate wine-colour, sometimes pale grey.
+
+Professor Keeler made an elaborate study of the planet with the Lick
+36-inch in 1889, and executed a series of valuable drawings, one of
+which we are privileged to reproduce (Fig. 15). With a power of 320, the
+disc, he tells us, “was a most beautiful object, covered with a wealth
+of detail which could not possibly be accurately represented in a
+drawing.” Most of the surface was then “mottled with flocculent and
+irregular cloud-masses. The edges of the equatorial zone were
+brilliantly white, and were formed of rounded, cloud-like masses, which,
+at certain places, extended into the red belt as long streamers. These
+formed the most remarkable and curious feature of the equatorial
+regions. They are the cause of the double or triple aspect which the red
+belts present in small telescopes.”[57]
+
+Near their starting-points the streamers were white and sharply defined,
+but became gradually diffused over the ruddy surface of the belts. When
+at all elongated, they invariably flowed backward _against_ the
+rotational drift, and were inferred to be cloud-like masses expelled
+from the equatorial region, and progressively left behind by its
+advance. This hypothesis was confirmed by the motion of some bright
+points, or knots, on the streamers. “The portions of the equatorial zone
+surrounding the roots of well-marked streamers were somewhat brighter,”
+Professor Keeler continues, “than at other places, and it is a curious
+circumstance that they were almost invariably suffused with a pale
+olive-green colour, which seemed to be associated with great
+disturbance, and was rarely seen elsewhere.”
+
+[Illustration:
+
+  FIG. 15.—_Jupiter, October 3, 1890. Drawn by Professor Keeler with the
+    great Lick Refractor. The Red Spot is visible._
+]
+
+Now, if the material of the streamers had been simply a superficial
+overflow, it should have carried with it into higher latitudes an excess
+of linear rotational speed, and should hence have pushed its way onwards
+as it proceeded north and south. But, instead, it fell behind; its
+velocity was less, not greater than that of the belts with which it
+eventually became incorporated. What are we to gather from this fact?
+Evidently that the currents issuing north and south were of eruptive
+origin. Their motion, in miles per second, was slow, because they
+belonged to profound strata of the planet’s interior. Their backward
+drift measured the depth from which they had been flung upward.
+
+The spots, red, white, and black, constantly visible on the Jovian
+surface, excite the highest curiosity. They are of all kinds and
+qualities, and their histories and adventures are as diverse as they are
+in themselves. Some are quite evanescent; others last for years. At
+times they come in undistinguished crowds, like flocks of sheep, then a
+solitary spot will acquire notoriety on its own account. White spots
+appear in both ways; black spots more often in communities; and it is
+remarkable that the former frequent distinctively, though not
+exclusively, the southern, the latter the northern hemisphere. Red
+spots, too, develop pretty freely; but the attention due to them has
+been mainly absorbed by one striking specimen.
+
+The Great Red Spot has been present with us for at least nineteen years;
+and it is a moot point whether its beginnings were not watched by
+Cassini more than two centuries ago. Its modern conspicuousness,
+however, dates from 1878. Then of a full brick-red hue, and
+strongly-marked contour, it measured 30,000 by nearly 7,000 miles, and
+might easily have enclosed three such bodies as the earth. It has since
+faded several times to the verge of extinction, and partially recovered;
+but there has never been a time when it ceased to dominate the planet’s
+surface-configuration. More than once it has been replaced by a bare
+elliptical outline, as if through an effusion of white matter into a
+mould previously filled with red matter; and just such a sketch was
+observed by Gledhill in 1870. The red spot is attached, on the polar
+side, to the southern equatorial belt. It might almost be described as
+jammed down upon it; for a huge gulf, bounded at one end by a jutting
+promontory, appears as if scooped out of the chocolate-coloured material
+of the belt to make room for it. Absolute contact, nevertheless, seems
+impossible. The spot is surrounded by a shining aureola, which seemingly
+defends it against encroachments, and acts as a _chevaux de frise_ to
+preserve its integrity. The formation thus constituted behaves like an
+irremovable obstacle in a strong current. The belt-stuff encounters its
+resistance, and rears itself up into a promontory or “shoulder,”
+testifying to the solid presence of the spot, even though it be
+temporarily submerged. The great red spot, the white aureola, and the
+brownish shoulder are indissolubly connected.
+
+The spot is then no mere cloudy condensation. Yet it has no real fixity.
+Its period of rotation is inconstant. In 1879–80, it was of 9 hours, 55
+minutes, 34 seconds; in 1885–86, it was longer by 7 seconds. The object
+had retrograded at a rate corresponding to one complete circuit of
+Jupiter in six years, or of the earth in seven months.[58] It is not
+then fast moored, but floats at the mercy of the currents and breezes
+predominant in the strange region it navigates. A quiescent condition is
+implied by the approximate constancy of its rotation-period during the
+last ten years. With the paling of its colour, its “proper motion”
+slackens or ceases. This must mean that, at its maxima of agitation, it
+is the scene of uprushes from great depths, which, bringing with them a
+slower linear velocity, occasion the observed laggings. It is not
+self-luminous, and shows no symptom of being depressed below the general
+level of the Jovian surface. A promising opportunity was offered in 1891
+of determining its altitude relative to a small dark spot on the same
+parallel, by which, after months of pursuit, it was finally overtaken.
+An occultation appeared to be the only alternative from a transit; yet
+neither occurred. The dark spot chose a third. It coasted round the
+obstacle in its way, and got damaged beyond recognition in the process.
+Its material, as Mr. Stanley Williams observed, “was diverted and forced
+bodily southwards, and obliged to pass round the southern side of the
+red spot as if it were an island projecting above a stream.”
+
+Jupiter has no certain and single period of rotation. Nearly all the
+spots that from time to time come into view on its disc are in relative
+motion, and thus give only individual results. The great red spot has
+the slowest drift of all (with the rarest exceptions), while the black
+cohorts of the northern hemisphere outmarch all competitors. Mr. Stanley
+Williams,[59] as the upshot of long study, has delimitated nine
+atmospheric surfaces with definite periods. They are well marked, and
+evidently have some degree of permanence, yet the velocities severally
+belonging to them are distributed with extreme irregularity. Thus, two
+narrow, adjacent zones differ in movement by 400 miles an hour. This
+state of things must obviously be maintained by some constantly acting
+force, since friction, if unchecked, would very quickly abolish such
+enormous discrepancies. The rotational zones are unsymmetrically placed;
+there is no correspondence between those north and south of the Jovian
+equator; and, although the equatorial drift is quicker than that of
+either tropic, it is outdone in 20° to 24° north latitude. The stability
+of this anomalous mode of rotation was remarkably illustrated by Dr.
+Rambaud’s measurements of the “Garnet Spot” of October, 1895. Its
+movement proved to be strictly conformable to that of the zone in which
+it was situated (10° to 20° north latitude), and to agree, moreover,
+within a fifth of a second with the value deduced by Schröter in 1787
+for that of a spot in the same “zenographical” district.[60]
+
+Jupiter’s equatorial rotation, as indicated by observations of spots, is
+accomplished in 9 hours 50 minutes; but Bélopolsky’s and Deslandres’
+spectrographic determinations gave rates of approach and recession
+falling somewhat short of the corresponding velocity.[61] Possibly the
+spots forge ahead in the medium that sustains them; or it may be, as M.
+Bélopolsky suggests, that the planetary sphere itself has been measured
+too large, owing to refraction in its atmosphere.
+
+However this be, the rotation of the great planet, albeit ill-regulated
+(if the expression be permissible), is distinctly of the solar type. It
+is itself a “semi-sun,” showing no trace of a solid surface, but a
+continual succession of cloud-like masses belched forth from within.
+Each series, in fact, of certain classes of markings, such as the
+equatorial “port-holes,” plainly owes its origin to the rhythmical
+activity of a solitary, deep-buried focus.[62] Jupiter’s low mean
+density, considered apart from every other circumstance, suffices to
+demonstrate the primitive nature of his state. Under the enormous
+pressure reigning in his interior, the same materials should be vastly
+more massive, specifically, than within our own small globe; their
+fourfold expansion gives us to understand the intensity of that heat by
+which pressure has been so much more than neutralised. Moreover, the
+agitations due to the cooling of a fluid globe make their mark on its
+turbulent surface. On a solidified body like the earth, circulation is
+kept up by heat received from without, and is purely atmospheric, and
+essentially horizontal. In a sun-like body, the circulation is bodily
+and vertical. That the processes going on in Jupiter are of this kind is
+beyond question. Exchanges of hot and colder substances are effected,
+not by surface-flows, but by up and down rushes. The parallelism of his
+belts to his equator makes this visible to the eye. An occasional
+oblique streak[63] betokens a current in latitude, but it is
+exceptional, and might be called out of character.
+
+Jupiter’s true atmosphere encompasses the disturbed shell of vapours
+observed telescopically. Its general absorptive action upon light is
+betrayed by the darkening of the planet’s limb—another point of
+resemblance to the sun; while its special, or selective, absorption can
+only be detected with the spectroscope. The arresting effect of
+water-vapour was early noticed by Huggins and Vogel, and they measured a
+strong line in the red of unknown origin, but contained in banded star
+spectra. Atmospheric absorption is strongest above the ruddy equatorial
+belts, which are hence concluded to be placed at a lower level than the
+white surface.
+
+Planetary photography was set on foot by Dr. Gould of Boston, in 1879,
+when he obtained some promise of success with Mars, Jupiter, and Saturn;
+and Dr. Lohse prosecuted the subject in 1883. The actinic power of
+Jupiter’s light is very remarkable. It surpasses that of moonlight nine
+times, and that of Mars twenty-four times. Dr. Lohse further ascertained
+that the southern hemisphere is twice as chemically effective as the
+northern.[64] This superiority is doubtless connected with the greater
+physical agitation of the same region. A series of photographs of
+Jupiter, taken in 1891 with the great Lick refractor, were the first of
+any value for purposes of investigation. Each is one inch in diameter;
+the image of the planet having been enlarged eight times before being
+received upon the plate. Mr. Stanley Williams found them full of
+interesting detail. Figure 16 shows an enlargement of a striking
+photograph taken by Professor E. C. Pickering.
+
+Jupiter’s satellites were the first trophies of telescopic observation.
+They are, indeed, bright enough for naked eye perception, could they be
+removed from the disc which obscures them with its excessive splendour;
+and the first and third have actually been seen, in despite of the
+glare, by a few persons with phenomenally good eyesight. The
+mythological titles of the Galilean group—Io, Europa, Ganymede, and
+Calypso (proceeding from within outward) have been superseded by prosaic
+numbers. The change was unlucky, but is now probably irremediable.
+
+The Jovian family presents an animated and attractive spectacle. The
+smallest of its original members (No. II.) is almost exactly the size of
+our moon; the largest (No. III.), with its diameter of 3,550 miles,
+considerably exceeds the modest proportions of Mercury. Satellite I.
+revolves in 42½ hours at the same average distance from Jupiter’s
+surface that our moon does from that of the earth. No. II. has a period
+of 3 days 13 hours, and its distance from Jupiter’s centre is 415,000
+miles. Both these orbits are sensibly circular; and Nos. III. and IV.
+travel in ellipses of very small eccentricity, the one at a mean
+distance of 664,000, the other at 1,167,000 miles, in periods
+respectively of 7 days 4 hours, and 16 days 16½ hours. All four revolve
+strictly in the plane of Jupiter’s equator.
+
+[Illustration:
+
+  FIG. 16.—_Photograph of Jupiter. Exposure, 87 seconds._
+
+  (From _Knowledge_, November, 1889.)
+]
+
+They constitute a system bound together by peculiar dynamical relations,
+in consequence of which they can never be all either eclipsed, or seen
+aligned at one side of their primary, at the same time. They can all,
+however, be simultaneously hidden behind it, or in its shadow; although
+this moonless condition is looked out for as a telescopic rarity.
+
+The varied phenomena of eclipses, occultations, and transits, offer the
+interest, not only of predictions fulfilled, but sometimes of
+discrepancies detected. The three inner satellites plunge through the
+huge neighbouring shadow-cone at every revolution; the fourth, owing to
+its greater distance, escapes eclipse when the shadow makes an
+appreciable angle with the plane of its orbit. When Jupiter is in
+opposition or conjunction, occultations, but no eclipses, of his moons
+take place; at other periods, the two kinds of obscuration merge into,
+or succeed each other. “Time cannot stale their infinite variety.”
+
+From observations of the eclipses of Jupiter’s satellites, Olaus Römer
+gathered, in 1675, the first intimations of the finite velocity of
+light. He noticed that their visibility was alternately retarded and
+accelerated as the earth withdrew from, and approached the scene of
+their occurrence; and he designated half the extreme difference, or the
+time occupied by light in travelling from the earth to the sun, the
+“equation of light.” Its value is 500 seconds; and until recently, no
+other measure was available of that fundamental constant of nature—the
+rate of luminous transmission.
+
+The transits of the satellites across the Jovian disc present many
+curious appearances, due to complicated and changeable effects of light
+and shade both upon the planetary background, and upon the little
+circular objects self-compared with it. These, in the ordinary course,
+show bright while near the dusky limb, then vanish during the central
+passage, and re-emerge again bright at the opposite side. But, instead
+of duly vanishing, they now and then darken even to the point of
+becoming indistinguishable from their own shadows, by which they are
+preceded or followed. This difference of behaviour cannot be attributed
+wholly to varieties of lustre in the sections of the disc transited;
+otherwise, it could be predicted. But this has never been attempted;
+“black transits” come when least expected. The third and fourth
+satellites are those chiefly subject to these phases; the second has
+never been known to exhibit them; and they but slightly affect the
+first. A drawing by Professor Barnard of one of its bright transits with
+an attendant shadow that Peter Schlemyl might have envied, is reproduced
+in Figure 17. Its belted appearance, detected by that eminent observer,
+will be noted. Indeed, all the satellites, except perhaps No. II. are
+striped or spotted; and this leads to seeming deformations in their
+shape, as well as fluctuations in their brightness, the markings being
+evidently of atmospheric origin, and hence changeable. Their distinct
+and accurate perception has been made possible by the excellence of the
+Lick thirty-six inch refractor.
+
+[Illustration:
+
+  FIG. 17.—_Transit of Jupiter’s first Satellite, with Shadow, drawn by
+    Prof. Barnard, November 19, 1893._ (From _Monthly Notices_, January,
+    1894.)
+]
+
+Jupiter’s moons seem to resemble him in constitution. The three first
+possess the same high reflective power. No. II. is as bright as the
+planet’s brightest parts, so that its albedo cannot fall short of 0·70.
+And even No. IV. (formerly designated “Calypso” in reference to its
+frequent obscurations) exactly matches, during its darkest phases, the
+blue-grey polar hoods of its primary. On an average, too, the satellites
+seem to be of about the same mean density as Jupiter, No. I. being
+considerably the lightest for its bulk; and their spectra, according to
+Vogel’s observations in 1873, are composed of solar rays modified in
+precisely the same way as those reflected by the planet. Nothing is
+known quite certainly about their rotation-periods. Sir William Herschel
+concluded them to be of the same length with their periods of
+revolution; but recent work throws some doubt upon the reality of this
+agreement.
+
+The discovery, September 9, 1892, of Jupiter’s “fifth satellite” was one
+of the keenest astronomical surprises on record. An accession to a
+system so symmetrically arranged, so complete, to our judgment, as it
+stood, appeared superfluous, and, considering the eager scrutiny devoted
+to it during 282 years, well-nigh incredible. But the extra member was
+in truth out of reach until it was found; original discovery being, as
+every one knows, a greatly more arduous feat than subsequent
+verification. Nor could it have been casually detected. Professor
+Barnard seized the opportunity, lent by the specially favourable
+opposition of 1892, to rummage the system for novelties. Keeping the
+telescopic field dark by means of a metallic bar placed so as to occult
+the gorgeous planetary round, he sought, night after night, for what
+might appear. At length, on September 9, he caught the glimmer he
+wanted, and made sure, September 10, that it truly intimated the
+presence of a new satellite.
+
+This small body revolves in a period of 11 hours, 57 minutes, 23
+seconds, at a mean distance of 112,160 miles from Jupiter’s centre, or
+67,000 from his bulged equatorial surface. Hence, it should by right be
+called “No. I.” instead of “No. V.” The major axis of the ellipse in
+which it circulates advances so rapidly, owing to the disturbance caused
+by Jupiter’s spheroidal figure, as to complete a revolution in five
+months. The implied eccentricity of its orbit, as M. Tisserand has
+shown,[65] very slightly exceeds that of the orbit of Venus, yet it has
+been made obvious by Barnard’s observations of the differences between
+its east and west elongations. Its orbital velocity of 16½ miles a
+second far surpasses that of any other satellite in the solar system.
+Close vicinity to a mass so vast as Jupiter’s demands counter-balancing
+swiftness. Its period of revolution being, however, longer by one hour
+than Jupiter’s period of rotation, it so far conducts itself normally as
+to rise in the east and set in the west. On the other hand, since its
+progress over the sphere is measured by the difference between the two
+periods, it spends five Jovian days in journeying from one horizon to
+the other, running, in the meantime, four times through all its phases.
+Yet it never appears full. Jupiter’s voluminous shadow cuts off sunlight
+from it during nearly one-fifth of each circuit.
+
+It is an exceedingly elusive telescopic object. There is no chance of
+catching a glimpse of it except with a powerful and perfect telescope at
+its “elongations,” or furthest excursions of about eight seconds of arc
+on either side of the planet For the most part, it lurks within the
+blaze as closely as Teucer behind the shield of Ajax. It is far too
+small to be discerned in projection upon the disc, which, viewed from it
+in mid-transit, is _full_ with a diameter of 42° 2′, and an area 6,440
+times that of our moon. Yet, since its intrinsic lustre is less in the
+proportion of 2 to 15, the light shed by Jupiter upon the “fifth
+satellite” equals the joint radiance of no more than 860 full moons.
+
+The new satellite is indistinguishable in aspect from a star of the
+thirteenth magnitude. And its neighbour No. I. being of 5·6 magnitude,
+we receive from it 910 times more light than from the stranger. If both
+be equally reflective, the diameter of the latter is ¹⁄₃₀th the diameter
+of the former, or, approximately, 80 miles. But its albedo is unlikely
+to exceed that of Mars. By a rough estimate, therefore, this interesting
+object measures 120 miles across, and 9000 such miniature globes would
+go to the making of one full-sized Jovian attendant. Instead of being a
+late addition to the system, or, so to speak, an afterthought, it may be
+presumed, from the perceptible eccentricity of its path, to be the
+senior member of the family. But the subject of its origin is not yet
+ripe for discussion.
+
+
+
+
+                              CHAPTER IX.
+                         THE SATURNIAN SYSTEM.
+
+
+Nearly twice as far from the sun as Jupiter revolves a planet, the
+spacious orbit of which was, until 1781, supposed to mark the uttermost
+boundary of the solar system. The mean radius of that orbit is 886
+millions of miles; but in consequence of its eccentricity, the sun is
+displaced from its middle point to the extent of 50 million miles, and
+Saturn is accordingly 100 million miles nearer to him at perihelion than
+at aphelion. The immense round assigned to the “saturnine” planet is
+traversed in 29½ years, at the tardy pace of six miles a second. His
+seasons are thus twenty-nine times more protracted than ours, and are
+nominally more accentuated, since his axis of rotation deviates from the
+vertical by 27°. But solar heat, however distributed, plays an
+insignificant part in his internal economy. In the first place, its
+amount is only ¹⁄₉₁th its amount on the earth; in the second, Saturn,
+like Jupiter—even more than Jupiter—is thermally self-supporting. The
+bulk of his globe comparatively to its mass suffices in itself to make
+this certain. The mean diameter of Saturn is 71,000 miles, or nine times
+(very nearly) that of the earth; if of equal density, its mass should
+then be nine cubed, or 729 times the same unit The actual proportion,
+however, is 95; hence the planet has a mean density of only ⁹⁵⁄₇₂₉, or
+between ⅐th and ⅛th the terrestrial, and being thus composed of matter
+as light as cork, would float in water. Professor G. H. Darwin has
+moreover demonstrated, from the movements of its largest satellite, that
+its density gains markedly with descent into the interior, so that its
+surface-materials must be lighter than any known solid or liquid.
+
+When at its nearest to the earth, Saturn is as large as a sixpence held
+up at a distance of 210 yards.[66] But instead of being round like a
+sixpence, it is strongly compressed—more compressed even than Jupiter.
+The spectra of the two planets are almost identical. Both are impressed
+with traces of aqueous absorption, and include the “red star line.”
+About the albedo of Saturn there is some uncertainty. Zöllner made it
+0·50, a very probable value; Müller of Potsdam determined it at 3·3
+times that of Mars, the unit of his scale. For the value of the unit,
+the only authority is Zöllner, who found Mars to give back 0·26 of the
+light dispensed to him. Multiplying then 0·26 by 3·3 we get for the
+albedo of Saturn 0·86, an impossible number for a non-luminous body, the
+albedo of “untrodden snow” being, as already stated, 0·78.
+
+Saturn resembles to the eye a large, dull star; its rays are entirely
+devoid of the sparkling quality which distinguishes those of Jupiter.
+But it shows telescopically an analogous surface-structure. Its most
+conspicuous markings are tropical dark belts of a greyish or greenish
+hue; the equatorial region is light yellow, diversified by vague white
+spots; while the poles carry extensive pale blue canopies. The apparent
+tranquillity of the disc may be attributed in part to the vast distance
+from which it is viewed; yet not wholly. For lack of fiducial points, no
+attempt was made to determine the planet’s rotation until 1794, when the
+elder Herschel, by following an identified irregularity in a complex
+banded formation, arrived at a period of 10 hours 16 minutes. The first
+possibility of checking this result offered itself to Professor Hall of
+Washington, after fourteen years of vain expectation, in the emergence
+of a white spot just north of the equator, the movement of which gave
+for the length of the Saturnian day, 10 hours, 14 minutes, 24 seconds.
+In 1891–2, Mr. Stanley Williams made observations upon a good many such
+objects; and their discussion by Mr. Denning afforded a mean period two
+seconds longer than Hall’s. Individual variations, however, to the
+extent of 14 seconds were brought out by it, proving that Saturnian,
+like Jovian, spots have “proper motions,” and cannot be depended upon to
+give the true rotation of the planet. Its compound nature may be
+suspected, but has not yet been proved.
+
+From measures executed by Barnard in 1895, it appears that the
+equatorial diameter of Saturn is 76,470, its polar diameter 69,770
+miles, giving a mean diameter of 74,240, and a compression of about
+¹⁄₁₂. Gravity, at its surface, is only one-fifth more powerful than on
+the earth.
+
+Thus, Saturn not only belongs to the same celestial species as Jupiter,
+but is a closely-related individual of that species. There is no
+probability that either is to any extent solid. Both exhibit the same
+type of markings; both betray internal tumults by eruptions of spots
+which, by their varying movements, supply a measure for the profundity
+of their origin; both possess identically constituted atmospheres, and
+are darkened marginally by atmospheric absorption.
+
+[Illustration:
+
+  FIG. 18.—_Saturn and its Rings. Drawn by Prof. Barnard, July 2, 1894._
+]
+
+Saturn is, however, distinguished by the possession of an unique set of
+appendages. Nothing like them is to be seen elsewhere in the heavens;
+and when well opened (as in Fig. 18) they form, with the globe they
+enclose, and the retinue of satellites in waiting outside, a strange and
+wonderful telescopic object. The rings, since they lie in the plane of
+Saturn’s equator, are inclined 27° to the Saturnian orbit, and 28° to
+the ecliptic. The earth is, however, comparatively to Saturn, so near
+the sun, that their variations in aspect, as viewed from it, may in a
+rough way be considered the same as if seen from the sun. They
+correspond exactly with the Saturnian seasons. At the Saturnian
+equinoxes, the rings are illuminated edgewise, and disappear, totally or
+approximately; at the Saturnian solstices, sunlight strikes them nearly
+at the full angle of 27°, first from _below_, then from _above_. At
+these epochs, we perceive the appendage expanded into an ellipse about
+half as wide as it is long. Two concentric rings (generally called A and
+B) are then very plainly distinguishable, the inner being the brighter.
+The black fissure which separates them is called “Cassini’s division,”
+because that eminent observer was, in 1675, the first to perceive it. A
+chasm known as “Encke’s division,” in the outer ring (A), is a thinning
+out rather than an empty space; and temporary gaps frequently appear in
+A, while B is entirely exempt from them. There are then two definite and
+permanent bright rings, and no more; but with them is associated the
+dusky formation discovered by W. C. Bond, November 15, 1850, and
+described by Lassell as “something like a crape veil covering a part of
+the sky within the inner ring.” It is semi-transparent the limb of
+Saturn showing distinctly through it.
+
+The exterior diameter of the ring-system is 172,800, while its breadth
+is 42,300 miles.[67] The rings A and C are each 11,000 miles wide; while
+B measures 18,000, Cassini’s division 2,270, and the clear interval
+between C and the planetary surface somewhat less than 6,000 miles. Each
+ring, C included, is brightest at its outer edge; but there is no gap
+between the shining and the dusky structures, B shading by insensible
+gradations up to C, yet maintaining distinctness from it. The earliest
+exact determinations of the former were made by Bradley in 1719, since
+when they have been affected by no appreciable change.[68] The
+theoretically inevitable subversion of the system is progressing with
+extreme slowness.
+
+The thickness of the rings is quite inconsiderable. They are flat
+sheets, without (so to speak) a third dimension. For this reason, they
+disappear utterly in most telescopes, when their plane passes through
+the earth, as it does twice in each Saturnian year. Only under
+exceptional conditions, a narrow, knotted, often nebulous, streak
+survives as an index to their whereabouts. On October 26, 1891,
+Professor Barnard,[69] armed with the Lick refractor, found it
+impossible to see them projected upon the sky, notwithstanding that
+their shadow lay heavily on the planet It was not until three days
+later, that “slender threads of light” came into view. The corresponding
+thickness of the formation was estimated at less than fifty miles. The
+phenomenon of the disappearance of the rings will not recur until July
+29, 1907.
+
+The constitution of this marvellous structure is no longer doubtful. It
+represents what might be called the fixed form of a revolving multitude
+of diminutive bodies. This was demonstrated by Clerk Maxwell in the
+Adams Prize Essay of 1857. His conclusion proved irreversible. The
+pulverulent composition of Saturn’s rings is one of the acquired truths
+of science. An incalculable number of tiny satellites, revolving
+independently in distinct orbits, in the precise periods prescribed by
+their several distances from the planet, are aggregated into the
+unmatched appendages of Galileo’s _tergeminus planeta_. The local
+differences in their brightness depend upon the distribution of the
+component satelloids. Where they are closely packed, as in the outer
+margins of rings A and B, sunlight is copiously reflected; where the
+interspaces are wide, the blackness of the sky is barely veiled by the
+scanty rays thrown back from the thinly scattered cosmic dust. The
+appearance of the crape ring as a _dark_ stripe on the planet results—as
+M. Seeliger has pointed out—not from the transits of the objects
+themselves, but from the flitting of their shadows in continual
+procession across the disc.
+
+The albedo of these particles is so high as to render it improbable that
+they are of an earthy or rocky nature, such as the meteorites which
+penetrate our atmosphere. The rings they form are, on the whole, more
+lustrous than Saturn’s globe; but this superiority is held to be due to
+the absence of atmospheric absorption. Their spectrum is that of
+unmodified sunlight.
+
+An eclipse of Japetus, the eighth Saturnian moon, by the globe and
+rings, November 1, 1889, was highly instructive as to the nature of the
+dusky appendage. The satellite was never lost sight of during its
+passage behind it; but became more and more deeply obscured as it
+travelled outward; then, at the moment of ingress into the shadow of
+ring B, suddenly disappeared. Certainty was thus acquired that the
+particles forming the crape ring are most sparsely strewn at its inner
+edge—which is, nevertheless, perfectly definite—and gradually reach a
+maximum of density at its outer edge. Yet, while there is not the
+smallest clear interval, a sharp line of demarcation separates it from
+the contiguous bright ring. Professor Barnard was the only observer of
+these curious appearances. The distribution of the ring-constituents,
+like that of the asteroids, was governed by the law of commensurable
+periods, Saturn’s moons replacing Jupiter as the perturbing and
+regulating power. Kirkwood showed in 1867, that Cassini’s division
+represents a region of peculiarly strong disturbance; since a body
+revolving there would have a period connected by a simple relation with
+the periods of no less than _four_ satellites. Encke’s division, too, as
+Dr. Meyer has indicated, and other lines of scanty occupation and
+occasional vacancy, coincide with districts of space where similar
+combinations occur.
+
+The “satellite-theory” of Saturn’s rings has received confirmation from
+apparently the least promising quarters. Professor Seeliger of Munich
+showed, from photometric experiments in 1888, that their constant lustre
+under angles of illumination ranging from 0° to 30° was proof positive
+of their composition out of discrete small bodies.[70] And Professor
+Keeler of Alleghany, by a beautiful and refined application of the
+spectroscopic method, arrived at the same result in April, 1895.[71]
+“Under the two different hypotheses,” he remarked, “that the ring is a
+rigid body, and that it is a swarm of satellites, the relative motion of
+its parts would be essentially different.” The former would necessarily
+involve increasing velocity _outward_, the latter, increase of velocity
+_inward_, just for the same reason that Mercury moves more swiftly than
+the earth, and the earth than Saturn; while the sections of a solid
+body, which could have but one period of rotation, should move faster,
+_in miles per second_, the farther they were from the centre of
+attraction. The line of sight test is then theoretically available; but
+it was an arduous task to render it practically so. The difficulties
+were, however, one by one overcome; and a successful photograph of the
+spectra of Saturn and its rings gave the required information in
+unmistakable shape. From measurements of the inclinations of five dusky
+rays contained in it with reference to a standard horizontal line, rates
+of movement were derived of 12½ miles per second for the inner edge of
+ring B, and of 10 miles for the outer edge of ring A. The agreement with
+theory was, as nearly as possible, exact; the components of the rings
+were experimentally demonstrated to be moving, each independently of
+every other, under the dominion of Kepler’s laws.
+
+For the globe of Saturn, Professor Keeler obtained, by the same
+exquisite method, a rotational period of 10 hours, 14 minutes, 24
+seconds, in precise accordance with that indicated by the white spot of
+1876, which thus seems to have had no proper motion, but to have floated
+on the ochreous equatorial surface as tranquilly as a water-lily upon a
+stagnant pool. The result, so far as it goes, hints that Saturn may be
+really, as well as apparently, less ebullient than Jupiter.
+
+Seers into the future of the heavenly bodies consider that the rings of
+Saturn, like the gills of a tadpole, are symptomatic of an early stage
+of development; and will be disposed of before he arrives at maturity.
+They cannot be regarded otherwise than as abnormal excrescences. No
+other planet retains matter circulating round it in such close relative
+vicinity. It was proved by Roche of Montpellier that no secondary body
+of importance can exist within less than 2·44 mean radii of its primary;
+inside of that limit, it would be rent asunder by tidal strain. But the
+entire ring-system lies within the assigned boundary; hence, being
+_where_ it is, it can only exist _as_ it is—in flights of discrete
+particles. Will it, however, always remain where it is?
+
+“Clerk Maxwell,” wrote Mr. Cowper Ranyard,[72] “used to describe the
+matter of the rings as a shower of brickbats, amongst which there would
+inevitably be continual collisions. The theoretical results of such
+impacts would be a spreading of the ring both inwards and outwards. The
+outward spreading will in time carry the meteorites beyond Roche’s
+limit, where, in all probability, they will, as Professor Darwin
+suggests, slowly aggregate, and a minute satellite will be formed. The
+inward spreading will in time carry the meteorites at the inner edge of
+the ring into the atmosphere of the planet, where they will become
+incandescent, and disappear as meteorites do in our atmosphere.”
+
+Yet it may be that collisions are infrequent in this conglomeration of
+“brickbats.” There is the strongest presumption that they all circulate
+in the same direction, in orbits nearly circular, and scarcely deviating
+from the plane of the Saturnian equator. Those pursuing markedly
+eccentric tracks must long ago have been eliminated. Thus, encounters
+can only occur through gravitational disturbances by Saturn’s moons, and
+they must be of a mild character, depending upon very small differences
+of velocity. The first sign of a “spreading outwards” should be the
+formation of an exterior “crape ring,” of which no faintest trace has
+yet been perceived.
+
+Saturn’s rings are entirely invisible from its polar regions, but
+occasion prolonged and complex eclipse-effects in its temperate and
+equatorial zones. They have been fully treated of from the geometrical
+point of view by Mr. Proctor in “Saturn and its System.”
+
+Of this planet’s eight satellites, the largest, Titan (No. VI.), was
+discovered first (by Huygens in 1655), and the smallest, Hyperion (No.
+VII.), last (by Lassell and Bond in 1848). The five others were detected
+by J. D. Cassini and William Herschel. Titan, alone of the entire group,
+equals our moon in size. It measures, according to Professor Barnard,
+2,720 miles across. Its period of revolution is nearly sixteen days, its
+distance from Saturn’s centre, 771,000 miles. The orbit of Japetus (No.
+VIII.) is the largest, and its period the longest of any secondary body
+in the solar system. It circulates in 79⅓ days at a distance of
+2,225,000 miles, equal to 59½ of Saturn’s equatorial radii. Hence its
+path is of about the same _proportional_ dimensions as that of our moon.
+Japetus is remarkable for its variability in light. It is capable of
+tripling or quadrupling its minimum lustre. Sir William Herschel noticed
+that these maxima coincided with a position on the western side of the
+planet, and inferred rotation of the lunar kind. “From the changes in
+this body,” he argued in 1792,[73] “we may conclude that some part of
+its surface, and this by far the largest, reflects much less light than
+the rest; and that neither the darkest nor the brightest side is turned
+towards the planet, but partly one and partly the other, though probably
+less of the bright side.”
+
+This explanation, however, he admitted to be incomplete. There was, and
+is, outstanding variability, which seems to intimate the presence of an
+atmosphere and the formation of clouds. But no positive knowledge has
+yet been gained regarding the physical state of Saturn’s moons. We may
+nevertheless conjecture that, since tidal friction has destroyed the
+rotation (as regards Saturn) of the remotest member of the family, it
+has not spared those more exposed to its grinding-down action. All
+presumably rotate in the same time that they revolve.
+
+The five inner satellites move in approximately circular orbits; the
+three outer in ellipses about twice as eccentric as the terrestrial
+path. All, Japetus only excepted, keep strictly to the plane of the
+rings. And since this makes an angle of 270 with the planet’s orbit,
+eclipses are much less frequent here than in the Jovian system. They can
+only occur when Saturn is within a certain distance (different for each)
+from the node of the satellite-orbit. Even Mimas (No. I.), although it
+wheels round the ring at an interval of only 34,000 miles, often slips
+outside the obliquely-projected shadow-cone. Its distance from Saturn’s
+centre is 118,000 miles, and it completes a circuit in 22½ hours.
+Perpetually wrapped in the glare of its magnificent primary, it is a
+very shy object, only to be caught sight of in its timid excursions by
+the very finest telescopes. Like all the Saturnian moons, except Titan,
+and, by a rare conjuncture, Japetus, it is far too much contracted to be
+visible in transit across the disc.
+
+The movements of these bodies have been carefully studied, and their
+mutual perturbations to some extent unravelled. They have proved
+exceedingly interesting to students of celestial mechanics. Titan has,
+in this department, chiefly to be reckoned with. He exercises in the
+Saturnian system a similar overpowering influence to that wielded by
+Jupiter in the solar system. Mr. Stone finds its mass to be ¹⁄₇₆₀₀th
+that of Saturn, showing that its density is nearly equal to that of our
+moon. This seems to indicate an advanced stage of cooling. On the other
+hand, its albedo is evidently very high. The other satellites appear in
+the largest telescopes as mere stellar points.
+
+
+
+
+                               CHAPTER X.
+                          URANUS AND NEPTUNE.
+
+
+The four giant planets, closely allied as they are, and strongly
+distinguished in physical constitution from the terrestrial planets,
+divide again of themselves into two sub-groups. Jupiter and Saturn have
+much more in common than either has with Uranus or Neptune; while Uranus
+and Neptune present peculiar analogies. Conclusions concerning one may
+almost be said to apply to the other. Their enormous distance, it is
+true, tends to efface minor differences; yet it is insufficient to
+obliterate similarities of a peculiar kind.
+
+Uranus is a globe 32,000 miles in mean diameter, and decidedly
+elliptical in shape. Mädler and Schiaparelli agreed in assigning to it a
+compression of ¹⁄₁₁; Barnard, in 1894, uninformed of their results,
+noticed the disc to be more oval than Saturn’s. The indicated rotational
+movement must be very swift; and a lucid spot watched by MM. Perrotin
+and Thollon at Nice in 1884, seemed to fix it at about ten hours. This
+was, however, only a vague estimate. Faint equatorial belts, too, have
+with difficulty been seen. Remembering, indeed, that the object they
+diversify is just large enough to be _annularly eclipsed_ by a cricket
+ball two miles off, there is little cause for surprise at the
+indistinctness of its surface-markings. They probably consist, like
+those of Jupiter and Saturn, in dusky polar hoods, a brilliant
+equatorial zone, and obscure intermediate bands. The last were seen as
+“the merest shades on the planet’s surface,” and under a somewhat
+deformed aspect, by the Lick observers in 1890 and 1891.[74] By
+Professor Young in 1883, on the other hand, and by the MM. Henry at
+Paris in 1884, they were observed to be symmetrically placed, parallel
+one to the other, and of what might be called the normal type for great
+planets. That they constitute, with the bright space they enclose, an
+equatorial scheme of marking, was proved by Barnard’s comparison of the
+trend (or position angle), determined for them by Young, with the
+direction of the shortest axis of the little disc they traverse.[75]
+Their considerable foreshortening in 1894 was, doubtless, the reason why
+Barnard, with his acute vision, was compelled to rely upon earlier
+observations, brought up to date by computation. Unless, indeed, the
+markings are intrinsically variable.
+
+This was suspected at Nice in 1889, when a thirty-inch refractor was
+available for their scrutiny.[76] Dusky rulings were obvious on a
+strongly compressed spheroid; and they ran parallel to the major axis of
+the spheroid—that is, to the planet’s equator. But their appearance
+varied, and their width seemed irregular. At the same establishment, but
+with a fourteen-inch telescope, Uranus was observed, under particularly
+favourable circumstances, March 18, 1884.[77] An unexpected resemblance
+to Mars was apparent. The ordinarily sea-green disc was divided into a
+sombre north-western and a bluish-white south-eastern hemisphere. Dark
+spots were visible, and a conspicuous white one at the limb simulated a
+snow-cap. But ulterior observations resolved the spots into belts, and
+showed the shining patch to be, not polar, but equatorial. It was
+presumably of an eruptive nature.
+
+The axis upon which Uranus rotates is very much bowed towards the plane
+of its orbit. Its seasons are hence abnormal; but their vicissitudes can
+scarcely be sensible at a distance from the sun more than twice that of
+Saturn. This, as Mr. Proctor noticed, is the only case in which the
+ratio of one to two is exceeded in the radii of two adjacent planetary
+orbits. The radius of the Uranian track, pursued at the leisurely pace
+of 4⅕ miles a second, is 1,782 millions of miles, or more than 19
+astronomical units. It consequently receives from the sun 370 times less
+warmth and light than the earth does. Area for area, it is true, the sun
+shines with the same intensity there as here; the difference lies in its
+apparent size. Instead of the broad eye of day to which we are
+accustomed, the luminary of Uranus presents a surface only 2¼ times that
+of Jupiter, as seen from the earth at an _unfavourable_ opposition; and
+although Uranus is 166 millions of miles nearer to the sun at perihelion
+than at aphelion, no conspicuous difference would mark the passage from
+one to the opposite point. This is accomplished in 42, the entire round
+in 84 years.
+
+In point of size, as Professor Young remarks, Uranus compares with the
+earth very much as the earth compares with the moon. For its surface
+exceeds the terrestrial surface about sixteen times, and its volume
+amounts to sixty-six times the terrestrial volume. Its mass, however, is
+less than fifteen times that of the earth, whence its density is
+represented (in round numbers) by the fraction ¹⁵⁄₆₆. The large globe is
+then nearly five times less dense than the small one, its materials
+exceeding the weight of an equal bulk of water by only one-fifth.
+Gravity is actually less at its surface than at the sea-level on the
+earth. Every ton of coal, for instance, delivered in that remote globe
+would fall short by two hundred pounds. The albedo of Uranus differs
+little from that of Jupiter; if anything, it is somewhat higher, and is
+nearly represented by the brilliancy of white paper.
+
+The spectrum of Uranus indicates an emphatic departure from the
+planetary conditions so far met with. This body is obviously surrounded
+by a powerfully absorptive atmosphere, of a constitution foreign to our
+experience. The greenish hue of the light which has traversed some of
+its strata gives a preliminary indication of the manner in which it has
+been affected. This its spectrum, first inspected by Secchi in 1869,
+expounds in detail. He noticed a number of heavy dark bands in the red,
+while the green and blue sections remaining open gave to the planet its
+characteristic colour. A couple of years later, Huggins and Vogel
+executed concordant measurements of six pronounced bands, besides some
+faint streaks; and on June 3, 1889, the former obtained, with two hours’
+exposure, a beautiful spectrographic impression extending far up into
+the ultra-violet. A corroborative, though less comprehensive, photograph
+was taken by Mr. Frost at Potsdam, April 23, 1892. Both included many
+Fraunhofer lines, the presence of which demonstrates that the light of
+Uranus, although more powerfully stamped with original absorption than
+that of the rest of the planets, consists essentially of reflected solar
+rays. Professor Keeler’s admirable series of visual observations with
+the Lick refractor were undertaken in 1889 to test the truth of a
+suggestion that this peculiar spectrum consisted of bright bands upon a
+dark ground, and not of dark bands upon a bright ground. His decision in
+favour of the latter alternative was without appeal.
+
+Of the six principal dark bands representing the arresting action upon
+light of the planetary atmosphere, four are quite distinctive; the fifth
+is the “red star line” common to the spectra of Jupiter and Saturn; the
+sixth is the hydrogen “F” (Hβ)—not definite and narrow as it is seen in
+the solar spectrum, but hazy, and graduating in darkness towards the
+middle, an undoubted outcome of native absorption.[78] Now, this is a
+fact that implies a great deal. It gives direct evidence of a very high
+temperature. Free hydrogen ceases to be present in a body upon which
+water can form—given, of course, the presence of oxygen, which it would
+be in the highest degree arbitrary to exclude. At one epoch of its
+development, the earth must have been surrounded by immense volumes of
+hydrogen. But with the diminution of heat, union with oxygen became
+possible, and the gas vanished to reappear in the form of liquid oceans,
+with their related hydrographic and cloud-systems. Uranus is
+presumably—almost certainly—still too hot to permit the combination of
+hydrogen and oxygen; and the absence from its spectrum of the slightest
+trace of aqueous absorption strengthens this inference. Doubtless, the
+time will come when the two elements will no longer be held at arms’
+length; their affinities will come into play; the familiar,
+all-important terrestrial liquid will be formed, and the geological
+history of Uranus will begin.
+
+Uranus is attended by four moons. They are named Ariel, Umbriel, Titania
+and Oberon. Titania—the third in order of distance from the primary—is
+the brightest of the group, and has a diameter of possibly one thousand
+miles. Oberon is slightly inferior. Both were detected by Herschel in
+1787. Ariel and Umbriel, captured by Lassell at Malta in 1851, are
+insignificant bodies in themselves—their dimensions probably differing
+but slightly from those of Hyperion, the seventh and least Saturnian
+moon, estimated to measure five hundred miles across. They are among the
+most difficult of telescopic objects, since they circulate about as
+close to Uranus as Mimas and Enceladus do to Saturn, are physically
+smaller, and more than twice as remote from the earth. Both were
+believed variable by Lassell, and Newcomb obtained in 1875 plausible,
+though not convincing, evidence that Ariel, at any rate, is subject to
+light changes in the period of its orbital circulation, showing that,
+here again, tidal friction has done its work of synchronising rotation
+and revolution.[79] None of the four orbits are appreciably eccentric;
+they all lie in the same plane, and are described in periods ranging
+from 2½ to 13½ days.
+
+The position of that plane is, however, exceedingly remarkable. It is
+tilted at an angle of 98° to the ecliptic. This means that the
+satellites move _backward_, against the succession of the zodiacal
+signs. For direct becomes retrograde motion automatically, so to speak,
+by turning the plane in which it is performed beyond the limit of the
+vertical. The same fact is merely expressed in two different ways by
+saying that the bodies in question travel from west to east at an angle
+of 98°, or from east to west at an angle of 82° to the ecliptic. The
+planes of the ecliptic and of the Uranian orbit deviate, it should be
+mentioned, by only two-thirds of a degree. The disturbance by which the
+Uranian system was set topsy-turvy did not in the least affect the
+motion of Uranus itself.
+
+Another unusual circumstance about that system is that the
+satellite-plane departs widely from the equatorial plane. Our own moon,
+it is true, is similarly circumstanced; but, on the Uranian scale, it is
+nearly eight times farther from its primary than Ariel, and 2·6 times
+farther than Oberon; while the enormous equatorial protuberance of
+Uranus almost seems to impose conformity upon bodies revolving so close
+to it. Conformity, none the less, is absent. The direction taken by the
+equator of Uranus, as we have seen, is indicated in a two-fold manner:
+first, by the trend of the belts; secondly, by the lie of the major
+axis. And these indications agree. Supposed discrepancies between them
+have been reconciled by improvements in the conditions of observation.
+But with the equatorial line the plane of satellite-revolution cannot be
+brought to coincide. The angle of divergence is uncertain, but may be
+put roughly at 20°. This would give 78° for the inclination of the
+Uranian equator, so that the rotation of the planet is likely to be
+direct. If so, the extraordinary anomaly is here met with of a
+satellite-system circulating in a direction opposite to that of its
+primary’s rotation.
+
+Uranus can at times be perceived with the naked eye. Indian traditions
+of an eighth “dark” planet have been thought to refer to it, and its
+slow course among the stars had been noted by savage tribes long before
+Herschel singled it out from them by its tiny disc. It is about three
+times brighter than Vesta; and Mr. Proctor stated that “in the summer of
+1887 they were comparable under favourable conditions,” when both, in
+the transparent skies of Florida, were “quite conspicuous without
+telescopic aid.” Twenty chances of discovering Uranus were missed before
+it came to Herschel’s turn. So many times it had been located or
+catalogued as a fixed star by astronomers far from indifferent to
+immortal fame.
+
+Neptune is much nearer to the sun than it ought to be. Both Leverrier
+and Adams assumed that Bode’s law would hold good for the planet still
+below the horizon of knowledge; they could do no otherwise; yet the rule
+played them false. Some have even asserted paradoxically that the planet
+found was not the planet sought. In point of fact, the distance of the
+theoretical Neptune is thirty-eight, that of the real Neptune thirty
+astronomical units. The mean radius of its orbit measures 2,792 million
+miles. Hence the sun is reduced to ¹⁄₉₀₀th its terrestrial brilliancy,
+and could be replaced by 687 full moons. “As seen from Neptune,”
+Professor Young remarks, “the sun would look very much like a large
+electric arc lamp at a distance of a few feet. It would give about
+forty-four millions the light of a first-magnitude star.”[80]
+Accordingly, Neptune does not circulate by any means in outer darkness.
+His orbit, although very slightly eccentric, brings him at perihelion
+fifty millions of miles nearer to the sun than at aphelion. It makes an
+angle of less than 2° with the ecliptic, and is traversed, at the rate
+of 3⅓ miles a second, in a period of 165 years.
+
+Neptune, being fainter than the eighth stellar magnitude, is quite
+inaccessible to unaided vision. But a good telescope at once displays
+the seeming star in the guise of a small planetary nebula with a
+diameter of 2″·433. This mean value, reduced to the mean distance of the
+planet from the sun, was afforded by Barnard’s measures in 1895 with a
+power of 1,000 on the Lick refractor.[81] It corresponds to a linear
+diameter of 32,900 miles. Neptune accordingly, although only 17 times
+more massive than the earth, is 72 times more bulky, and composed of
+materials 4·2 times specifically lighter. Gravity at its surface has
+almost precisely its terrestrial power. The albedo of Neptune, combining
+Zöllner’s with Müller’s results, is 0·65; and its spectrum appears
+identical with that of Uranus. It may be inferred that this planet also
+is too hot to contain water.
+
+Its satellite is believed to be of about the size of the moon; but since
+it is 12,000 times more distant, it can be distinguished only with the
+most powerful telescopes as a star of the fourteenth magnitude. The
+radius of its orbit measures 225,000, that of our moon 238,000 miles;
+but Neptune’s attendant completes a circuit in 5 days 21 hours; and it
+is through this rapidity of movement that the large mass of its primary
+has been learned. It resembles the moon besides in being solitary, so
+far as can be ascertained by the most diligent researches; and it is
+beyond doubt that if any companion-bodies exist they are comparatively
+small or obscure. That they do exist, appears probable on the face of
+it.
+
+The one Neptunian satellite emphasises the problems set by the Uranian
+four. These problems are concerned with the origin and early mechanical
+relations of the solar system. Here, at its utmost verge, we encounter a
+decided reversal in the direction of systemic motion—a reversal prepared
+for, as it might seem, by the nearly vertical position of the Uranian
+plane of satellite-revolution. This diversity is in no sense
+“accidental,” as some have unwisely asserted, invoking impacts of
+comets, and such like futile devices, to account for it; it belongs
+fundamentally to the design of planetary evolution. Laplace’s scheme has
+no room for it; Faye’s, constructed expressly to include it, requires
+that Uranus and Neptune, instead of being the first, should have been
+the latest formed of all the solar train. And their obviously
+rudimentary condition favours the suggestion. Neptune’s satellite
+revolves from east to west in a quasi-circular path, inclined to the
+ecliptic at an angle of 35°; or, putting it otherwise, it revolves from
+west to east at an angle of 145°.
+
+As the only member of the solar system exempt from perturbations by a
+third body (the sun being too remote to cause perceptible deflections),
+it seemed admirably fitted to discharge the functions of a standard
+celestial clock, greatly needed, but nowhere to be found in our
+system.[82] But in 1886 Mr. Marth drew attention to certain divagations
+of this “ideal time-keeper” resulting from conspicuous changes in the
+position and plane of its orbit. They were explained almost
+simultaneously in 1888 by M. Tisserand,[83] late director of the Paris
+Observatory, and by Professor Newcomb.[84] The disturbance, which, in
+its mode of production, is analogous to the precession of the equinoxes,
+results from the polar compression of the Neptunian globe combined with
+a deviation of the satellite’s motion from its equatorial plane. By the
+action of the protuberant girdle, a slow gyration of the secondary
+body’s orbital plane is produced, its inclination to the primary’s
+equator remaining unchanged. Viewed under a different aspect, the same
+phenomenon may be described as a retrograde movement, in a period of at
+least five hundred years, of the pole of the satellite’s orbit round the
+pole of the planet’s equator. The radius of the circle described cannot
+be less than 20°, implying a flattening of the Neptunian globe of
+¹⁄₈₅th, and may easily amount to 30°, with which an ellipticity of ¹⁄₁₁₅
+should be associated. But before the centre of this circle—that is, the
+pole of Neptune’s axial movement—can be satisfactorily located, several
+centuries must elapse. At present we may affirm with reasonable
+certainty: first, that the rotation in question is retrograde, like the
+satellite’s revolution; secondly, basing the inference upon the
+comparatively slight ellipticity of Neptune’s figure, that it is much
+slower than the vertiginous spinning of Jupiter, Saturn, and Uranus.
+
+Uranus and Neptune are, as has been said, companion globes. In bulk and
+density they differ very slightly; their albedoes are virtually the
+same, their spectra indistinguishable. They seem perfectly alike in
+chemical and physical constitution, and to be situated at precisely the
+same stage of development. Both govern retrograde systems. In Uranus the
+peculiarity appears as if in an incipient form; in Neptune, strongly
+accentuated.
+
+Viewed from the position of Neptune, all the planets are morning and
+evening stars. They are tethered to the chariot-wheels of the sun,
+instead of having the run of the sky. “The four terrestrial planets,”
+Professor Young writes, “would be hopelessly invisible, unless with
+powerful telescopes, and by carefully screening off sunlight. Mars would
+never reach an elongation of three degrees from the sun; the maximum
+elongation of the earth would be two, and that of Venus about one and a
+half degrees. Jupiter, attaining an elongation of about ten degrees,
+would probably be easily seen somewhat as we see Mercury. Saturn and
+Uranus would be conspicuous, though the latter is the only planet of the
+whole system that can be better seen from Neptune than it can be from
+the earth.”[85]
+
+To a spectator retreating with the velocity of light, all the planetary
+cortège would in a few hours disappear, and the sun would shine alone.
+No sign would remain that his office is purely ministerial—that he
+exists only to enlighten, rule, and vivify the relatively minute globes
+shred from his mass in the beginning, maintaining by his attractive
+power the adjusted movements of the complicated piece of mechanism they
+constitute. The skies perhaps hold millions of his stamp; every solitary
+star telescopically visible may be the centre of a planetary scheme like
+our own; or, on the other hand, our own may, quite conceivably, have no
+counterpart in the wide universe.
+
+
+
+
+                              CHAPTER XI.
+                             FAMOUS COMETS.
+
+
+In the fourth year of the 101st Olympiad (373 B.C.), the Greeks were
+startled by a celestial portent. They did not, at that time, draw fine
+distinctions, and posterity would have remained ignorant that the
+terrifying object was a great comet but for the description of it left
+by Aristotle, who saw it as a boy at Stagira. It was mid-winter when it
+flared up from due west at sunset, its narrow, definite tail running
+“like a road through the constellations” over a third of the heavens.
+Diodorus relates that it cast shadows like the moon, which implies a
+very unusual, yet not impossible, degree of brightness. The prompt
+engulfment by an earthquake and its attendant tidal wave of the Achaean
+towns, Helice and Bura, justified the apprehensions it aroused. It never
+came back to retrieve its reputation. During at least two thousand
+subsequent years, such objects lay under the ban of popular
+superstition; and the counts upon which they were accused of malefic
+influence were so many and so vague that acquittal was impossible. Their
+respect of persons was notorious; nor were they consistent in their
+dealings with the great, to whom alone they paid individual attention. A
+comet marked the apotheosis of the great Julius; a comet announced the
+death of Constantine; a comet illuminated the cradle of Napoleon.
+
+The very word “comet” takes us back to the Stagyrite; for it is derived
+from the Greek word κόμη, hair, and signifies a _hirsute_ star.
+Shakspeare’s “crystal tresses” represent what we now, in homely fashion,
+call the “tail,” while the “nucleus” and “coma” make up the “head.” The
+nucleus, in great comets, shines like a star of the first magnitude,
+sometimes indeed surpassing the brilliancy of Jupiter. It is usually of
+measurable dimensions, often of granular texture. The planetary disc,
+round which the filmy appendages of the comet of December 1618 were
+displayed, was observed by Cysatus, a Jesuit astronomer at Ingolstadt,
+to become transformed into the semblance of a star cluster; Hevelius
+noticed a double nucleus in the comet of 1652; and modern instances of
+the same kind abound. There is indeed no likelihood that substantial
+globes are ever included in the construction of comets.
+
+The coma is of immense volume, and extreme tenuity. The rays of faint
+stars traverse, undimmed and unrefracted, strata of it tens of thousands
+of miles in thickness. Yet strong lines of structure develop in it
+through the influence of forces emanating from the sun. As they approach
+our system out of the depths of space, comets are scarcely
+distinguishable from round nebulæ, and they relapse into a similar
+quiescent condition on leaving it. Their temperature must then be very
+near the absolute zero of cold, since they cannot be supposed either to
+contain stores of native heat, or to retain stores of borrowed heat.
+Thus the rapidly augmenting power of solar radiation, as they rush with
+accelerated velocity nearer and nearer to its source, produce upon them
+stupendous effects. The nucleus blazes out into a coruscating star; the
+coma, violently driven off from it, forms multiple envelopes like thin
+gauze veils, one outside the other, flung round the nucleus on the side
+next the sun, separated by intervening dark spaces, and diversified by
+brilliant jets and sectors. The tail is the outcome of a double
+repulsion. Matter expelled by the nucleus towards the sun is, at a
+certain point, thrown back to form an immense, oppositely directed
+appendage, usually convex on the forward side. Some tails resemble
+hollow cones, being bright at the edges, and dark within: others are
+traversed by a shining _backbone_; many, perhaps all, are composite. The
+magnificent object first seen by Klinkenberg at Haarlem, December 9,
+1743, was supplied with six, varying in length from 30° to 44°, each,
+according to the extant representations, being separately _rooted_ in
+the head. Grouped into a lustrous fan, they presented a very beautiful
+and surprising appearance, not again to be displayed until the world and
+humanity have undergone some unlooked-for changes. For the period of the
+comet was computed to be one hundred thousand years! Tails, less
+obviously and splendidly multiplex, are rather the rule than an
+exception. Or rather, closer observations, chiefly photographic, have
+made it manifest that the single efflux of nebulous stuff generally
+designated as a comet’s tail can be analysed into bundles of fibres,
+into straight rays and curved plumes of light, or into knotted and
+branching emanations. Homogeneous outflows, such as are seen in
+drawings, do not really exist. Tails pointing _towards_ the sun have
+also been occasionally noticed; but they are always feeble. Olbers
+recorded, however, that, during eight days of January, 1824, the comet
+then visible had a solar tail of 7°, while its anti-solar tail was only
+3½° long.
+
+The great comet of 1680 will always be memorable for having had its
+orbit calculated by Newton on gravitational principles. It was not
+unworthy of the distinction. Approaching the sun almost in a straight
+line, it penetrated the corona at the rate of 370 miles a second, and
+passing within 140,000 miles of the photosphere, escaped by means of its
+extraordinary velocity from those perilous precincts. Resulting internal
+commotions became evident through the rapid development of a tail more
+than a hundred million miles in length. Newton calculated that particles
+from the head reached its extremity in two days. He assigned to the
+comet a highly elliptical orbit traversed in six centuries. But, since
+its speed might be called parabolic, millenniums may be nearer the mark
+than centuries. It cannot, therefore, be identified with any earlier
+apparition.
+
+The comet of 1682 was Halley’s, the predicted return of which, in 1759,
+was unprecedented and memorable. At its apparition in 1835, valuable
+observations of a physical kind were made upon it by Bessel at
+Königsberg, and by Sir John Herschel at the Cape. They were facilitated
+by the circumstance that this far-travelling body, the perihelion
+distance of which is 55 million miles, and the aphelion-distance 2½
+times that of Neptune, approached the earth on this occasion within 4½
+million miles. It was remarkable for singular and sudden changes of
+aspect. To Bessel the nucleus seemed like a burning rocket. Divergent
+flames issued from it towards the sun, and he took especial note of a
+blazing “sector,” which swung like a pendulum to and fro, in a period of
+4⅗ days. These emanations, accumulating at the surface where the solar
+balanced the cometary repulsive force, were then swept back, as if by a
+tempestuous wind, to form a tail, which, on October 15, measured at
+least 24°. The conviction was forced upon him that the body in which
+these wonderful processes were going on was affected by opposite
+polarities; and he fully concurred with Olbers in the opinion that
+tail-production was a purely electrical phenomenon.
+
+During some time before and after its perihelion passage on November 16,
+the comet wore the disguise of a star. All its hairy appendages had
+vanished. On the 23rd of January, 1836, it was sharply stellar;
+twenty-four hours later it had acquired, besides a twenty-fold increase
+of light, a disc like that of the planet Neptune, enclosed in a nebulous
+sheath of about fourfold breadth. Later in its career, Sir John
+Herschel[86] observed the nucleus under the form of “a miniature comet,
+having a nucleus, head, and tail of its own, perfectly distinct, and
+considerably exceeding in intensity of light the nebulous disc or
+envelope” containing it, which was, properly speaking, the “head” of the
+comet. At last, on May 5, through the progress of distension, the last
+thin shred of its substance melted into the sky. The next return of
+Halley’s comet, somewhat accelerated by Jupiter’s influence, is looked
+for in the year 1910.
+
+The “vintage comet” lingered in northern skies during 510 days—from
+March 26, 1811, until August 17, 1812. It was attentively observed by
+Sir William Herschel, who gathered from it the then new truth that
+comets are self-luminous bodies. “The quality of giving out light,” he
+acutely remarked, “is immensely increased by an approach to the sun.”
+But he failed to persuade his contemporaries or successors. His
+inference had to wait for spectroscopic demonstration. The nucleus of
+the comet of 1811 he found to measure 428 miles. It showed a ruddy hue,
+and was eccentrically placed within a greenish-blue “planetary body”
+127,000 miles in diameter. This was again enclosed in a shining
+atmosphere about four times as wide, round which was flung an envelope
+of a yellow tint, forming a thin hemispherical shell on the side next
+the sun, and continued indefinitely away from the sun as the hollow cone
+of the tail. Owing to this mode of construction, the space between the
+head and the hemispherical sheath, as well as the central part of the
+tail, appeared dark. The latter extended, in October, over 100 million
+miles of space, and was 15 million miles broad. Its soft radiance
+resembled that of the Milky Way, side by side with which it ran on
+November 9, 1811. The comet’s path lay entirely outside the earth’s
+orbit, and Argelander assigned to it a period of 3,065 years. The
+restriction was needless. Between a period of infinite length, and one
+of 3,000, or 1,000 years, no valid distinction can, where comets are in
+question, be drawn. The short sections of their tracks observable from
+the earth might belong equally well to parabolas or to the
+far-stretching ellipses which such protracted periods imply.
+
+The apparition of 1811 suggested to Olbers the “electrical theory” of
+comets’ tails. The uncommon impressiveness with which it displayed not
+uncommon phenomena, was perhaps a result of its considerable distance
+from the sun, owing to which the _interior_ force obtained an advantage
+over the _exterior_, and the locus of equilibrium between solar and
+cometary repulsion was pushed back further than usual from the
+nucleus.[87] He calculated that the materials of the tail spent 11
+minutes in the journey from its root to its tip, indicating ejection by
+a force greatly more powerful than the opposing force of gravity. Olbers
+anticipated the modern view that chemical differences determine the
+shapes of comets’ tails, the various species of matter being diversely
+acted upon by electrical repulsion. The long, straight ray, for
+instance, issuing from the comet of 1807, must, he perceived, have been
+composed of particles much more energetically repelled than those
+aggregated in the inflected plume with which it was associated. The
+curvature of these appendages, in fact, depends upon the relation
+between the orbital velocity of the comet and the velocity of ejection
+imparted to their constituent molecules. It has to be borne in mind,
+however, that while curved tails may appear straight in projection,
+straight tails can never appear curved
+
+Olbers’ classification of comets is still of great significance. He
+divided them into:
+
+(1.) Comets which develop no matter subject to solar repulsion. These
+are without tails, and may be regarded as simple nebulosities devoid of
+solid nuclei.
+
+(2.) Comets showing no trace of nuclear, while subject to solar
+repulsion. They throw out no matter _towards_ the sun; the heads are
+consequently left bare of envelopes, and are of simple structure. The
+comet of 1807 was of this kind.
+
+(3.) Comets manifesting the effects of both species of action. They are
+characterised by the presence of a dark hoop round the head, and of a
+dark rift in the tail, by which it may be judged to be a hollow conoid.
+
+On February 28, 1843, a “short, dagger-like object” blazed out at an
+interval of only fifty-two minutes of arc from the sun’s limb. It was
+viewed with amazement in various parts of the world; and spectators in
+Italy, by shielding their eyes from the direct mid-day glare, were able
+to discern a tail already several degrees long. The proportions of the
+appendage rapidly grew. On March 3, it measured twenty-five degrees; on
+March 11, an adjunct to it shot out, within twenty-four hours, to nearly
+twice the apparent length of the main structure, conveying, as Sir John
+Herschel said, “an astounding impression of the intensity of the forces
+at work.” It was first seen in this country after sunset on March 17, as
+“a perfectly straight, narrow band of white cloud, thirty degrees in
+length, and about one and a half in width.” On the following night, Sir
+John identified this “luminous appearance” as the tail of a grand comet,
+stretching over an extent of space (as it afterwards proved) of no less
+than two hundred millions of miles.
+
+The movements of this body were as surprising as its aspect. It rushed
+past perihelion with a speed of 366 miles a second, leaving an interval
+of 100,000 miles between its centre and the sun’s surface, and swinging
+through two right angles in two hours and eleven minutes. The northern
+part of its course was finished in two hours and a half; hence, it was a
+“southern” comet. Very curiously, it seems to have remained obscure
+throughout its journey towards the sun, reserving its outburst for the
+day _after_ perihelion. Periods were assigned to it ranging from seven
+to six hundred years.
+
+Strangest of all, it turned out to be but one member of a whole family
+of similarly-conditioned bodies. The “great southern comet” of February,
+1880, seemed like its ghost. It had no perceptible nucleus, but an
+inordinately extended train, which rapidly faded; and it scarcely
+deviated by a hair’s breadth from the track of its predecessor. That is
+to say, so far as could be ascertained; for the object was so indefinite
+as to elude exact observation. Its period could not even be conjectured.
+The nature of the relationship between the comets was thus left
+uncertain.
+
+But after the lapse of two years and a half, the question was reopened
+by the appearance of the leading constituent of the group. Like the
+comet of 1843, the “great September comet” of 1882, was first seen close
+beside the sun. At Ealing, shortly before noon, on September 17, Dr.
+Common was struck with the astonishing spectacle of a brilliant comet
+hurrying up to perihelion. A transit was evidently imminent, but clouds
+veiled the scene. Its completion was, however, fortunately witnessed six
+thousand miles away by Mr. Finlay and Dr. Elkin at the Cape Observatory.
+The comet was watched by them “right into the boiling of the limb,”
+which it had no sooner touched, than it utterly disappeared. This cannot
+have been through the absence of contrast; for although its intrinsic
+brilliancy was excessive, it must either have shown bright against the
+sun’s dusky margin, or dark when projected upon his dazzling centre.
+Since neither effect was produced, it can only be inferred that the
+object was translucent owing to insubstantiality. That it had not passed
+_behind_ the sun was later fully ascertained. During three subsequent
+days the “blazing star near the sun” drew popular attention in the
+southern hemisphere, and many parts of Europe. Nothing quite so
+extraordinary had ever been seen before. The spectacle of 1843 was
+renewed, but outdone.
+
+Meanwhile, an astonished public hung on the dicta of perplexed
+astronomers. The speculation which obtained most currency was that the
+three successive southern comets were accelerated returns of the same
+body, destined, after a few short, spiral circuits, to make fiery
+shipwreck in the glowing solar ocean. The effects upon terrestrial life
+were unwarrantably described as likely to prove disastrous; but only an
+abortive panic ensued. Data, however, to serve as the basis of a
+determinate conclusion, were on this occasion collected in abundance.
+The comet of 1882 was not lost sight of until June 1, 1883, when its
+distance from the earth was more than five astronomical units—the
+greatest at which any previous comet except that of 1729 had been
+observed. Hence the general character of its orbit became thoroughly
+known. It proved to deviate somewhat from the tracks pursued by the
+comets of 1843 and 1880; it gave the sun a slightly wider berth; above
+all, its period had unmistakably a duration of several centuries. There
+could then be no further question of its being a return of either, or
+both of those bodies, although its close connexion with them was
+assured. This can be most rationally explained by supposing them to have
+primitively constituted a single body. According to Professor Kreutz’s
+able and exhaustive research, the period of the September comet is 772,
+that of the comet of 1843, between five and six hundred years; and the
+relative situation of their orbits indicates that the supposed
+catastrophe of their disruption took place at perihelion, where a large
+incoherent mass could scarcely fail to yield to the strain of the sun’s
+unequal attraction at the excessively close quarters it was brought into
+by the conditions of its movement. The comet of 1880 is another splinter
+from the same trunk; and yet one more fragment presented itself to M.
+Thome at Cordoba, January 18, 1887, when he observed literally a “nine
+days’ wonder” in the guise of a shadowy ray, thirty-five degrees in
+extent, following the lead of the other “southern comets,” and taking
+rank (so far) as the last and least of their company.
+
+A tendency to still further disaggregation was evident in the comet of
+1882. It did not pass with impunity through the fiery ordeal of its
+visit to the sun; internal agitations supervened; abnormal appendages of
+rarefied texture, but prodigious dimensions, issued from it sunward; the
+nucleus broke up into six spherules like strung pearls; and it was
+noticed in October to be surrounded by detached nebulous masses, just
+launched perhaps on independent cometary careers. The tail was two-fold.
+It consisted of a dim, straight ray which temporarily attained a length
+of a couple of hundred millions of miles, and a massive forked
+appendage, strongly luminous and unusually permanent. Fig. 19 shows one
+of a series of photographs of this comet taken with an ordinary portrait
+lens under Dr. Gill’s direction in October, 1882. The observations of
+its transit proved to be of great importance. Having been made just
+before perihelion, they availed to demonstrate that no loss of motion
+had been suffered in its plunge through the corona. This
+incontrovertible fact implies an inconceivable degree of rarity in the
+solar surroundings.
+
+[Illustration:
+
+  FIG. 19.—_Great Comet of September, 1882. Photographed at the Royal
+    Observatory, Cape of Good Hope._ (From Clerke’s “History of
+    Astronomy,” 3rd ed.)
+]
+
+So long ago as 1831, Clausen pointed out that many comets are grouped
+together after the manner incomparably exemplified later by the southern
+comets. An analogous system, composed of only two known members, is
+formed by the comet of 1807, and Tebbutt’s comet of 1881. The former,
+made by Bessel the subject of a masterly investigation, was not again
+due at perihelion until the remote epoch 3346 A.D., so that the
+announcement of a reappearance so exceedingly premature was startling.
+But when the new comet was also found to have a period of several
+thousand years, it became clear that no return had been observed, but
+only a companion recognised. Tebbutt’s comet was a beautiful object. Its
+head, adorned with interlacing arcs of light, was an overmatch for
+Capella, while so translucent that a star of the seventh magnitude
+seemed rather to gain than to lose brightness by shining centrally
+through it. As the upshot of these singular experiences, the difficulty
+of identifying comets has been increased tenfold. Their aspects were
+always perceived to be well-nigh interchangeable, but their movements
+were held to be distinctive; now their very orbits are found to be, to a
+considerable extent, common property.
+
+A small, glimmering nebulosity descried at Florence by Donati, June 2,
+1858, gave little promise of coming splendour. Yet few more picturesque
+celestial effects have been witnessed than it presented, October 5, when
+Arcturus blazed undimmed through the denser part of the tail, in
+brilliant conjunction with the equal splendour of the nucleus. The
+ineffable grace with which the comet spread its luminous plumage was set
+off by the juxtaposition, as if for the purpose of determining the
+amount of its curvature, of a long, perfectly straight ray. The aspect
+of this beautiful object on October 3, is represented in Fig. 20; some
+idea of its rapid development in size and brilliancy can be gathered
+from an inspection of the Frontispiece to this Section. The apparition
+lasted, to the naked eye, for 112 days, and will not again be visible
+for 2,000 years. So that Donati’s comet may be reckoned an “irrevocable
+traveller.”
+
+[Illustration:
+
+  FIG. 20.—_Donati’s Comet, October 3, 1858._ (From Langley’s, “New
+    Astronomy.”) _The Star to the left of the Comet’s head is Arcturus._
+]
+
+Twice during the present century the earth has traversed, with impunity,
+the tail of a comet. First, on June 26, 1819, when a comet passed
+invisibly between us and the sun, sending its tail our way. Again on
+June 30, 1861. The sun had scarcely set that evening when a yellowish
+disc became apparent at the horizon, from which issued an enormous
+double train, enclosing our planet within its folds. The closing-up and
+withdrawal of the “outspread fan” to which they were compared was
+accomplished in a few hours. The head of the comet had as many envelopes
+as a Chinese puzzle.
+
+The first recognised “short-period” comet approached within one and a
+half million miles of the earth, July 1, 1770. Had it possessed ¹⁄₅₀₀₀th
+the mass of the globe which rushed by it with entire indifference, a
+perceptible lengthening of the year should have ensued; and its
+gravitational insignificance was confirmed by the fact that it passed,
+in 1779, right through the Jovian system without troubling the mutual
+relations of its members. Lexell (with whose name it has continued to be
+associated) fixed its period of revolution at five and a half years; yet
+it had never been seen before. Astronomers, in fact, caught it on its
+trial trip along a fresh orbit to which it had been transported in 1767
+by the disturbing power of Jupiter, and whence it was removed by the
+same influence in 1779. An intermediate return in 1776 had doubtless
+occurred; but circumstances precluded its observation. Further
+encounters with the giant planet may, however, bring back the vagrant,
+and the possibility was thought to have been realised when the history
+of a comet discovered by Mr. Brooks of Geneva, N.Y., July 6, 1889, came
+to be inquired into. Its return about the predicted time in 1896
+afforded an opportunity for revising the laborious inquiry, with the
+result of disproving the case for identity.
+
+A comet, lost under very different circumstances, was picked up February
+27, 1826, by an Austrian officer, Wilhelm von Biela. His calculations
+led him to the unlooked-for discovery that it travelled in an orbit with
+a period of 6½ years, and had already been observed in 1772 and in 1805.
+On its return in 1832, when it had become reduced to the status of a
+telescopic object, Sir John Herschel watched its conjunction with a knot
+of minute stars, the rays of which traversed it without the smallest
+obstruction. It had neither tail nor nucleus; its aspect was that of the
+commonest type of nebula. On December 29, 1845, however, a curious
+change was seen to have affected it. The comet had split into two, each
+of which immediately assumed the characteristic cometary shape, by
+providing itself with a tail and bright nucleus. Thus divided and
+regenerated, the pair advanced side by side, 157,000 miles apart,
+without the least trace of mutual action through gravity, but displaying
+vivid interchanges of brightness, reasonably attributed to the play of
+electrical forces.[88] They re-visited the sun in 1852, but have never
+since, and most probably will never again, be seen. Their end came
+through senile decay. It was that predicted by Newton for all such
+bodies. _Diffundi tandem et spargi per universos cœlos._
+
+The most rapidly-revolving comet of our acquaintance was investigated in
+1819 by Johann Franz Encke, of the Seeberg Observatory, who assigned to
+it a period of 3½ years, and predicted its return in May, 1822. It was
+punctually recaptured at Sir Thomas Brisbane’s Observatory in New South
+Wales. Encke traced back its appearances to 1786, and identified it with
+a comet detected by Caroline Herschel in 1795. At its last return in
+1894–5, it was just at the limit of naked eye visibility. It fluctuates,
+however, considerably, at successive apparitions. M. Berberich[89] has
+sought to associate these perplexing changes with solar vicissitudes;
+but his arguments are not entirely convincing. Encke’s comet, even if
+45,000 billion times less dense than air at atmospheric pressure—the
+consistence attributed by Babinet to cometary matter—would still weigh
+twelve hundred tons.[90] Its excessive rarefaction is a matter of ocular
+proof. On October 21, 1881, Barnard observed a central passage of this
+comet, then more than usually bright and condensed, over a ninth
+magnitude star, which “remained so remarkably distinct during the entire
+progress of occultation, that it formally impressed me with the idea of
+a transit of the star _across_ the comet—a pearly point floating between
+me and the bright mass of vapour.”[91]
+
+This object signally exemplifies the cometary peculiarity of contracting
+near perihelion, and re-expanding after the critical point has been
+passed. Thus, it measured 312,000 miles across, October 28, 1828, when
+135 million miles from the sun, but only 14,000 on December 24, when its
+distance had been reduced to 50 millions; and in passing perihelion,
+December 17, 1838, at an interval of 32 millions, its diameter had
+shrunk to 3,000 miles. It fulfils, as regards Mercury, the function of
+spying upon the planets, assigned to comets by Airy; for, only through
+the Mercurian disturbances of its motion has the Mercurian mass been at
+all definitely ascertained; and a residual acceleration, which, at each
+circuit, brings it back to perihelion a couple of hours before the
+appointed time, has long been regarded as an index to the condition of
+planetary space. Encke explained this shortening of period by the action
+of an hypothetical “resisting medium” augmenting in density towards the
+sun; but accumulated facts have swept it out of existence. The southern
+comets performed for our benefit, one after the other, an _experimentum
+crucis_ in the matter. The chief of them, on September 17, 1882, swept
+through a region where Encke’s medium should be _two hundred thousand_
+times denser than it is at the perihelion distance of Encke’s comet; yet
+suffered no appreciable loss of motion. Nor has the comet itself of late
+complied with the requirements of the theory it suggested. At its return
+to the sun in 1868, the acceleration had fallen to one-half its
+customary, and until then, constant value. And the change has proved to
+be permanent. But the influence of the postulated medium is evidently
+incapable of diminution. Thus, the movements of Encke’s comet still
+remain problematical.
+
+
+
+
+                              CHAPTER XII.
+                      NATURE AND ORIGIN OF COMETS.
+
+
+Comets reflect sunlight, and also emit light of their own. But the
+combination was scarcely thought of as possible until the spectroscope
+gave its verdict. The first analysis of cometary rays was made by Donati
+at Florence, August 5, 1864. They were dispersed by his prisms into a
+yellow, a green, and a blue band, with wide intervals between. Their
+chemical interpretation was afforded by Dr. Huggins in 1868. The subject
+of his experiments was Winnecke’s comet, an insignificant object with a
+period of five and a half years. He found it to be composed—at least in
+part—of acetylene, or some other hydro-carbon gas. The coloured bands
+agreed precisely in position with those in the spectrum of the blue
+light at the base of a candle-flame, or of a gas-jet. The spectra of the
+immense majority of comets is of this pattern, with more or less of
+continuous light added. A portion of this is borrowed, a portion
+inherent. A photograph of the spectrum of Tebbutt’s comet (1881, III.),
+taken by Dr. Huggins, June 24, 1881, demonstrated by its distinct
+impression with several Fraunhofer lines the presence of solar radiance;
+the association of which with native emissions of the continuous sort
+has been made evident in various comets by sudden outbursts of white
+light.
+
+Comets do not then consist entirely of carbon-compounds; but their
+remaining constituents make no distinctive show in their spectra unless
+when sun-raised agitation is particularly vehement. Thus, an approach
+within five million miles of the sun evoked in comet Wells (1882, I.),
+sodium-luminosity, detected by Dr. Copeland at Dunecht, June 17, 1882.
+The blaze was so vivid that a crocus-tinted image of the entire head
+with the beginning of the tail was visible, like a solar prominence,
+through the open slit of the spectroscope. The same observer witnessed
+an outbreak of both sodium and iron lines in the September comet (1882,
+II.). In both cases, the newly-kindled emissions effaced the old, and,
+after a time, were replaced by them. This mode of procedure is
+characteristic of electrical action, and combines with other symptoms to
+assure us that cometary illumination is produced by interior electrical
+disruptive discharges due to solar induction.
+
+Olbers’s felicitous conjecture has been developed into a plausible
+theory of comets’ tails by M. Bredichin, late director of the Pulkowa
+Observatory. He divided them into three “types,” distinguished by the
+values of the repulsive forces employed severally in their production.
+Those belonging to type I. imply the exertion of a counter-influence
+fourteen times stronger than gravity. They are long, straight rays, the
+constituent particles of which are carried, in a torrent too swift to be
+deflected, to the observed extraordinary distances. Their outward
+velocity of five miles a second to start with is, we must remember,
+constantly accelerated, and finally becomes enormous. Halley’s comet and
+the great comets of 1811 and 1861 had tails of this type. Donati’s great
+plume exemplified the second, in which the average strength of repulsion
+exceeds that of gravity one and a half times. Tails of the third type
+correspond to a ratio varying from three-tenths to one-tenth. Solar
+attraction is, in them, only partially neutralised. They are short,
+strongly-bent, brush-like appendages, seldom seen apart from those of a
+more striking kind.
+
+These three types have a physical meaning of great interest. The
+attractive force of gravity varies as the mass, the repulsive force of
+electricity as the surface of the molecules they sway; hence the ratio
+of repulsion is inversely as the ratio of molecular weight, the lightest
+particles being the most violently driven away from the sun. Assuming
+them to be hydrogen-molecules, Bredichin found that the atomic weights
+of hydro-carbon gases and iron would correspond fairly well with the
+speed of projection signified respectively by the curvatures of the
+second and third types of tail. Materials of other kinds are not
+excluded; their presence is, indeed, demanded by the width of these
+appendages, which obviously consist of bundles of emanations differently
+influenced, and presumably of a different chemical nature. Bredichin’s
+theory works admirably from a geometrical point of view. All the
+varieties of cometary trains can be constructed by strict calculation
+from the basis it supplies. Yet there are spectroscopic difficulties in
+the way of accepting it unreservedly. No evidence is at present
+forthcoming of any connexion between the chemistry of tails and their
+shapes; and hydrogen rays are conspicuously absent from cometary
+spectra.
+
+“Short period,” or “planetary” comets may be defined as those revolving
+in periods of less than eight years. They have much more in common,
+however, than the quickness of their successive returns to the sun. All
+move from west to east; they show some preference for the plane of the
+ecliptic; and none of their orbits are excessively elongated. Thus, they
+tend towards conformity with the regular ordinances of the solar system,
+which its less accustomed visitants completely ignore. All, too, have a
+_used-up_ appearance. This is easily understood. They have wasted their
+substance spinning out nebulous appendages—_sicut bombyces filo
+fundendo_, as Kepler said—at their frequent returns to perihelion. They
+are thus visibly effete bodies. Before long, they will drop out of
+individual existence, and survive obscurely, reduced to the “dust of
+death.” Yet the supply is not likely to become exhausted. Discovery
+proceeds faster than disappearance.
+
+“Lost comets” belong, without exception, to this class. Two typical
+instances have already been mentioned in the disaggregation of Biela’s,
+and the removal of Lexell’s comet. The fate of Biela may have been
+shared by Brorsen’s, a comet with an established period of five and a
+half years, which has, nevertheless, remained submerged since 1879. It
+is believed by Dr. Lamp to have exploded through internal forces in
+1881, and he recognises as one of its fragments a faint comet detected
+by Mr. Denning at Bristol, March 26, 1894. The adventures of displaced
+comets, such as Lexell’s can be traced only by arduous and delicate
+inquiries. They depend upon a single cause. Unsettled comets are those
+which pass near Jupiter’s orbit, and are subject to encounters with his
+mighty mass. And since they must necessarily return to the point of
+disturbance, the series of their vicissitudes can come to an end only by
+their being driven off finally from the solar system along a hyperbolic
+path.
+
+The condition of these bodies might be described by saying that, in the
+regular course of things, they revolve round the sun disturbed by
+Jupiter; while, during brief but energetic crises, they revolve round
+Jupiter disturbed by the sun. Their abnormal condition results from the
+situation of their aphelia close to the Jovian track. This is the case,
+in a minor degree, with many comets of comparatively settled habits.
+They escape eviction and exile, and suffer only disquietment. Such are
+Winnecke’s, D’Arrest’s, Faye’s comets, which, having been continuously
+observed during half a century, are, as Mr. Plummer expresses it, “well
+under control.”[92]
+
+Short-period comets, with the solitary exception of Encke’s, appear to
+be inevitably connected with Jupiter. The peculiarity is rendered more
+significant by the circumstance that the other great planets are also
+provided with cometary clients. The Jovian group is the largest; it
+includes more than two dozen recognised individuals. Saturn claims nine,
+Uranus eight, and Neptune five. Halley’s comet belongs to the Neptunian
+family. Another of its members was discovered by Pons in 1812, and
+re-discovered by Brooks in 1883, so that it has a period of 71 years.
+And the reappearance in 1887 of a comet first seen by Olbers in 1815,
+bore reassuring testimony to the regularity with which Neptune’s comets
+conduct themselves during their long periods of invisibility.
+
+The nature of these planetary relationships was at once conjectured. It
+seemed an open secret that the comets had been taken prisoners by the
+attractive force of the great globes they flitted past on their way to
+the sun. But astronomers can take nothing for granted; and preliminary
+mathematical inquiries served rather to discredit the first and easy
+surmise. The case had to be thoroughly sifted; and it was only through
+the profound researches of Tisserand, Callandreau, and Newton of Yale,
+that the “capture-theory” has taken its place as a highly probable
+truth. With an unstinted allowance of time and _comets_, it can perform
+all that is required of it. “Captures” are not effected all at once; the
+lasso is thrown many times over the escaping body before it is
+definitively secured. Moreover, at each such effort, the chances are
+even of its being made in the wrong direction. We observe only the
+outcome of the hits; the misses are beyond our reckoning. A multitude of
+happy accidents have led to the domestication in our system of Faye’s,
+Tuttle’s, Winnecke’s, D’Arrest’s comets. Mr. Plummer has adverted to the
+likelihood that we are indebted to some slight but well-directed pulls
+from Mercury for the permanent addition of Encke to the solar company;
+and Neptune exerted itself ages ago with similar success as regards
+Halley’s comet, yet under great difficulties, since retrograde comets,
+and those with highly inclined orbits are, as a rule, exempt from
+capture. This is one of the reasons why short-period comets show some
+degree of conformity to planetary modes of motion.
+
+These investigations remove all doubt as to the foreign origin of
+comets. Those that are in the solar system are not of it. They assuredly
+remained unaffected by the gradual processes of its development. Yet
+they, as well as the multitude of parabolic comets, belong to it in a
+wider sense. That is to say, they accompany its march through space.
+Otherwise, as M. Fabry has demonstrated, most of their orbits should be
+strongly hyperbolic; and no such cometary orbits are known. They should,
+besides, if casually encountered, present themselves chiefly along the
+line of the sun’s way; they arrive, on the contrary, indifferently from
+all quarters of the heavens. They are then subject to the same
+mysterious influences which govern his motion, and drift with the cosmic
+current which bears the solar family along, we know not how or whither.
+
+[Illustration:
+
+  FIG. 21.—_Photograph of Swift’s Comet. Taken by Prof. Barnard, April
+    6, 1892. Exposure, 1h. 5m._
+]
+
+[Illustration:
+
+  FIG. 22.—_Photograph of Swift’s Comet. Taken by Prof. Barnard 24h.
+    later. Exposure, 50m._
+]
+
+Comet-photography became possible only through the introduction of
+highly-sensitive gelatine plates; and even with them, exposures of an
+hour and upwards are necessary in order to obtain the desired results.
+But these results are of such importance as to deserve the closest
+attention. For investigating either the forms or the spectra of comets,
+the camera is unrivalled. Its systematic employment for these purposes
+dates from 1892. It can also serve as an engine of discovery. On October
+12, 1892, a comet so faint that, had it not been photographed, it would
+most likely never have been seen, appeared as a nebulous trail on a
+plate exposed by Professor Barnard to the Milky Way in Aquila. It proved
+to be one of Jupiter’s dependents, pursuing, in a period of 6·3 years, a
+track so closely resembling the orbit of Wolf’s comet in 1884, that
+Schulhof regarded them as the offspring of one parent body.
+
+In the year 1892, seven comets were detected; and all, by one of those
+picturesque coincidences with which nature loves to entertain her
+devotees, were, towards its close, visible in the sky together. One of
+them was first noticed by Lewis Swift—a specialist in that line—and
+passed perihelion April 6.[93] The head competed in brightness with a
+third-magnitude star; the tail was 20° long, and came out, in a
+photograph taken by Mr. Russell at Sydney, on March 22, self-analysed
+into eight perfectly distinct rays. _No such structure could be seen
+with the telescope._ Figs. 21 and 22 reproduce two pictures of this
+object obtained by Professor Barnard, April 6 and 7 respectively. During
+the interval, a striking change had occurred. In the first photograph,
+the tail is sharply separated into two branches, and shows traces of
+further indefinite subdivisions. The uneven, knotty texture of the main
+stream is obvious. The matter composing it seems as if it had rushed in
+a torrent over a rocky bed, whirling and foaming round the obstacles it
+encountered. Twenty-four hours later, this powerful emanation left
+scarcely a trace on the plate. Its dwindled remnant had split up into
+two faint streaks, while the almost negligeable offset of the previous
+night had sprung into unlooked-for prominence. A unique feature was
+added in the apparent development of a secondary comet two degrees
+behind the head. The anomalous enlargement brightened gradually inwards,
+and can readily be seen upon the plate to be the centre of an entirely
+new system of tails.[94]
+
+Owing to moonlight and clouds, the autobiography of this planetary _bud_
+unfortunately remained a fragment; and since Swift’s comet has an
+indefinitely long period, it will never again exhibit for our benefit
+any of its caprices of change.
+
+[Illustration:
+
+  FIG. 23.—_Photograph by Prof. Barnard of Holmes’ Comet near the
+    Andromeda Nebula._
+]
+
+On November 8, 1892, Professor Barnard secured a very perfect
+representation (shown in Fig. 23) of a peculiar-looking comet grouped
+with the great Andromeda and its attendant nebula. Discovered only two
+days previously by Mr. Edwin Holmes of London, it presented a great
+round disc with definite edges visible to the naked eye. This contained
+a tail in embryo, which subsequently opened out into a feeble brush, the
+head being then pear-shaped, and granulated like a remote star
+cluster.[95] A strictly continuous spectrum was derived from it. “Its
+appearance,” Professor Barnard wrote, “was absolutely different from
+that of any comet I had ever seen. It was a perfectly circular and
+clean-cut disc of dense light, almost planetary in outline. There was a
+faint, hazy nucleus.”[96] A photograph taken by him, November 10,
+showed, distant about one degree to the south-east, “a large irregular
+mass of nebulosity covering an area of one square degree or more, and
+noticeably connected with the comet by a short, hazy tail.”
+
+This object underwent extraordinary vicissitudes of aspect. From a
+seeming planet it quickly degenerated by distension into the thinnest of
+nebulosities; then suddenly, on January 16, 1893, gathered itself
+together into an ill-defined star of the eighth magnitude. This
+evanescent outburst was simultaneously observed in several parts of the
+world. After some minor rallies and relapses, the comet finally, on
+April 6, 1893, melted into the sky-ground. Jupiter is responsible for
+its introduction into the solar system, and it will again be due at
+perihelion in May, 1899. Yet its reappearance is considered doubtful.
+
+It was perhaps caught sight of during a temporary crisis of internal
+agitation, which may not recur. Certainly it could not, if as bright as
+when discerned by Mr. Holmes, have remained many nights unnoticed.
+Nevertheless, it had passed the sun five months previously. Its orbit is
+more nearly circular than that of any previously observed comet, and it
+revolves wholly within the asteroidal zone. That is to say, its
+perihelion lies outside the orbit of Mars, its aphelion inside that of
+Jupiter. Hence, it ought to be visible like a planet, at every
+opposition. Professor Barnard, however, sought vainly for it, when thus
+situated. The apparition was in many ways enigmatical.
+
+A comet discovered by Brooks, October 16, 1893, was photographed by
+Barnard three nights later, when a tail was disclosed, 3½° long, and
+flowing off in two branches with a spine-like ray attached to each. A
+series of impressions were fortunately taken, and that of October 21
+(reproduced in Fig. 24) proved to be of peculiar interest. Since the
+night before, the tail had apparently met with an accident. It imprinted
+itself upon the plate shattered, deformed, and affected by a double
+curvature. A collision with some external body was at first suggested as
+the cause of this untoward state of things; but, knowing all that we do
+about the violent interior paroxysms of comets, it seems more rational
+to attribute it to extreme irregularities in the quantity and direction
+of effluences from the nucleus. The following night’s photograph gave
+evidence of a partial return to normal conditions. Yet the appendage
+still looked badly damaged; and an elliptical fragment, wrenched from it
+during the convulsion, showed no tendency towards reunion. At the time
+of this incident, Brooks’ comet was situated well outside the orbit of
+the earth.
+
+The facts already collected by the photographic study of comets are
+concordant, and easily interpreted. One obvious inference from them is
+“that the matter of a comet’s tail is driven away from the nucleus in a
+very irregular and spasmodic manner.”[97] At certain crises, outflows
+are only accomplished by convulsions, compared by Mr. Ranyard to the
+explosions of terrestrial volcanoes, or solar prominences. Moreover,
+capricious as cometary forms are to the eye, they are still more
+inconstant as recorded chemically. “The appearance one day,” Professor
+Hussey says, “affords no indication as to what it may be the next. The
+most radical changes of form have been observed in almost every
+reasonably bright comet that has been photographed; and they sometimes
+take place so rapidly as to become conspicuous in an hour or two.”[98]
+
+[Illustration:
+
+  FIG. 24.—_Brooks’ Comet, photographed by Prof. Barnard, October 21,
+    1893. Exposure, 35m._
+]
+
+Comets’ tails appear very different in structure photographically and
+visually. On the sensitive plate, they are perceived to be composed of
+innumerable, distinct filaments, sometimes tied up, as it were, into
+sheaves. The filaments, or streamers may, however, according to the same
+authority, “leave the coma in a single compressed bundle, or they may
+spring from it in widely divergent and loosely connected groups; they
+may be smooth, and straight, and distinct, or they may be lumpy,
+crooked, interlacing, and spirally twisted; or again, they may be broken
+into fragments, and scattered as though they were smoke driven by the
+wind.” And these effects often swiftly succeed each other in the same
+comet.
+
+In photographs of Swift’s and Rordame’s comets in 1892 and 1893 (taken
+by Barnard and Hussey respectively), the effects of a spiral outward
+movement in the grouped streamers of the tail can be plainly recognised.
+They are indistinguishable from “the twisted forms produced by an
+electrical discharge in a magnetic field.”[99] Another much more common
+peculiarity of such appendages brought into prominence by chemical
+portraiture, is the occurrence upon them of knots, or condensations.
+These are evidently accumulations of outflowing matter. Again, in most
+of the comets recently photographed, the tails start directly from the
+nuclei, which appear destitute of genuine envelopes. This is the precise
+criterion of Olbers’ first cometary division, in which solar repulsion
+acts alone, nuclear repulsion being ineffective, or non-existent. It
+comes out remarkably in Barnard’s photographs of Gale’s comet in 1894.
+
+We may now resume in a few words what we have learned about comets. To
+begin with, they are of such small mass that no gravitational effects
+from their closest vicinity have ever yet been detected. Their bulk, on
+the other hand, is enormous. The great comet of 1811 comprised a
+nebulous globe 2½ times larger than the sun, with a tail many thousand
+times more voluminous. Hence the extraordinary tenuity of such bodies.
+They must indeed contain solid matter; otherwise they could not hold
+together even in the imperfect way that they do; but it is probably in a
+state of very loose aggregation. Their permeability to light may thus be
+accounted for. The visibly granular texture of their nuclei is
+confirmatory of the supposition. If, then, the nuclei of comets are
+essentially “meteor-swarms,” all the constituent particles must revolve
+round the centre of gravity of the whole, in a common period, but with a
+velocity directly proportional to distance from the centre—that is,
+increasing outward. And the joint mass being so small, the utmost speed
+attained would perhaps rarely exceed a couple of hundred yards a second.
+Moreover, towards the centre, where the components of the swarm would
+crowd most closely together, motion would become so slow as to be
+scarcely perceptible. Hence collisions would be infrequent and of slight
+effect; while the probability of their occurrence should diminish with
+the comet’s approach to the sun, which, by its unequal attraction, would
+draw the revolving particles asunder, and amplify their allowance of
+space. Internal collisions may then fairly be left out of the account in
+considering the phenomena of comets. The expansion of their nuclear
+parts, due to tidal forces, is, however, usually disguised by the
+contraction, near perihelion, of their nebulous surroundings. The latter
+effect can be explained by the immense predominance at that conjuncture
+of solar over cometary electrical repulsion.
+
+That the light-emissions of comets are largely of electrical origin is
+no longer doubtful; so that the present rush-ahead in this branch of
+knowledge cannot but help to elucidate many of the still mysterious
+circumstances connected with these strange visitants from the uttermost
+verge of the sun’s empire. The tie of allegiance hangs loosely there;
+but by the persevering efforts of the great planets it is sometimes
+drawn closer, with the result of domiciling under their control a train
+of dilapidated comets, verging towards dissolution.
+
+Carbon, sodium, and iron, are the only substances directly known to
+exist in these bodies. Spectroscopic evidence also suggests the presence
+of nitrogen or hydrogen; and a number of chemical elements which make no
+show in their light doubtless enter into their composition. The state of
+comets when remote from the sun can only be surmised. Their gaseous
+constituents may be solidified by cold. They can, in any case, scarcely
+be other than obscure and inert bodies.
+
+
+
+
+                             CHAPTER XIII.
+                     METEORITES AND SHOOTING STARS.
+
+
+At Madrid, on the morning of February 10, 1896, the sunshine was at 9.30
+overpowered by a vivid flash of bluish light, succeeded by a violent
+explosion. Much glass was broken, and other devastation of a minor kind
+wrought; above all, some hundreds of thousands of people were thoroughly
+frightened. The origin of the commotion was visible in a white cloud
+rushing across the sky, and leaving behind a dusty train. Of this
+débris, scattered from a height of fifteen miles, some fragments were
+picked up and analysed. They were composed of silicates of magnesia and
+iron, with very small quantities of aluminium, nickel, and calcium.
+These specimens were strictly “aerolites,” a term used to designate any
+solid meteoritic matter that reaches the earth.
+
+Equally conspicuous apparitions of the sort are not always equally
+clamorous. There are silent, as well as detonating fire-balls. The cause
+of the difference cannot certainly be assigned. It resides, perhaps, in
+the diverse constitution of the exploding bodies; it is, beyond doubt,
+unconnected with their height in the atmosphere. Thus, a remarkable
+meteor was seen, but not heard, by Dr. Rambaud, the astronomer-royal for
+Ireland, at Dunsink, February 8, 1894. The object, he says, “suddenly
+burst into view with an intense brilliance, and shone out against the
+cloudless blue sky with a greenish metallic lustre. It fell in a
+vertical direction until it disappeared behind some trees. In shape it
+resembled a very elongated pear, like most fire-balls of the sort. It
+emitted no visible sparks, and disappeared quite noiselessly.” When
+first observed, it was at a height of about 87 miles above the Irish
+Channel; then crossing Lancashire, it descended so rapidly on its way,
+probably, to engulfment in the North Sea, that, when last noticed, it
+was scarcely, if at all, higher above the earth’s surface than the
+Madrid meteorite at the moment of its formidable disruption. Astonished
+rustic beholders at Kingswood and Dudley averred that it burst “in the
+next field”; but this is a common illusion. Professor Langley relates
+that some witnesses of a marvellously swift meteor at a presumable
+elevation of some fifty miles, sallied out of their houses next day to
+make sure that it had not struck their chimneys.
+
+Such phenomena are tolerably frequent, and have been recorded from the
+remotest antiquity. Homer lends a meteoric aspect to Athene, when she
+descends from Olympus to take the war-path by the shore of Scamander.
+Chronicles abound with accounts substantially identical with the
+telegrams supplied by Reuter’s Agency on February 10, 1896. The fall of
+the “Crema meteorite” has a special interest as having been depicted by
+Raphael in his “Madonna di Foligno.”[100] A multitude of stones were
+discharged by it on the banks of the Adda, six of which weighed each one
+hundred pounds and upwards; the sulphurous smell characteristic of
+fresh-fallen aerolites is mentioned in contemporary accounts of the
+event, which occurred September 4, 1511; and it is further said that
+“sheep were killed in the fields, birds in the air, and fishes in the
+streams.” No specimen of this sky-volley is known to exist. In elder
+times, objects of this class were worshipped; and Professor Newton[101]
+has collected many curious facts about the meteoric cult traceable in
+classical history. To this day, indeed, the central sanctuary of
+Mahometanism—the Kaaba—owes its sacredness to the embedment in its
+masonry of a blackened aerolite.
+
+Until the beginning of the present century, only the ignorant believed
+it possible that stones could come from heaven; philosophers regarded
+them as generated in the clouds. They were at last convinced that the
+popular view was correct by Biot’s investigation of the meteoric tempest
+which broke over L’Aigle, in the department of the Orne, April 26, 1803.
+He estimated at two thousand the number of fragments scattered over an
+area six by two and a half miles, one of which, weighing five pounds, is
+now in the South Kensington Museum. And at Pultulsk, January 30, 1869,
+one hundred thousand stones were reported to have been showered upon the
+earth. It is not often, indeed, that largesse from space is so lavishly
+made. Yet all meteors (with the rarest exceptions) rendered luminous by
+the resistance of its atmosphere, become, in one way or another,
+incorporated with its mass. Their materials are no doubt often reduced
+to fine dust and gas; yet six or seven hundred solid masses per annum
+are computed to reach the surface of sea or land, for the most part
+“unrecked-of and in vain.” Of late, the scientific demand for them has
+grown keen, and their enhanced value has raised the legal question of
+their ownership. The decision of the American courts is that aerolites
+are not “wild game,” but “real estate,” and, as such, belong to the
+owner of the land upon which they fall.
+
+No wonder they should be at a premium, those blackened and wasted
+samples of immeasurably distant globes. The velocities with which they
+entered our atmosphere alone suffice to prove their cosmical origin. Had
+it not trapped them, many, circuiting the sun in a hyperbolic curve,
+would have escaped for ever from our system. Their primitive
+disconnexion from it is implied by their swift motions, which
+considerably exceed, on an average, those of comets, and point to
+interstellar space as their proper habitat. The earth’s orbital pacing
+has, however, to be added or subtracted as the case may be; so that the
+actual rate of encounter varies from ten to forty-five miles a second.
+Most of this is spent before the earth’s surface is reached. Only
+considerable masses travelling at express speed bring any sensible
+proportion of it with them to the ground. But what is lost as motion
+reappears in other forms of energy, as light, heat, and sound. In front
+of the rushing body, the air—despite its inconceivable tenuity at
+elevations of fully one hundred miles—is suddenly compressed and raised
+to an exceedingly high temperature, while a corresponding vacuum behind
+gives rise to violent reactive currents. Professor Dewar calculated, by
+way of example, in 1887, that a body, three feet in diameter, moving
+eighteen miles a second at an altitude of twenty-three miles, where
+barometric pressure is reduced to one-fifth of an inch, would compress
+the air in its path 5,600 times, the resistance offered to its passage
+thus equalling that of thirty-seven atmospheres. The abrupt increase of
+heat accompanying compressions of this order amounts to thousands of
+degrees, and tends to rend in pieces a body arriving from frigid abysses
+where matter can only exist in a stark and, so to speak, lifeless state.
+Explosions of occluded gases ensue; vaporised and incandescent particles
+are blown behind in a luminous train; and, at the most, some shattered
+solid remnants tumble to our continents, or plunge into our oceans. The
+few that are rescued for examination look much the worse for their final
+adventure. The signs of the furnace and the hurricane (both
+self-created), are visible in their jetty and fused surfaces,
+“thumb-marked,” probably through the continual and irregular changes in
+the pressure exerted upon them. The crust is, however, a mere varnish,
+the interior, which is usually of a greyish hue, being entirely
+unaffected by heat. It remains, on the contrary, sunk in the depths of
+cold. Agassiz compared the aerolite which fell at Dhurmsala in India, in
+1860, to the Chinese _chef d’œuvre_, a “fried ice”;[102] and a large
+fragment of it, which fell in moist earth, was found coated with
+ice.[103]
+
+Aerolites, or meteorites, as they may equally well be called, are
+roughly divided into “stones” and “irons”; the former being composed of
+various and peculiar minerals, the latter of iron, with a considerable
+percentage of nickel.[104] All show a more or less distinctive
+crystalline structure. Meteoric chemistry includes about thirty of the
+seventy or so terrestrial elements. The chief of them are: iron, nickel,
+carbon, oxygen, silicon, magnesium, sulphur, aluminium, phosphorus, with
+smaller quantities of chromium, cobalt, tin, copper, titanium,
+manganese, antimony, arsenic, lithium, hydrogen, nitrogen, argon, and
+helium. Argon and helium were expelled by heat from a piece of meteoric
+iron picked up in Augusta County, Virginia, the former coming off nearly
+a hundred times more plentifully than the latter. As the light of argon
+makes no show in the spectrum of any heavenly body, the proof of its
+cosmical diffusion thus obtained by Professor Ramsay is of great value.
+Besides argon and helium, hydrogen, carbonic acid, and carbonic oxide
+gases are found included in meteorites. They seem, as it were, to
+hybernate in the stony or metallic enclosures from which they can only
+be _boiled out_.
+
+Although these wind-falls from space contain no strange elements, the
+manner of their composition is special to themselves. Their study
+constitutes a separate branch of mineralogy. They are certainly of
+igneous origin. They show no sign of water-action, and but little of
+oxidation. The nearest affinities of the minerals aggregated in them are
+with volcanic products from great depths. Thus meteorites seem broken up
+fragments of the interior parts of globes like our own. A few among them
+contain solid carbon, either amorphous, or in the shape of graphite, or
+even crystallised into minute diamonds. In the Cañon Diablo siderite, or
+meteoric iron, all three varieties occurred together, some of the
+translucent particles proving, when put to the test of actual
+combustion, to be indeed “gems of purest ray serene,” dwelling incognito
+in a strange environment!
+
+The thin streaks of light called “shooting stars” differ in several
+respects from explosive meteorites. In the first place, they—probably
+without exception—form systems. Innumerable multitudes of them travel in
+the same paths round the sun. Moreover, those paths resemble cometary
+orbits; they are very elongated ellipses, inclined at all angles to the
+plane of the ecliptic, and traversed indifferently in either direction.
+Their velocities are thus sensibly parabolic, while fire-balls commonly
+attain hyperbolic speed. Finally, they are soundless. They slide by in
+ghostly silence. Most of them are probably not larger than a pea, yet
+were the shield of its atmosphere withdrawn, the earth would be rendered
+well-nigh uninhabitable by their pelting. Incredible numbers of them are
+encountered. They come by the million daily to be burnt, visibly to the
+naked eye, in the thin upper air. Kleiber’s allowance is eleven,
+Newton’s twenty millions; and these figures should be multiplied a score
+of times to include telescopic fire-specks. Now, the combined mass of
+all these particles goes to reinforce the mass of the earth; but it is
+relatively so small that ages must elapse before the contribution can
+become sensible. Our defeated meteoric assailants surrender to us also
+the heat of their arrested motion; which is, however, only as a spark
+added to the furnace of our supply from the sun.
+
+Shooting stars, as we have seen, move in closed orbits. They are, then,
+a periodical phenomenon. Not that we ever see the same individual twice;
+its visibility implies its dissolution, but its companions are as the
+sands of the seashore. Their association is recognised by their
+agreement in direction and date. Unless their orbits intersected that of
+the earth, nothing could be known of them terrestrially; they come to
+our notice only through actual encounters, and encounters are possible
+only at the time of year when our planet is passing through the node.
+This is the given rendezvous, different, speaking generally, for each
+system; although, speaking particularly, many meteoric streams are so
+wide that the earth takes days, even weeks, to cut its way through them,
+and so may be overtaken by fresh onsets before the original one is
+exhausted. Each community is distinguished by the lie of its orbit—that
+is, by the point in the sky from which the flying arrows of light seem
+to diverge. This is known as the “radiant-point” of the system, and is
+its special characteristic.
+
+The August meteors are a familiar example of such an association. Their
+annual recurrence is no new discovery. Long ago, in mediæval times, they
+were called the “tears of Saint Lawrence,” because never looked for
+vainly on the 10th of August. But they are so far from being limited to
+that particular night, that Mr. Denning has picked up skirmishers and
+stragglers from the main body all the way from July 8 to August 22. They
+are distributed with tolerable evenness along an immensely long ellipse,
+traversed in 120 years; and, because they radiate from near the star η
+Persei, are known to science as the “Perseids.”
+
+The scattering of the November meteors—or “Leonids,” since their point
+of emanation is marked by ζ Leonis—is on the same plan, with a
+difference: the Perseids might be compared to a plain gold ring; the
+Leonids, to a ring with a gem on it They send us some shots every year
+on the 13th and 14th of November; but three times in a century they open
+fire for a regular bombardment. An early Leonid display took place in
+902 A.D., noted in old chronicles as “the year of the stars.” All night
+long on October 19—the node advances 14½ degrees in a thousand
+years—while the tyrant Ibrahim lay dying “by the judgment of God” before
+Cosenza, beholders far and near viewed with consternation the stars
+precipitating themselves from the sky. Recurrences of the phenomenon
+every thirty-three years received curiously little attention until
+Humboldt described, and insisted on the periodic nature of the meteoric
+tempest witnessed by him at Cumana on the morning of November 12, 1799.
+One scarcely less violent broke over Europe and Asia in 1832, and the
+American continent in 1833. From the Gulf of Mexico to Halifax the stars
+were seen to fall as silently as snow-flakes, and almost as thickly, yet
+after a less undirected fashion. Rather they darted and swooped, like
+falcons, with a purpose; and it was noticed that the lines of their
+flight could, with essential invariability, be traced back to one point,
+or small area in the heavens. This remark gave the clue to their nature.
+They were perceived to be necessarily cosmical bodies. For since the
+focus of the meteors remained unaffected by the earth’s rotation, they
+showed themselves plainly extraneous to its domestic arrangements. “A
+new planetary world,” exclaimed Arago, “has been disclosed to us!”
+
+The anticipated repetition, in 1866, of the November shower of 1833,
+came off with _éclat_. Many still remember the amazing spectacle
+presented by the heavens in the early morning of November 14, in that
+year. In 1867, when the earth came round again to the same point of its
+orbit, the star-rain was still falling heavily; and even in 1868 it
+amounted to a fair sprinkle. Thus the swarm was, thirty years ago,
+already so extended that it spent three years in sweeping past the node,
+at the rate of twenty-seven miles a second. “The meteors themselves,”
+according to Dr. Johnstone Stoney,[105] “are probably little pebbles,
+the larger about an ounce, or perhaps two ounces, in weight, and spaced
+in the densest part of the swarm at intervals of one or two miles
+asunder every way. The thickness of the stream is about 100,000 miles,
+which, however, is a mere nothing compared with its enormous length. The
+width is such that the earth, when it passes obliquely through the
+stream, is exposed to the downpour of meteors for about five hours.”
+Each “pebble” revolves round the sun, and suffers planetary
+perturbation, in complete independence of its fellows, their orbits
+being only alike, not identical. The next full encounter with them will
+take place November 14, 1899; but avant-couriers may be looked for at
+the critical dates in 1897 and 1898, as well as a strong rear-guard in
+1900.
+
+The orbit of the November meteors is roughly bounded by the orbits of
+the earth and of Uranus. They pass perihelion very near our
+meeting-place with them; and since they run counter to the earth’s
+motion, the velocity of collision is nearly equal to the sum of the two
+orbital velocities, or forty-four miles a second. They are almost the
+swiftest shooting stars of our acquaintance.
+
+The successful calculation of meteoric orbits by Adams, Schiaparelli,
+and Leverrier, promptly led to a discovery as important as it was
+unexpected. Late in 1866, Schiaparelli announced that the August meteors
+follow precisely the same track with a bright comet (1862, III.)
+discovered in 1862 by Tuttle, an American astronomer; and the reality of
+this singular relationship was, in the following year, verified by the
+detection of three similar examples. The Leonids, with a period of 33¼
+years, proved to be close associates of Tempel’s comet (1866, I.); a
+meteoric stream flowing down upon the earth annually on April 20, from
+the direction of the constellation Lyra, was perceived to move in the
+vast ellipse traced out in 415 years by the comet 1861, I.; finally a
+star-drift, first noticed December 6, 1798, was rightfully claimed as an
+appurtenance of Biela’s comet.
+
+Thus the fact of a close connexion between comets and meteors was at
+once rendered patent; and as to the nature of the connexion, the history
+of Biela’s comet is particularly instructive. Since its disappearance,
+the meteor-swarm sharing its orbit has received a notable accession. The
+comet seems to have broken up into meteors. And this, we can scarcely
+doubt, is what has really occurred. Hence, when the earth passes
+moderately, near where the comet _would_ have been, had it survived in
+cometary shape (a conjuncture happening once in thirteen years), a
+vehement outburst of shooting stars is observed. On November 27, 1872,
+the “Bielids,” or “Andromedes,” came in tens of thousands from near γ
+Andromedæ, the very point whence the track of the disaggregated comet
+intersects the earth’s orbit at an angle of twelve degrees. Their
+movements were leisurely; for they came up with our globe, instead of,
+like the Leonids, rushing to meet it. They seemed to sail, rather than
+shoot, across the sky. The calculated position of the originating body
+was, at this date, two hundred millions of miles _in advance_ of the
+node, and it was three hundreds of miles _behind_ the same point when
+the display was renewed in 1885. It is then certain[106] that at least
+five hundred millions of miles of Biela’s route are densely strewn with
+meteoric fragments. The entire multitude, moreover, necessarily
+separated from the comet subsequently to an episode of disturbance by
+Jupiter in 1841. This is plainly shown by the fact that the members of
+the associated company pursue the modified track. The perturbation of
+1841 was exerted upon them no less than upon the comet, with which,
+accordingly, they must then have formed one mass.
+
+Biela’s comet has thus taught us that such bodies meet their end by
+getting pulverised into meteoric particles; and further, that the
+particles disperse with extraordinary rapidity along the length of their
+orbits. Solar and planetary _differential_ action produce this kind of
+effect, although they hardly explain its amount. Subordinate swarms are
+also created by disturbance. Such an one met the earth November 23,
+1892, when Professor Young estimated that at least 30,000 Andromedes
+furrowed the sky at Princeton. Heavy star-showers, however, are
+perishable phenomena. They thin out with comparative rapidity into a
+continuous drizzle. At each recurrence, diffusion is seen to have made
+progress, until at last the “gem on the ring” has vanished. With the
+Perseids this is already the case. The stream flows without material
+interruption over a bed a hundred times wider than that of the Leonids.
+These meteors, too, will no doubt eventually reach a similar condition.
+In the course of a couple of centuries, their thirty-three year period
+will be completely effaced. In 1799, the main body of them crossed the
+node in less than a year; at the close of the present century, the earth
+will probably make her annual round at least four times, before the
+march-past comes to an end. Obviously, it is about to become perennial.
+Leverrier concluded from his researches that the Leonid comet and the
+Leonid meteors, which then made part of its substance, were “captured”
+by Uranus in 126 A.D., and so introduced into the solar domain. The
+truth of the supposition may still be tested; should it be established,
+this remarkable system affords yet another example of the rapidity with
+which cometary materials become disintegrated and scattered.
+
+The number of meteoric radiants now distinctly known is estimated by Mr.
+Denning at about three thousand; and we need not hesitate to ascribe to
+all these streams a cometary origin. It is true that the three thousand
+generating comets have, all but three, “gone over to the majority.” But
+we have witnessed the obsequies of Biela, and it seems only logical to
+infer that those of its 2996 congeners were, in old times, celebrated
+after the same fashion, and are still kept in mind by the annual blaze,
+in their honour, of a few representative sky-rockets.
+
+No component of a star-burst has so far _undoubtedly_ come to the
+ground. The fire-works shown are of the most innocuous kind. Two
+_possible_ exceptions are, however, on record. On April 4, 1095, a
+shower of Lyraids was visible in Western Europe. The stars, according to
+the Saxon Chronicle,[107] crowded “so thickly that no man could count
+them.” And in France, one of the throng fell so accessibly that a
+bystander, having noted the spot, “cast water upon it, which was raised
+in steam with a great noise of boiling.” But, unless the aerolite came
+from the same radiant as the stars, their simultaneous arrival was an
+unmeaning coincidence. It implied no connexion, physical or dynamical,
+between them. The same coincidence was renewed during the Andromede
+shower of November 27, 1885. Just before it began, a “ball of fire”
+struck the ground at Mazapil in Mexico, and proved to be a substantial
+piece of iron containing nodules of graphite. It weighed eight pounds.
+Yet here again that essential circumstance, the direction of its fall,
+remained unknown. We must then, for the present, suspend our judgment as
+to whether aerolites may be regarded, like shooting stars, as actual
+cometary débris.
+
+Mr. Denning’s patient watch of thirty years has led him to the singular
+discovery of “stationary radiants.” The direction in which meteors
+appear to approach the earth is determined by the combination of theirs
+with the earth’s movements. The effect is strictly analogous to the
+aberration of light. Meteoric radiants ought accordingly to shift on the
+sphere just as the heavenly bodies change their apparent places by the
+prescribed measure of aberration. And most do in this respect conform to
+theory, the Perseid radiant notably. On the other hand, certain
+well-known radiants continue fixed night after night in seeming
+independence of the earth’s orbital advance; and there are a good many
+points in the sky whence shooting stars continue to _dribble_ without
+sensible interruption during many months of each year. The fact is
+undeniable, although inexplicable.
+
+The future progress of meteoric astronomy depends largely upon the
+introduction of the photographic mode of observation. Only by its aid
+can the precise determination of radiant-points be effected; and this is
+the chief desideratum. Its realisation before the close of the century
+may safely be predicted. Dr. Elkin, director of Yale College
+Observatory, had a “meteorograph” constructed for the purpose in 1894,
+and hopes to use it for the registration of the Leonids now hastening to
+meet us. Hitherto, only casual fire-balls have printed their tracks on
+sensitive plates. Success in obtaining permanent records of shooting
+stars diverging from a radiant will mark a turning-point in meteoric
+investigations.
+
+
+
+
+                               ASTRONOMY
+
+
+[Illustration:
+
+  NEBULA IN ANDROMEDA. 31 MESSIER.
+
+  (_From a Photograph, by Dr. Roberts._)
+]
+
+
+
+
+                   SECTION IV.—THE SIDEREAL HEAVENS.
+
+                        BY J. E. GORE, F.R.A.S.
+
+
+
+
+                               CHAPTER I.
+                     THE STARS AND CONSTELLATIONS.
+
+
+The study of the sidereal heavens is one of surpassing interest, and
+tends to raise our minds above the sordid things of time and the petty
+affairs of the little planet on which we dwell,—a globe absolutely
+large, of course, when compared with objects around us, but relatively
+very small in comparison with the vast stellar universe which surrounds
+us on all sides, a universe so vast that even the largest telescopes can
+only partially fathom its immeasurable depths.
+
+For the study of the sidereal heavens, as revealed to us by the giant
+telescopes of modern times, it will be advisable to begin by a
+consideration of the starry sky as seen by the naked eye, without
+optical assistance of any kind. On a clear and moonless night, when the
+vault of heaven is spangled over with shining points of light, some
+bright, others fainter, and many more barely perceptible to the unaided
+vision, we are inclined to imagine that the stars visible to the naked
+eye are innumerable, and that any attempt to count them would be a
+hopeless task. This idea, however, is quite a mistake, and, indeed,
+merely an optical illusion, due partly to the scintillation or twinkling
+of the brighter stars, and stars near the limit of vision, and partly to
+their irregular distribution over the surface of the heavens. As a
+matter of fact, the stars visible to the naked eye can be easily
+counted; and they have been counted and catalogued. As every book in the
+catalogue of a large library can be identified, so every star visible to
+the unaided vision—and thousands even fainter, and only visible in
+telescopes—have been mapped, and their exact positions are as well known
+to astronomers as those of every town and village in Great Britain are
+known to geographers. The number of stars which can be seen with
+ordinary eyesight is, in fact, very limited, and does not exceed the
+number of inhabitants in a small town. Some years ago, a German
+astronomer, Heis, who was gifted with excellent eyesight carefully
+mapped down all the stars visible to his eye without optical aid, and
+found the total number visible in the middle of Europe to be only 3,903.
+A similar work was undertaken for the Southern Hemisphere by Behrmann,
+another German astronomer, and the total number distinctly seen by both
+astronomers in both hemispheres of the star sphere is 7,249. Of course,
+at any given time and place only one half the star sphere is visible,
+the other half being below the horizon. It follows, therefore, that
+about 3,600 stars are visible at one time from any point on the earth’s
+surface. As, however, everyone does not possess the keen vision of the
+astronomers referred to above, we may safely say that not more than
+3,000 stars are, on the average, visible at a time to ordinary eyesight.
+On the other hand, persons gifted with exceptionally keen vision may
+possibly see even more than Heis and Behrmann did; but even to such
+eyes, the total number distinctly visible on a clear night without a
+moon would probably not exceed 5,000. We may easily satisfy ourselves as
+to the truth of this statement by taking a portion of the sky, and
+counting the number of stars which can be steadily seen. Everybody knows
+the Great Bear, sometimes called the “Plough,” or “Charles’ Wain.” Four
+of the well-known stars in this remarkable group form a four-sided
+figure. Well, let the reader look carefully at this figure, and see how
+many stars can be detected within the space formed by imaginary lines
+joining the bright stars. Probably surprise will be felt at the small
+number which can be distinctly seen. Heis, with his keen vision, only
+shows eight on his map, and of these, four are very faint, and near the
+limit of even good eyesight. Probably very few eyes will see more than
+eight, and perhaps most persons will fail to see so many. As the whole
+hemisphere is roughly five hundred times larger than this spot, the
+number seen by Heis in the quadrilateral of the Plough would give a
+total of 4,000 stars visible at one time. Of course, some portions of
+the sky are much richer in stars than the spot selected; but, on the
+other hand, others are much poorer, so that perhaps this may be taken as
+a spot of average richness. From this single example it will be seen
+that the idea of countless multitudes of stars visible to the naked eye
+is a mistake. Probably the effect of a great number is partly due to our
+catching glimpses by “averted vision” of still fainter stars, which
+cannot, however, be seen steadily when the eye is turned directly
+towards them.
+
+[Illustration:
+
+  FIG. 1.—_Stars visible in the Northern Hemisphere._
+
+  (From “Visible Universe.”)
+]
+
+In speaking of stars visible to the naked eye, we do not, of course,
+include the stars in the Milky Way, that arch of cloudy light which
+spans the heavens; for although this wonderful zone is composed of faint
+stars, these stars are not individually visible without a telescope.
+
+Notwithstanding the limited number of the visible, or lucid, stars, as
+they are called, the aspect of the starry sky still presents a spectacle
+of marvellous beauty and interest, and may be viewed with pleasure and
+profit even without a telescope. There are many interesting objects
+which may be seen without optical assistance of any kind. Look at the
+middle star of the three forming the “tail” of the Great Bear, or
+“handle” of the Plough. This star was called Mizar by the old Arabian
+astronomers. Close to it, good eyesight will see a small star, known as
+Alcor. This little star was called by the Arabians Alsuha, which means
+“the neglected small star.” The name Alcor means the “test,” and is
+supposed to indicate that the old astronomers considered it a test for
+keen vision; but the Arabians had a proverb, “I show him Alsuha, and he
+shows me the moon,” a saying which seems to imply that it could be
+easily seen by these old astronomers. The faintest star of the seven,
+the one at the root of the tail, was called Megrez by the Arabian
+astronomers. This star is supposed to have diminished in brightness
+since ancient times, as it was rated of the third magnitude by Ptolemy,
+and of the second by Tycho Brahé, while at present it is not much above
+the fourth magnitude. It may possibly be variable in its light, like
+many other stars in the heavens.
+
+[Illustration:
+
+  FIG. 2.—_Stars visible in the Southern Hemisphere._
+
+  (From “Visible Universe.”)
+]
+
+Here it may be mentioned that the stars were divided into magnitudes or
+classes according to their brightness by the ancient astronomers, all
+the brightest stars being placed in the first magnitude, those
+considerably fainter being called second magnitude, those fainter still
+third magnitude, and so on to the sixth magnitude, or those just visible
+to ordinary eyesight. This classification has been practically retained
+by modern astronomers, but, of course, there are stars of all degrees of
+brightness from Sirius down to the faintest stars visible in the largest
+telescopes. Sirius is the brightest star in the heavens, and is equal to
+about six average stars of the first magnitude, such as Altair or
+Aldebaran. According to the Harvard photometric measures, the following
+are the brightest stars in the heavens in order of magnitude:—(1)
+Sirius, (2) Canopus, (3) Arcturus, (4) Capella, (5) Vega, (6) Alpha
+Centauri, (7) Rigel, (8) Procyon, (9) Achernar, (10) Beta Centauri, (11)
+Betelgeuse (slightly variable), (12) Altair, and (13) Aldebaran. Of
+these Canopus, Alpha, and Beta Centauri, and Achernar, do not rise above
+the horizon of London. Of those brighter than the second magnitude, the
+following are north of the Equator: Alpha Cygni, Pollux, Castor, Eta
+Ursæ Majoris, Gamma Orionis, Beta Tauri, Epsilon Ursæ Majoris, Alpha
+Ursæ Majoris, Alpha Persei, and Beta Aurigæ; and south of the Equator:
+Alpha Crucis, Fomalhaut, Antares, Spica, Beta Crucis, Gamma Crucis,
+Epsilon Orionis, Zeta Orionis, Epsilon Canis Majoris, Beta Carinæ,
+Epsilon Carinæ, Lambda Scorpii, Alpha Triangulum Australis, Gamma Argûs,
+Alpha Gruis, Epsilon Sagittarii, Alpha Hydræ, Theta Scorpii, and Delta
+Velorum. Of those below the second magnitude, and brighter than the
+third, there are about 34 in the Northern Hemisphere, and 61 in the
+Southern. As the brightness decreases, the numbers increase rapidly.
+Indeed, the increase is in geometrical progression, the number in each
+class of magnitude being about three times as many as those in the class
+one magnitude brighter. The exact magnitudes of all stars visible to the
+naked eye in both hemispheres have now been determined by the aid of
+photometers. These instruments are described in Section II. of the
+present work, Chapter XVII.
+
+The stars were divided by the ancient astronomers into groups called
+constellations. Some of these were formed in the earliest ages of
+antiquity. Orion and the Pleiades are mentioned in Job (Chapter
+XXXVIII.), which is believed to be one of the oldest books in existence.
+Josephus ascribes the division of the stars into constellations to the
+family of Seth, the son of Adam; and according to the Book of Enoch the
+constellations were already known and named in the time of that
+patriarch. The brightest stars of each constellation are designated by
+the letters of the Greek alphabet, which were assigned to them by Bayer
+in the year 1603, Alpha generally denoting the brightest star, Beta the
+next in lustre, and so on. This is not, however, invariably the case,
+and Bayer seems in many cases to have followed the outline of the
+imaginary figure from which the constellation derives its name, rather
+than the relative brightness of the stars composing the constellation.
+For example, the seven stars in the Plough are known as Alpha, Beta,
+Gamma, Delta (the faint one), Epsilon, Zeta, and Eta, beginning with the
+northern of the two in the square farthest from the tail, thus evidently
+following the shape of the figure, and not the order of relative
+brightness. When the letters of the Greek alphabet are exhausted,
+recourse is had to numbers, those in Flamsteed’s catalogue being usually
+employed. Those only visible in telescopes are known by their numbers in
+various catalogues. The exact positions of the stars are fixed by
+determining their right ascensions and declinations, terms which on the
+celestial sphere correspond to longitude and latitude on the earth.
+
+The stars Alpha and Beta of the Plough are called “the pointers,”
+because a line drawn from Beta through Alpha points nearly to a star of
+the second magnitude, called the Pole Star, which lies near the pole of
+the celestial sphere, or the point round which the whole star sphere
+seems to rotate, owing to the rotation of the earth on its axis, in
+twenty-four hours. The distance from Alpha to the Pole Star is about
+five times the distance between Alpha and Beta.
+
+If we draw an imaginary line from the star Epsilon through the Pole
+Star, and produce it to about the same distance on the opposite side of
+the Pole, it will pass through a well-known group called Cassiopeia’s
+Chair. This consists of five fairly bright stars arranged in the form of
+an irregular W. A sixth star, much fainter than the others, forms with
+three of them a quadrilateral figure. It was near this faint star—known
+to astronomers as Kappa—that the famous “new,” or temporary, star of
+Tycho Brahé, sometimes called the “Pilgrim Star,” suddenly appeared in
+November, 1572, of which more hereafter.
+
+If we continue the curve formed by the three stars in the tail of the
+Great Bear, it will pass near a very bright star of an orange colour.
+This is Arcturus, one of the brightest stars in the sky. If we can rely
+on the measures of distance which have been made of this brilliant star,
+it must be one of the largest bodies in the universe, much larger than
+our sun, which, placed at the distance assigned to Arcturus, would only
+shine as a small star, quite invisible indeed to the naked eye.
+
+Returning again to the Great Bear, if we draw a line from Gamma to Beta
+and produce it, it will pass near a bright star of a yellow colour. This
+is Capella. It was called by the Arabian astronomers the “Guardian of
+the Pleiades.” It is the brightest star of the constellation Auriga or
+“the Charioteer,” referred to by Tennyson in the lines:
+
+           “And the shining daffodil dies, and the Charioteer
+           And starry Gemini hang like glorious crowns
+           Over Orion’s grave low down in the West,”
+
+evidently referring to the disappearance of Orion below the western
+horizon in the evening sky of April. “Starry Gemini” is marked by two
+bright stars, Castor and Pollux, which may be found by drawing a line
+from Delta to Beta of the Great Bear, and producing it. Another line
+drawn from Delta to Gamma, and produced towards the south, will pass
+near a bright star called Regulus, the brightest star in the well-known
+“Sickle” in Leo or the Lion. Again, a line drawn from Regulus to Gamma
+in the Great Bear, and produced, will pass near another bright star,
+Vega in the Lyre. This is one of the brightest stars in the Northern
+Hemisphere, the three, Arcturus, Capella, and Vega, being nearly equal
+in brightness. The name Vega seems to be a corruption of the Arabic name
+_vaki_, or _al-nasr al-vaki_, “the falling eagle,” the wings of the bird
+being represented by the stars Epsilon and Zeta Lyræ, which form, with
+Vega, a small triangle, called by the Arabians _al-alsafi_, “the
+trivet.” But what relation exists between a “falling eagle” and the
+musical instrument known as the Lyre (Persian _al-lûra_) is not very
+obvious. Possibly, however, as suggested by Schjellerup, the Arabic
+word, _al-schalzâk_ “a goose,”—also applied to the constellation—refers
+to the resemblance in shape between a plucked goose and a Greek lyre.
+The Greeks called the constellation χέλυς, a tortoise, which also
+somewhat resembles a lyre in shape.
+
+Of the two stars which form a triangle with Vega, the northern, Epsilon,
+is a double star, which is said to have been seen double with the naked
+eye by several astronomers, but, probably, most people would fail to see
+it as anything but a single star, as the component stars are very close.
+An opera-glass will, however, show it distinctly. Each of the components
+is again double, so that the object forms a most interesting quadruple
+star when viewed with a good telescope.
+
+To the east of Vega lies Cygnus, or the Swan, one of the finest of the
+constellations. It may be distinguished by the long cross formed by the
+principal stars which are known to astronomers as Alpha, Beta, Gamma,
+Delta, and Epsilon; Alpha, or Deneb, being the brightest and most
+northern of the five, and Beta the most southern and faintest. The name
+Deneb is derived from the Arabic word _dzanab al-dadjâdja_, or “the tail
+of the hen,” referring to its position in the ancient figure, which
+represents a hen or swan flying towards the south.
+
+To the south-east of Cassiopeia’s Chair, we find the well-known festoon
+of stars which marks the constellation Perseus. Its brightest star is
+sometimes called Mirfak, a name derived from the Arabic word _marfik_,
+the elbow, referring, perhaps, to its position in the curved line of
+stars. South of Perseus, and the nearest bright star to Mirfak in that
+direction, is Algol, the famous variable star. Further south, we come to
+the constellation of Taurus, or the Bull, with the well-known groups of
+the Pleiades and Hyades. The Pleiades form a remarkable cluster, and
+when once recognised can never be mistaken. To ordinary eyesight six
+stars are visible, but those having keener vision can see more. A little
+south of the Pleiades is a V-shaped figure, the Hyades, with a bright
+star of a reddish colour. This is Aldebaran, a name derived from the
+Arabic _al-dabarân_, the attendant or follower, because it appears to
+follow the Pleiades in the diurnal motion. It was also called _aïn
+al-tsaur_, “the eye of the bull,” and by several other names such as
+_al-fanîk_, “the great camel,” the other smaller stars forming the
+Hyades being called _al-kilas_, “the young camels!”
+
+South of Taurus and Gemini comes the magnificent constellation of Orion,
+perhaps the most splendid collection of stars in the sky. This brilliant
+asterism contains many fine objects. Looking at it when it is visible in
+the winter sky, we notice a large quadrilateral figure formed by four
+conspicuous stars. The upper one to the left is called Betelgeuse, and
+is decidedly reddish in colour—very much resembling Aldebaran both in
+tint and brightness. Its name is derived from our Arabic word meaning
+the shoulder, because it is situated on the right shoulder of the giant
+Orion on the old celestial globes. The upper one to the right is called
+Bellatrix, or the female warrior! The real significance of some of those
+old names is sometimes difficult to understand. Of the lower stars, the
+one on the right is a fine white star of the first magnitude known as
+Rigel. It is situated on the left foot of the ancient figure of Orion,
+and its name is derived from the first part of the compound Arabic name
+_ridjl-al-djauzâ_, “the leg of the giant.” The lower star on the left is
+known to astronomers by the Greek letter Kappa.
+
+In the middle of the four-sided figure referred to above are three stars
+of the second magnitude, nearly in a straight line, forming “Orion’s
+Belt.” The upper one of the three is slightly fainter than the others,
+and has been suspected of being slightly variable in its light, but the
+variability is doubtful. South of these three conspicuous stars are
+three fainter stars, forming a nearly vertical line. This is “the Sword
+of Orion.” The middle star of the three marks the position of “the great
+nebula in Orion,” one of the finest objects in the heavens, of which
+more hereafter. To some eyes a nebulous glow is visible round this star.
+Even in a small telescope the nebula is an interesting object. On a very
+clear night the southern star of the three may be seen double with good
+eyesight. The stars forming Orion’s Belt were called by the Arabian
+astronomers _mintakat al-djauza_, “the Belt of the Giant”; and the stars
+forming the “sword,” _al-lakat_, the “gleaned ears of corn,” and also
+_saif-al-djabbâr_, “the Sword of the Giant.” Perhaps the latter word is
+the origin of the name Algebar, formerly applied to Rigel.
+
+The three bright stars in Orion’s Belt nearly point (to the south-east)
+to Sirius, the brightest star in the heavens. This is a splendid white
+star, and is so much brighter than any other fixed star that its
+identity cannot be mistaken.
+
+If we draw a line from the star Gamma in the Plough to the Pole Star,
+and produce it, it will pass through a somewhat similar four-sided
+figure, but of much larger size, and the stars rather fainter. This is
+known as “the Square of Pegasus.” The upper stars are known as Beta
+Pegasi (the one to the right) and Alpha Andromeda. To the east of Alpha
+Andromedæ is a star of the third magnitude, Delta, and to the east of
+Delta, a star of the second magnitude called Beta Andromedæ. A little
+north of Beta are two small stars, Mu and Nu, nearly in a line with
+Beta, and to the north of Nu is the famous “nebula in Andromeda” “the
+queen of the nebulæ,” as it has been termed. It is just visible to the
+naked eye as a hazy spot of light, and it may be well seen in a good
+opera-glass or binocular. Even in a small telescope it is a really
+splendid object. The reader should fix its exact position carefully, as
+it has been frequently mistaken for a comet by observers whose knowledge
+of the heavens is not very accurate.
+
+The following alignments may be found useful by beginners in the study
+of the starry sky:—
+
+Castor and Pollux, already mentioned, nearly point south to the star
+Alpha Hydræ, an isolated reddish star of the second magnitude. It is
+also called Alphard, from the Arabic _al-fard_, “the solitary one,”
+because there is no other bright star near it. It is described by
+Al-Sûfi, the Persian astronomer, as red in the tenth century. In the
+Chinese annals it is called “the Red Bird.”
+
+An isosceles triangle is formed by Castor (at the vertex), Alphard and
+Sirius. Procyon is nearly in the centre of this triangle. Two other
+roughly isosceles triangles are formed, having Aldebaran at the vertex
+of each, namely: Aldebaran, Castor, and Procyon, and Aldebaran, Procyon,
+and Sirius.
+
+Castor, Alpha, Delta, and Beta Orionis are nearly in a straight line;
+also Beta Pegasi, Alpha Pegasi and Fomalhaut. A right-angled triangle is
+formed by Arcturus, Spica, and Regulus, Spica being at the right angle.
+
+In the Southern Hemisphere, the most remarkable group of stars is the
+well-known Southern Cross. It consists of four stars, known as Alpha,
+Beta, Gamma and Delta—Gamma being at the top of the cross, and Alpha at
+the bottom. These stars are popularly supposed to be of great
+brilliancy, but this is a mistake; their magnitudes, according to recent
+photometric measures, being Alpha, first magnitudes, Beta 1½, Gamma,
+second magnitude, and Delta, third magnitude. A little south of Delta is
+Epsilon, a star of the fourth magnitude, which rather spoils the
+symmetry of the cross-shaped figure. A little to the east of the
+Southern Cross are Alpha and Beta Centauri, two of the brightest stars
+in the sky. Another fine group of stars is Scorpio, or the Scorpion, of
+which the brightest star is Antares, a reddish star of about magnitude
+1½, which is visible near the southern horizon in the months of June and
+July in England.
+
+When the positions of the principal stars are known, it will be easy to
+find any other required object by means of star maps.
+
+
+
+
+                              CHAPTER II.
+                 DOUBLE, MULTIPLE, AND COLOURED STARS.
+
+
+Many of the stars when examined with a good telescope are seen to be
+double, some triple, and a few quadruple, and even multiple. These when
+viewed with the naked eye, or even a powerful binocular, seem to be
+single, and show no sign of consisting of two components. These
+telescopic double stars should be carefully distinguished from those
+which appear very close together with the naked eye, and which in
+opera-glasses or telescopes of small power might be mistaken for wide
+double stars by the inexperienced observer. These latter stars, such as
+Mizar—the middle star in the tail of the Great Bear, and its small
+companion, Alcor, referred to in the last chapter—have been called
+“naked eye doubles,” but they are not, properly speaking, double stars
+at all. Telescopic double stars are far closer, and even the widest of
+them could not possibly be seen double without optical aid, even by
+those who are gifted with the keenest vision. Of these so-called “naked
+eye doubles,” we may mention Alpha Capricorni, which on a very clear
+night may be seen with the naked eye to consist of two stars. On a very
+fine night two stars may be seen in Iota Orionis, the most southern star
+in Orion’s Sword. The star Zeta Ceti has near it a fifth magnitude star,
+Chi, which may be easily seen with the unaided vision. The star Epsilon
+Lyræ (near Vega), is, as mentioned in the last chapter, a severe test
+for naked eye vision. Bessel, the famous German astronomer, is said to
+have seen it when thirteen years of age. Omicron Cygni (north of Alpha
+and Delta Cygni) forms another naked eye double, and other objects of
+this class may be noticed by a sharp-eyed observer.
+
+The star Mizar, already referred to, is itself a wide telescopic double,
+and it seems to have been the first double star discovered with the
+telescope (by Riccioli in 1650). It consists of two components, of which
+one is considerably brighter than the other. It will give an idea of the
+closeness of even a “wide” telescopic double when we say that the
+apparent distance between Mizar and Alcor is nearly forty times the
+distance which separates the close components of the bright star. From
+this it will be seen that even a powerful binocular field-glass would
+fail to show Mizar as anything but a single star. The components may,
+however, be well seen with a 3-inch telescope, or even with a good
+2-inch. The colours of the two stars are pale green and white. Between
+Mizar and Alcor is a star of the eighth magnitude, and others fainter.
+Mizar was the first double star photographed by Bond.
+
+The Pole Star has a small companion at a little greater distance than
+that which separates the components of Mizar, but owing to the faintness
+of this small star, the object is not so easy as Mizar. A telescope with
+a good 3-inch aperture should, however, show it readily. Dawes saw it
+with a small telescope of 1³⁄₁₀-inch aperture, and Ward, who has
+wonderful vision, with only 1¼-inch.
+
+The star Beta Cygni is composed of a large and small star, of which the
+colours are described as “golden-yellow and smalt-blue.” This is a very
+wide double, and may be seen with quite a small telescope. Another fine
+double star is that known to astronomers as Gamma Andromedæ. The
+magnitudes of the components are about the same as those of Mizar, but a
+little closer. Their colours are beautiful (“gold and blue”). This is
+one of the prettiest double stars in the heavens. It is really a triple
+star, the fainter of the pair being a very close double star; but this
+is beyond the reach of all but the largest telescopes. The star Gamma
+Delphini is another beautiful object, the components being a little more
+unequal in magnitude, but the distance between them about the same as in
+Gamma Andromedæ. I have noted the colours with a 3-inch telescope as
+“reddish-yellow and greyish-lilac.” Gamma Arietis, the faintest of the
+three well-known stars in the head of Aries, is another fine double
+star, a little closer than Gamma Delphini. This is an interesting
+object, from the fact that it was one of the first double stars
+discovered with the telescope—by Hooke, in 1664, when following the
+comet of that year. He says:—“I took notice that it consisted of two
+small stars very near together, a like instance of which I have not else
+met with in all the heaven.” Eight years previous to this, however, in
+1656, Huygens is said to have seen three stars in Theta Orionis, the
+well-known multiple star in the Orion nebula; and in 1650, Riccioli, at
+Bologne, saw Zeta Ursæ Majoris (Mizar) double, as already stated.
+
+Another beautiful double star is Eta Cassiopeiæ, the components being
+about equal in brightness to those of Gamma Delphini, but the distance
+less than one half, so that a higher magnifying power will be required
+to see them well. The colours are, according to Webb, yellow and purple;
+but other observers have found the smaller star garnet or red. This is a
+very interesting object, the components revolving round each other, and
+forming what is called a binary star.
+
+Another fine double star is Castor, which is composed of two nearly
+equal stars separated by a distance about half that between the
+components of Gamma Andromedæ. This is also a binary or revolving double
+star, but the period is long. Gamma Virginis is another fine double
+star, with components at about the same distance as those of Castor, and
+the colours very similar. It is also a remarkable binary star, and
+further details respecting it will be given when we come to speak of the
+binary stars.
+
+Among double stars of which the components are closer than those
+mentioned above, but which are within the reach of a good 3-inch
+telescope—a common size with amateur observers—the following may be
+noticed:—Alpha Herculis, colours, orange or emerald green; the light of
+this star is slightly variable. Gamma Leonis, another binary star with a
+long period; colours, pale yellow and purple. Epsilon Boötis, a lovely
+double star, the colours of which Secchi described as “most beautiful
+yellow, superb blue.” This has been well seen with a 2¼-inch achromatic.
+
+For observers in the Southern Hemisphere, the following fine double
+stars may be seen with a 3-inch telescope:—Alpha Centauri; this famous
+star, the nearest of all the fixed stars to the earth, is also a
+remarkable binary; its period, as recently computed by Dr. See, is 81
+years, and the component stars are now at nearly their greatest distance
+apart, the distance being greater than that between the components of
+Mizar, so that any small telescope will show them. Theta Eridani is a
+splendid pair, but closer than Alpha Centauri. It is, however, an easy
+object with a 3-inch telescope, and with a telescope of this size I
+noted the colours in India as light yellow and dusky yellow. The star
+known as _f_ Eridani is a very similar double to Theta, but the
+components are fainter. I noted the colours in India as yellowish-white
+and very light green. There are, of course, many other double stars in
+both hemispheres within the reach of small telescopes; but those
+described above are perhaps the finest examples.
+
+In addition to these comparatively wide double stars, there are many of
+which the components are so close that they are quite beyond the reach
+of a 3-inch or even a 4-inch telescope. Some, indeed, are so excessively
+close as to tax the highest powers of the largest telescopes yet
+constructed.
+
+Of triple, quadruple, and multiple stars, there are several which may be
+well seen with a small telescope. Of these may be mentioned Iota
+Orionis, the lowest star in the Sword of Orion, which consists of a
+bright star accompanied by two small companions. In Theta Orionis, the
+middle star of the Sword, four stars may be seen forming a quadrilateral
+figure, known to observers as the “trapezium.” I have seen these in
+India—where the star is higher in the sky than in this country—with a
+3-inch refractor reduced by a “stop” over the object-glass to 1½ inch.
+There are two fainter stars in this curious object, which lie in the
+midst of the Orion nebula, but a somewhat larger telescope is required
+to see them. Within the trapezium are two very faint stars, which are
+only visible in the largest telescopes. In Sigma Orionis—a star closely
+south of Zeta, the lowest star in Orion’s Belt—six stars may be seen
+with a 3-inch telescope. Indeed, Ward has seen ten with a slightly
+smaller telescope. Epsilon Lyræ may be seen double with a low power, and
+each star of the pair again double with a high power; but this is more
+difficult than the other close stars mentioned above.
+
+When carefully examined, many of the stars show differences in colour.
+Among the brightest stars it will be noticed that Sirius, Rigel, and
+Vega, shine with a white or bluish-white light; Capella is distinctly
+yellowish; Arcturus yellow or orange; and Aldebaran and Betelgeuse have
+a well-marked reddish hue. There are no stars of a decided blue colour
+visible to the naked eye, at least in the Northern Hemisphere. The third
+magnitude star, Beta Lyræ, is said to be greenish, but its colour is not
+conspicuous. Betelgeuse is perhaps the ruddiest of the brighter stars,
+and its reddish tint contrasts strongly with the white light of Rigel,
+in the same constellation. Aldebaran, which lies not far from
+Betelgeuse, is of nearly the same hue. But the reddest star visible to
+the naked eye in the Northern Hemisphere is the fourth magnitude star,
+Mu Cephei. It is not, however, sufficiently bright to enable its colour
+to be well seen without optical aid, but with an opera-glass its reddish
+hue is beautiful and striking when compared with other stars in its
+immediate vicinity. It was called by Sir William Herschel the “garnet
+star,” and its colour is certainly remarkable. Like so many of the red
+stars, it is variable in light, but numerous observations by the present
+writer seem to show that there is no regular period, and its light often
+remains for many weeks with little or no perceptible change.
+
+Among other stars visible to the naked eye, the reddish colour is also
+conspicuous in Antares, Alphard, Eta, and Mu Geminorum, Mu and Nu Ursæ
+Majoris, Beta Ophiuchi, Gamma Aquilæ, and others in the Southern
+Hemisphere· Alphard was noted as red by the Persian astronomer, Al-Sûfi,
+in the tenth century, and it was called “the Red Bird,” by the old
+Chinese observers.
+
+Ptolemy, in his catalogue, calls the following stars “fiery red”:
+Arcturus, Aldebaran, Pollux, Antares, Betelgeuse, and, curious to say,
+Sirius, which is now white. There is some little doubt as to the reality
+of this change of hue in Sirius, but Al-Sûfi distinctly describes the
+variable star, Algol, as red, whereas it is now white, or only slightly
+yellowish.
+
+The finest examples of red stars are, however, found among those only
+visible with a telescope. Of these may be mentioned the star numbered
+713 in Espin’s edition of Birmingham’s “Catalogue of Red Stars,” which
+Franks describes as “orange vermilion,” and the star Birmingham 248,
+which Espin notes as “magnificent blood-red.” Another very fine red star
+is the variable R Crateris, which Sir John Herschel described as
+“scarlet, almost blood colour,” Birmingham “crimson,” and Webb “very
+intense ruby.” Observing it in India with a 3-inch telescope, I noted it
+as “full scarlet.” It has near it a star of the ninth magnitude of a
+pale bluish tint. No. 4 of Birmingham’s “Catalogue” is described by
+Espin as of an “intense red colour, most wonderful.” The variable star U
+Cygni is very red, and is described by Webb as showing “one of the
+loveliest hues in the sky.” Another red star is the remarkable, variable
+R Leonis, whose fluctuations in light will be described in the chapter
+on Variable Stars. Hind says: “It is one of the most fiery-looking
+variables on our list—fiery in every stage from maximum to minimum, and
+is really a fine telescopic object in a dark sky about the time of
+greatest brilliancy, when its colour forms a striking contrast with the
+steady white light of the sixth magnitude, a little to the north.”
+
+In the Southern Hemisphere there are some fine red stars. Epsilon
+Crucis, one of the stars of the Southern Cross, is said to be very red,
+and so are Mu Muscæ and Delta Gruis, the southern star of a naked eye
+double. Pi Gruis is also a wide double star, and Dr. Gould describes one
+of the pair as “deep crimson,” while the other is “conspicuously white.”
+The variable R Sculptoris is another fine red star, which Gould
+describes as “intense scarlet,” and Miss Clerke says it “glows like a
+live coal in the field,” a good description of these telescopic red
+stars. With reference to a small star in the field of view with Beta
+Crucis, one of the brightest stars in the Southern Cross, Sir John
+Herschel says: “The fullest and deepest maroon-red, the most intense
+blood-red of any star I have seen. It is like a drop of blood when
+contrasted with the whiteness of Beta Crucis.”
+
+Among the double stars there are numerous examples of coloured suns. Of
+these may be mentioned Alpha Herculis, the components of which are
+orange and emerald, or bluish-green, and described by Smith as “a lovely
+object, one of the finest in the heavens”; Epsilon Boötis, of which the
+colours are described by Secchi as “most beautiful yellow, superb blue”;
+Beta Cygni, “golden-yellow and smalt-blue”; Beta Cephei, “yellow and
+violet”; Delta Cephei, “yellow and blue”; Gamma Andromedæ, “gold and
+blue”; and Beta Piscis Australis, of which the colours were noted by the
+present writer in India as white and reddish-lilac.
+
+It has been found that the red stars are most numerous in or near the
+Milky Way, and one portion of the Galaxy—between Aquila, Lyra, and
+Cygnus—was called by Birmingham “the red region in Cygnus.” Yellow and
+orange stars seem to be most abundant in the constellations, Cetus,
+Pisces, Hydra, and Virgo, and the white stars in Orion, Cassiopeia, and
+Lyra.
+
+
+
+
+                              CHAPTER III.
+                THE DISTANCES AND MOTIONS OF THE STARS.
+
+
+The determination of the distances of the stars from the earth has
+always formed a subject of great interest to astronomers. The earlier
+observers appear to have thought that the problem was an insoluble one.
+The famous Kepler, judging from what he called the “harmony of
+relations,” came to the conclusion that the distance of the fixed stars
+should be about 2,000 times the distance of Saturn from the sun. Saturn
+was then the outermost planet of the solar system. The distance of even
+the nearest star, as now known, is about 14 times greater than that
+supposed by Kepler. Huygens thought the determination of stellar
+distance by observation to be impossible, but made an attempt at a
+solution of the problem by a photometric comparison between Sirius and
+the sun. By this method, he found that Sirius is probably about 28,000
+times the sun’s distance from the earth, but modern measures show that
+this estimate is far too small, the distance of Sirius being probably
+over 500,000 times the sun’s distance, or about 18 times greater than
+Huygens made it.
+
+When the Copernican theory of the earth’s motion round the sun was first
+advanced, it was objected that, if the earth moved in a large orbit, its
+real change of place should produce an _apparent_ change of position in
+the stars nearest to the earth, causing them to shift their relative
+position with reference to more distant stars. Copernicus replied to
+this objection—and we now know that his reply was correct—by saying that
+the distance of even the nearest stars was so great that the earth’s
+motion would have no perceptible effect in changing their apparent
+position in the heavens; in other words, the diameter of the earth’s
+orbit round the sun would be almost a vanishing point if viewed from the
+distance of the nearest stars. This explanation of Copernicus was at
+first ridiculed, and even the famous astronomer, Tycho Brahé, could not
+accept such a startling conclusion. This celebrated observer failed
+indeed to detect by his own observations any annual change of place in
+the stars, but he fancied that the brightest stars showed a perceptible
+disc, like the planets, a fact which, if true, would imply that, if the
+distance of the stars was so great as Copernicus supposed, their real
+diameter must be enormous. The invention of the telescope, however,
+dispelled this delusion of Tycho Brahé, and showed that even the
+brightest stars showed no perceptible disc. This was proved by Horrocks
+and Crabtree, who noticed that, in occultations of stars by the moon,
+the stars disappeared instantaneously, a fact which proved that the
+apparent diameter of the stars must be a very small fraction of a second
+of arc.
+
+Galileo suggested that possibly the distance of the nearer stars might
+be determined by careful measures of double stars, on the assumption
+that the brighter star of the pair—if the difference in brilliancy is
+considerable—is nearer the earth than the fainter star. He says (in his
+“_Opere di Galileo Galilei_”), “I do not believe that all the stars are
+scattered over a spherical superficies _at equal distances from a common
+centre_, but I am of opinion that their distances from us are so various
+that some of them may be two or three times as remote as others, so that
+when some minute star is discovered by the telescope close to one of the
+larger, and yet the former is highest, it may be that some sensible
+change might take place among them.” Acting on this idea, Sir William
+Herschel, at the close of the eighteenth century, made a careful series
+of measures of certain double stars. He did not, however, succeed in his
+attempt, as his instruments were not sufficiently accurate for such an
+investigation, but his labours were rewarded by the great discovery of
+binary or revolving double stars, most interesting objects, which will
+be considered in the next chapter.
+
+Numerous but unsuccessful attempts were made by Hooke, Flamsteed,
+Cassini, Molyneux, and Bradley, to find the distance of some of the
+stars. Hooke, in the year 1669, thought he had detected a parallax of 27
+to 30 seconds arc in the star Gamma Draconis, but we now know that no
+star in the heavens has anything like so large a parallax. It must be
+here explained that to find the distance of any star from the earth, we
+must first measure its “parallax,” which is the apparent change in its
+place due to the earth’s motion round the sun. As the earth makes half a
+revolution in six months, and as the earth’s mean distance from the
+sun—or the radius of the earth’s orbit—is about 93 millions of miles,
+the earth is, at any given time, about 186 millions of miles distant
+from the point in its orbit which it occupied six months previously. The
+apparent change of position in a star’s place, known as parallax, is
+_one-half_ the total displacement of the star as seen from opposite
+points of the earth’s orbit. In other words, it is the angle subtended
+at the star by the sun’s mean distance from the earth. The measured
+parallax of a star may be either “absolute” or “relative.” An “absolute
+parallax” is the actual parallax. A “relative parallax” is the parallax
+with reference to a faint star situated near a brighter star, the faint
+star being assumed to lie, as suggested by Galileo, at a much greater
+distance from the earth. As, however, the faint star may have a small
+parallax of its own, the “relative parallax” is the difference between
+the parallaxes of the two stars. Indeed, in some cases a “negative
+parallax” has been found, which, if not due to errors of observation,
+would imply that the faint star is actually the nearer of the two. From
+the observed parallax, the star’s distance in miles may be found by
+simply multiplying 93 millions of miles by 206,265 and dividing the
+result by the parallax. To find the time that light would take to reach
+us from the star—the light journey as it is called—it is only necessary
+to divide the number 3·258 by the parallax.
+
+In attempting to verify the result found by Hooke for the parallax of
+Gamma Draconis, Molyneux and Bradley found an apparent parallax of about
+20 seconds of arc, thus apparently confirming Hooke’s result, but
+observations of other stars showing a similar result, Bradley came to
+the conclusion that the apparent change of position was not really due
+to parallax, but was caused by a phenomenon now known as the “aberration
+of light,” an apparent displacement in the positions of the stars, due
+to the effect of the earth’s motion in its orbit round the sun combined
+with the progressive motion of light. The result is that “a star is
+displaced by aberration along a great circle, joining its true place to
+the point on the celestial sphere towards which the earth is moving.”
+The amount of aberration is a maximum for stars lying in a direction at
+right angles to that of the earth’s motion. The existence of aberration
+is an absolute proof that the earth does revolve round the sun, for were
+the earth at rest—as some paradoxes contend—there would be no aberration
+of the stars. This effect of aberration must, of course, be carefully
+allowed for in all measures of stellar parallax. To show that
+“aberration” could not possibly be due to “parallax,” it may be stated
+that aberration shifts the apparent place of a star in one direction,
+while parallax shifts it in the opposite direction.
+
+From photometric comparisons, the Rev. John Mitchell, in the year 1767,
+concluded that the parallax of Sirius is less than a second of arc; a
+result which has been fully confirmed by modern measures. He considered
+that stars of the sixth magnitude are probably 20 to 30 times the
+distance of Sirius, and judging from their relative brilliancy alone,
+this result would also be nearly correct. But recent measures have shown
+that some of the fainter stars are actually nearer to us than some of
+the brighter, and that the brightness of a star is no criterion of its
+distance.
+
+The first stars on which observations seem to have been made with a view
+to a determination of their distance seem to have been Aldebaran and
+Sirius. From observations made in the years 1792 to 1804 with a vertical
+circle and telescope of 3 inches aperture, Piazzi found for Aldebaran an
+“absolute” parallax of about 1½ seconds of arc. O. Struve and Shdanow,
+in 1857, using a refractor of 15 inches aperture, found a “relative”
+parallax of about half a second. This was further reduced by Hall with
+the 26-inch refractor of the Washington Observatory to about one-tenth
+of a second, and Elkin, with a heliometer of 6 inches aperture, finds a
+relative parallax of 0″·116, or about 30 years’ journey for light For
+Sirius, Piazzi found, in 1792–1804, an absolute parallax of four
+seconds, but this was certainly much too large. All subsequent observers
+find a much smaller parallax, recent measures giving a relative parallax
+of 0·370″ by Gill, and 0·407″ by Elkin. In the years 1802–1804, Piazzi
+and Cacciatori found an absolute parallax of 1′·31 for the Pole Star;
+but this has been much reduced by other observers. Pritchard, by means
+of photography, found a relative parallax of only 0·073″, which agrees
+closely with some other previous results, and indicates a “light
+journey” of about 44 years!
+
+For the bright star Procyon, Piazzi found a parallax of about three
+seconds, but this is also much too large, a recent determination by
+Elkin giving 0·266″, a figure in fair agreement with results found by
+Auwers and Wagner. For the bright star Vega, Calandrelli, in the years
+1805–6, found an absolute parallax of nearly four seconds, but this has
+also been much reduced by modern measures; Elkin, from observations in
+the years 1887–88, finding a relative parallax of only 0·034″. Brinkley
+found a parallax of over one second for Arcturus, but Elkin’s result is
+only 0·018″. If this minute parallax can be relied on, Arcturus must be
+a sun of vast size.
+
+Owing to the large “proper motion” of the star known as 61 Cygni, its
+comparative proximity to the earth was suspected, and in 1812, Arago and
+Mathieu found, from measures made with a repeating circle, a parallax of
+over half a second. Various measures of its parallax have since been
+made, ranging from about 0·27″ to 0·566″. Sir Robert Ball, at Dunsink,
+Ireland, found 0·468″, and Pritchard, by means of photography with a
+13-inch reflector, found 0·437″. We may, therefore, safely assume that
+the parallax of 61 Cygni is about 0·45″. This implies a distance of
+458,366 times the sun’s distance from the earth, or about 42 billions of
+miles, and a “light journey” of about 7¼ years.
+
+It is usually stated that 61 Cygni is the nearest star to the earth in
+the Northern Hemisphere, but for the star known as Lalande 21,185,
+Winnecke found 0·511″, and afterwards 0·501″. This has, however, been
+reduced by Kapteyn (1885–1887) to 0·434″; and recently a parallax of
+0·465″ has been found by the photographic method for the binary star,
+Eta Cassiopeiæ. 61 Cygni is a wide double star, but it seems doubtful
+whether the components are physically connected, although several orbits
+have been provisionally computed.
+
+Nearer to us than 61 Cygni is the bright southern star Alpha Centauri,
+which, so far as is known at present, is the nearest of all the fixed
+stars to the earth. The first attempt to find its distance was made by
+Henderson in the years 1832–33, using a mural circle of 4 inches
+aperture and a transit of 5 inches. He found an “absolute” parallax of
+about one second of arc, which subsequent measures have shown to be
+rather too large. Measures in recent years range from 0·512″ to 0·976″,
+but probably the most reliable are those made with a heliometer of 4½
+inches aperture by Dr. Gill (1881–82), who found a “relative” parallax
+of 0·76″, and by Dr. Elkin, using the same instrument, 0·671″. Gill’s
+result would place the star at a distance of 271,400 times the sun’s
+distance from the earth, or about 25 billions of miles, a distance which
+light, with its great velocity of 186,300 miles a second, would take
+over 4¼ years to traverse.
+
+It will be understood that the parallaxes found for even the nearest
+fixed stars are so small that their exact determination taxes the powers
+of the most perfect instruments and the skill of the most experienced
+observers. One thing, however, seems certain, that the brightest stars
+are not necessarily the nearest, and that comparatively faint stars may
+be actually nearer to the earth than some of the brightest gems which
+deck our midnight sky. Indeed, from a discussion of the observed
+parallaxes and “proper motions” of 11 stars, Gylden finds a mean
+parallax of only 0·083″ for stars of the first magnitude. This agrees
+closely with the value 0·089″ found by Dr. Elkin.
+
+In old times the stars were supposed to be absolutely fixed in the
+celestial vault, that is to say, that their relative positions did not
+change. This was a very natural conclusion, for before the invention of
+the telescope it would have been impossible to detect any “proper
+motion”—as it is called—by naked eye observations. Hence the term “fixed
+stars,” used to distinguish the stars from the planets, which are always
+shifting their positions in the heavens. The existence of proper motion,
+in some at least of the stars, seems to have been discovered by Halley,
+who found from his observations in 1715 that the bright stars, Sirius,
+Arcturus, and Aldebaran, had apparently shifted their positions since
+the date of the earliest observations. This discovery was confirmed by
+James Cassini in 1738. He found that Arcturus had apparently moved
+through some five minutes of arc in 152 years, or about two seconds a
+year, a result which agrees fairly well with more exact modern measures.
+
+This interesting discovery of stellar motion has been fully confirmed by
+modern observations, and we now know that, far from the stars being
+“fixed,” most of them have an apparent motion on the celestial vault.
+These motions are, however, very slow, and can only be detected by
+accurate measurements and a careful comparison of their positions after
+the lapse of a number of years. The largest proper motion hitherto
+detected is that of a star known as 1830 of Groombridge’s catalogue, a
+small star of about 6½ magnitude, which lies in the constellation Ursa
+Major. This star has an apparent motion of seven seconds per annum,
+which, though relatively large, is of course absolutely small, as the
+observed motion would only suffice to carry it through a space equal to
+the moon’s apparent diameter in about 266 years. Assuming a parallax of
+about one-sixth of a second found by Kapteyn, this apparent motion would
+indicate a real motion of about 128 miles a second at right angles to
+the line of sight. As, however, there may be also motion _in_ the line
+of sight, the above velocity would be a minimum—if the parallax can be
+relied upon—and the actual motion may be considerably more. From its
+rapidity, 1830 Groombridge has been called by Prof. Newcomb “the runaway
+star.”
+
+Next in order of rapidity of motion comes the southern star known as
+Lacaille 9352, which lies in the constellation Piscis Australis, a
+little south of Fomalhaut. This seventh magnitude star has an apparent
+motion of 6·9 seconds, which, with a parallax of 0·285″ found by Gill,
+indicates a velocity of 71 miles per second. Next comes 61 Cygni, with a
+velocity of 30 miles, and Epsilon Indi—another southern star—with a
+velocity of nearly 68 miles a second. These velocities are, however,
+exceeded by other stars if the measured parallaxes are correct. Thus the
+star Mu Cassiopeiæ, with a proper motion of 3·7 seconds, has, according
+to Pritchard’s photographic measures, a parallax of only 0·036″, which
+would indicate a velocity of no less than 302 miles a second! and the
+small parallax found by Elkin for Arcturus would imply the startling
+velocity of 376 miles a second!
+
+It is a remarkable fact that the eight stars with the largest proper
+motions are all below the fourth magnitude in brightness, and as a large
+proper motion probably indicates proximity to the earth, the conclusion
+seems evident that the brightest stars are not as a rule the nearest. Of
+twenty-five stars, with proper motions greater than two seconds of arc,
+there are only two—Arcturus and Alpha Centauri—whose magnitude exceeds
+the third. Indeed, more than half the stars with motions greater than
+one second are invisible to the naked eye!
+
+Many stars have proper motions of less than a second of arc per annum.
+Very small proper motions have also been detected, which only reveal
+themselves after the lapse of a great number of years, and it seems
+probable that there are no really “fixed stars” in the heavens. For
+stars of the sixth magnitude, M. Ludwig Struve finds an average motion
+of only eight seconds in a hundred years, or about one-twelfth of a
+second per annum. If we assume that stars of the sixth magnitude are, on
+the average, of the same size and brightness as stars of the first
+magnitude, their distance from the earth would be ten times greater.
+Consequently, stars of the first magnitude should have an average proper
+motion of about eighty seconds in one hundred years. This, however, is
+not the case. The twenty brightest stars show an average motion of only
+sixty seconds in a hundred years. And the motion of stars of the second
+magnitude is relatively still slower. Instead of an average motion of
+fifty seconds in a hundred years—which they should have if the
+brightness were inversely proportional to the distance—it has been found
+that twenty-two stars of the second magnitude show an average motion of
+only seventeen seconds. This result seems to show that the brighter
+stars are not so near us as their brilliancy would lead us to suppose, a
+conclusion which has been already proved by actual measures of their
+distance.
+
+From a consideration of the results found for stellar parallax, Mr.
+Thomas Lewis, F.R.A.S., of the Greenwich Observatory, comes to the
+following conclusions[108]:—
+
+
+“(1) Leaving out a few of the brightest stars, the parallaxes are
+constant down to 2·70 magnitude.
+
+“(2) After 2·70 mag. is reached, the parallaxes are doubled, and remain
+practically constant to 8·40 mag.
+
+“(3) Up to the 3rd mag. the velocities are very small, averaging about 9
+miles per second, while after the 3rd mag. the velocity is 38 miles per
+second.
+
+“Hence we may fairly deduce—
+
+“(1) That there are a few stars (about 8) of exceptional brilliancy in
+our immediate neighbourhood, and scattered about amongst these a number
+of small stars (at present about 40 are known).
+
+“(2) Stars of mag. 1·0 to 3·0 are, as a class, far outside this inner
+space, and have very small velocities.
+
+“(3) The small stars here dealt with have apparently large velocities
+across the line of sight.
+
+“These results show that the generally received idea that parallaxes are
+to be sought for in stars with large proper motion is correct, and we
+may add that this holds good, no matter what may be the star’s
+magnitude.”
+
+
+The “proper motion” of a star only indicates its motion at right angles
+to the line of sight—that is, its motion on the surface of the celestial
+vault—and gives us no information as to whether the star is approaching
+to or receding from the earth. This motion “in the line of sight” cannot
+be detected by micrometrical measures with an ordinary telescope, and
+would probably have remained for ever unknown had the spectroscope not
+been invented. Dr. Huggins was the first to show that motions in the
+line of sight could be determined by measuring the displacement of the
+spectral lines caused by the approach or recession of the source of
+light, the lines being slightly shifted towards the blue end of the
+spectrum when the star is approaching the earth, and towards the red end
+when it is receding from us. The effect would, of course, be exactly the
+same if the star were at rest and the earth in motion. By carefully
+measuring this observed displacement of the spectral lines, the velocity
+in the line of sight can be easily computed. Dr. Huggins’ observations
+were fully confirmed by Dr. Vogel.
+
+The earlier determinations of motion in the line of sight were made by
+eye measurements with a micrometer, and owing to the difficulty and
+delicacy of these measures, the results were very discordant. The method
+has recently been much improved by photographing the spectra and
+measuring the positions of the lines on the photograph. Both methods
+agree in showing that the following stars, among others, are certainly
+_approaching_ the earth: Arcturus, Vega, Procyon, Pollux, Altair, Spica,
+Alpha Cephei, Alpha Persei, Alpha Arietis, 61 Cygni, and the Pole Star;
+and the following are certainly _receding_: Capella, Rigel, Betelgeuse,
+Aldebaran, and Regulus.
+
+Measures of photographic stellar spectra have yielded much more accurate
+results than the old method. Some of the velocities found in this way by
+Dr. Vogel—who has given especial attention to this subject—are very
+considerable. For the bright star Rigel he finds a velocity of recession
+of about 39 miles a second, for Aldebaran 30 miles, and for Capella 15
+miles. He finds that the Pole Star is approaching the earth at the rate
+of 16 miles a second, and Procyon about 7 miles.
+
+Dr. Bélopolsky has recently investigated the _absolute_ velocity in
+space of the brighter component of 61 Cygni—that is, the motion across
+the line of sight combined with the motion _in_ the line of sight.
+Assuming a parallax of half a second and a proper motion of 5·2 seconds,
+he finds that the motion across the line of sight, corrected for the
+sun’s motion in space, is about 22½ miles per second. The motion _in_
+the line of sight, also corrected for the sun’s motion, he finds, from
+photographs taken at Pulkova, to be about 27 miles a second towards the
+earth. Combining these motions, he finds the absolute velocity of the
+star in space to be about 35 miles a second, or nearly double the
+velocity of the earth in its orbit
+
+This method of measuring velocities in the line of sight has also been
+applied to the nebulæ. Mr. Keeler has observed and measured a
+displacement of the line known as the chief nebular line in several
+planetary nebulæ, and finds considerable motion in the line of sight.
+For example, in the nebula numbered 6790 in the “New General Catalogue,”
+he finds a motion of recession of about 38 miles a second. Some of these
+motions may possibly be due, in part at least, to the sun’s motion in
+space, carrying the earth with it, a motion which will now be
+considered. The method has also led to the discovery of the so-called
+“spectroscopic binary stars,” a most interesting class of objects, which
+will be considered in the next chapter.
+
+The proper motions of the stars long since suggested the idea that
+possibly the observed motion may be—to some extent, at least—merely
+apparent, and due to the real motion of the sun and solar system through
+space. The first investigation of this interesting question was made by
+Sir William Herschel in 1783, and he came to the conclusion that the sun
+is moving towards a point near Lambda Herculis, a result not differing
+widely from modern determinations. The reality of Herschel’s result has
+been fully confirmed by subsequent investigations, and Argelander placed
+it beyond doubt by a comparison of the positions of a large number of
+stars determined at Abo with those found by Bradley in 1752. The
+accuracy of Argelander’s result was confirmed by Otto Struve. According
+to the elder Struve, the results arrived at by Argelander, O. Struve,
+and Peters, is to place the point towards which the sun is moving,
+between the stars Pi and Mu Herculis, “at a quarter of the apparent
+distance of these stars from Pi Herculis,” and they estimated the annual
+motion at about 33½ million miles geographical. The general accuracy of
+this conclusion has been verified by modern researches, although the
+results found by different astronomers vary to some extent. The
+accompanying diagram shows some of the different positions found by
+various computers. The later determinations seem to place the “apex of
+the solar motion,” as it is termed, not far from the bright star Vega,
+or further to the east than Herschel placed it. The velocity of the
+sun’s motion in space has not been so well determined as its direction.
+L. Struve’s computations would indicate a velocity of about 14 miles a
+second; but other results give a much smaller velocity.
+
+[Illustration:
+
+  FIG. 3.—_Diagram showing “Solar Apex,” and the different Positions
+    found by various Computers._
+
+  (From “Visible Universe.”)
+]
+
+From a recent investigation of the nature of the sun’s motion in space
+by Mr. G. C. Bompas,[109] he considers that the various positions of the
+sun’s “apex” show a tendency to a drift along the edge of the Milky Way,
+and that this drift “seems to point to a plane of motion of the sun
+nearly coinciding with the plane of the Milky Way, or, perhaps, more
+nearly with the plane of that great circle of bright stars first
+described by Sir Wm. Herschel as inclined about 20° to the galaxy, and
+which passes through Lyra, in or near which constellation the solar apex
+lies,” and he concludes, from the motion of the nearer stars, “that the
+sun moves in a retrograde orbit from east to west, and in a plane
+inclined a few degrees to that of the Milky Way.” With reference to this
+very interesting conclusion, which may, perhaps, be confirmed by further
+observations, Mr. Bompas quotes the following passage from “The Visible
+Universe,” p. 197, by the present writer:—“With reference to a possible
+motion of the stars in some general system, M. Rancken has found, from
+an examination of 106 stars, a tendency to drift along the course of the
+Milky Way from Aquila towards Cygnus and Cassiopeia, and past Capella
+through Orion to Argo. The _larger_ motions, shown in Proctor’s map of
+‘proper motions,’ exhibit this tendency in a marked degree between
+Cygnus and Capella, and less clearly on the Sirius, but the smaller
+motions not so well,” and Mr. Bompas points out that this apparent drift
+of the stars in the Milky Way, from west to east, “is just such as would
+be occasioned by a real motion of the sun in that plane, in a contrary
+direction from east to west.”
+
+
+
+
+                              CHAPTER IV.
+                             BINARY STARS.
+
+
+Double and multiple stars may be either optical or real. Optical double
+stars are those in which the component stars are merely apparently close
+together, owing to their being seen in nearly the same direction in
+space. Two stars may _seem_ to be close together, while, in reality, one
+of them may be placed at an immense distance behind the other. Just as
+two lighthouses at sea may, on a dark night, appear close together when
+viewed from a certain point, whereas they may be really miles apart. In
+the case of double stars it is, of course, always difficult to determine
+whether the apparent closeness of the stars is real or merely optical.
+But when, from a long series of observations of their relative position,
+we find that one is apparently moving round the other, we know that the
+stars must be comparatively close, and linked together by some physical
+bond of union. These most interesting objects are known to astronomers
+as binary or revolving double stars. The probable existence of such
+objects was predicted from abstract reasoning by Mitchell in the
+eighteenth century; but the discovery of their actual existence was made
+by Sir William Herschel, while engaged on an attempt to determine the
+distance of some of the double stars from the earth. “Instead of
+finding, as he expected, that annual fluctuation to and fro of one
+component of a double star with respect to the other—that alternate
+increase and decrease of their distance and angle of position, which the
+parallax of the earth’s annual motion would produce—he observed, in many
+cases, a regular progressive change; in some cases bearing chiefly on
+their distance, in others on their position, and advancing steadily in
+one direction, so as clearly to indicate a real motion of the stars
+themselves,” and measurements made during the subsequent 25 years fully
+proved the truth of the illustrious astronomer’s discovery. It was found
+that in many double stars an orbital motion round each other was evident
+after a number of years of careful observation of their relative
+positions. Unlike the planetary orbits, which are nearly circular, at
+least those of the larger planets of the solar system, it was found that
+the orbits of these double stars differ, in many cases, widely from the
+circular form, in some cases, indeed, approaching in shape more the
+orbit of a comet than a planet.
+
+The binary stars are among the most interesting objects in the heavens.
+The number now known probably amounts to nearly one thousand. In most of
+them, however, the motion is very slow, and in only about seventy cases
+has the change of position, since their discovery, been sufficient to
+enable an orbit to be computed. In most cases the plane of the real
+orbit, or ellipse, described by the companion round the principal star,
+is inclined to the line of sight. We therefore see the orbit
+foreshortened into a more elongated ellipse.
+
+The relation of the apparent ellipse—or the ellipse we see described by
+one star round the other—to the real ellipse will be easily understood
+by the following illustration. Suppose a cylinder or rod of an
+elliptical, not circular, section to be cut across obliquely to its
+axis. This oblique section will represent the _real_ orbit of a binary
+star, and the section at right angles to the axis, the _apparent_ orbit.
+The angle between these two sections will represent the inclination of
+the real orbit to the plane of projection, or background of the sky. In
+the apparent orbit, the primary star, which is assumed to be situated in
+one of the foci of the real ellipse, does not lie in the focus of the
+apparent ellipse, and from its observed position in this latter ellipse
+we can deduce, mathematically, the particular angle at which the oblique
+section must be made to agree with the observed place of the primary
+star, and other details respecting the real ellipse.
+
+Savary, in 1830, was the first astronomer who attempted to compute the
+orbit of a binary star, namely, the star Xi Ursæ Majoris. This
+remarkable pair was discovered by Sir William Herschel in 1780, and as
+the period of revolution is about 61 years, a considerable portion of
+the ellipse had been described in 1830, when it was attacked by Savary.
+Since that year, orbits have been computed for a number of binary stars
+by several computers, among whom may be mentioned Sir R. Ball, Behrmann,
+Casey, Celoria, Doberck, Dunér, Elkin, Fritsche, Glasenapp, Sir J.
+Herschel, Hind, Jacob, Mädler, Mann, Schur, See, Thiele, Villarceau, and
+the present writer. The computation of a double star orbit is a matter
+of considerable trouble and difficulty, and cannot be described here. An
+account of the principal results arrived at by astronomers in this
+interesting branch of sidereal astronomy may, however, prove of interest
+to the general reader.
+
+We will first consider the binary stars with short periods of
+revolution, which are, of course, the most interesting, and those whose
+orbits can be computed with greater accuracy than binaries having
+periods of considerable length. The binary star with the shortest period
+known at present seems to be the fourth magnitude star Kappa Pegasi. It
+was discovered as a wide double star by Sir William Herschel in 1786,
+the companion star being of the ninth magnitude. In August, 1880, Mr.
+Burnham, the famous American double star observer, examining the star
+with the 18½ inch refractor of the Dearborn Observatory, found the
+brighter star to be a very close double, with a distance between the
+components of only a quarter of a second of arc. A few years’
+observations showed that this pair were in rapid motion round each
+other, and from measures up to the year 1892, Burnham finds a period of
+11·37 years. A later determination by Dr. See makes the period 11·42
+years, so that we may conclude that the orbit is now pretty accurately
+determined. The plane of the orbit is highly inclined to the line of
+sight. Dr. See makes the inclination 81°.
+
+Another binary star, with a period of about the same length, is Delta
+Equulei, which was discovered to be a close double by Otto Struve in
+1851. As in the case of Kappa Pegasi, the orbit is highly inclined to
+the line of sight. In the year 1887, Wrublewsky, the Russian computer,
+found a period of about 11½ years, with an orbit nearly circular. A new
+orbit was published in 1895 by Dr. See, who finds a period of 11·45
+years, and an orbit agreeing fairly well with that of Wrublewsky, the
+orbit differing little from the circular form, and inclined to the line
+of sight at the high angle of 79 degrees. Burnham found only a “slight
+elongation” in the star with the great 36-inch telescope of the Lick
+Observatory in July, 1889. The distance between the components does not
+at any time exceed half a second of arc, so that it is always beyond the
+reach of all but the largest telescopes.
+
+Next in order of shortness of period comes the southern binary star Zeta
+Sagittarii, for which an orbit was first computed in the year 1886 by
+the present writer, who found a period of 18·69 years. The orbit was
+re-computed in 1893, with the aid of recent measures by Mr. J. W.
+Froley, who finds a period of 17·71 years. The orbit of this star will,
+I think, require still further revision, but the period of about 18
+years is probably not far from the truth.
+
+Another remarkably rapid binary star is 85 Pegasi, for which Schaeberle
+computed a period of 22·3 years, but a later orbit by Prof. Glasenapp
+makes the period 17½ years, and Burnham thinks it will certainly be less
+than 20 years. Dr. See, however, finds a period of 24 years. The primary
+star is about the sixth magnitude, and the companion only the eleventh,
+a difference of five magnitudes, which implies that the larger star is
+one hundred times brighter than the companion.
+
+Next in order of rapidity of motion we have the southern binary star 9
+Argûs. For this pair, Burnham finds a period of 23·3 years, and Dr. See
+22 years, the other elements of the orbit being also in close agreement.
+In this case also the orbit plane is highly inclined to the line of
+sight.
+
+The star 42 Comæ Berenices has a period of about 25¾ years, according to
+Otto Struve. The orbit is remarkable from the fact that its plane passes
+through or nearly through the earth, and is, therefore, projected into a
+straight line, the companion star oscillating backwards and forwards on
+each side of its primary. I find that the plane of the orbit is at right
+angles to the general plane of the Milky Way.
+
+The star Beta Delphini—the most southern of the four stars in the
+“Dolphin’s Rhomb”—is also a fast-moving binary, discovered by Burnham in
+1873, for which periods have been computed of 22·97 years by Glasenapp,
+26·07 years by Dubjago, 27·66 years by Dr. See, and 30·91 years by the
+present writer. Burnham thinks the period will prove to be about 28
+years. The spectrum of the light of Beta Delphini is similar to that of
+our sun, so that the two bodies should be comparable in intrinsic
+brilliancy. From my orbit of the pair, the “hypothetical parallax” is
+0·052″—that is, this is the parallax the star would have on the
+supposition that the combined mass of its components is equal to the
+mass of the sun. Now, assuming the value of the sun’s stellar magnitude
+which I have recently computed (_Knowledge_, June, 1895)—namely, 27·15—I
+find that the sun, if placed at the distance indicated for Beta
+Delphini, would be reduced to a star of 5·84 magnitude. As the star was
+measured 3·74 at Harvard, we have a difference of 2·1 magnitude,
+denoting that the binary—if of the same mass as the sun—must be about
+seven times brighter. As the spectrum is of the same type, this seems
+improbable, and we must conclude that the star’s parallax is more than
+0·052″.
+
+Another remarkable binary star with a comparatively short period is Zeta
+Herculis. This pair have now performed three complete revolutions since
+their discovery in 1782 by Sir William Herschel. Several orbits have
+been computed, but Dr. See’s period of 35 years is probably the best The
+companion is now not far from its maximum distance (1½ seconds) from the
+primary star, and is within the reach of moderate-sized telescopes. The
+companion is, however, rather faint, being only 6½ magnitude, while the
+primary star is of the third. When at their nearest, some observers have
+spoken of an “occultation” of one star by the other, but no real
+occultation ever takes place, the components never approaching within
+half a second of arc. The companion merely disappears owing to its
+faintness in telescopes of moderate power. An occultation of one
+component of a binary star by the other cannot take place except—as in
+the case of 42 Comæ—when the plane of the orbit passes through the
+earth.
+
+[Illustration:
+
+  FIG. 4.—_Apparent Orbit of Zeta Herculis._ (From “Worlds of Space.”)
+]
+
+In the case of the binary star, Eta Coronæ Borealis, it was, some forty
+years ago, uncertain whether its period was 43 or 66 years, but now that
+two complete revolutions have been performed since its discovery by Sir
+William Herschel in 1781, the question has been finally decided in
+favour of the shorter period. Numerous orbits have been computed, but
+these by Dr. Doberck and Dr. Dunér are probably the best. Those give a
+period of about 41½ years. The components are nearly equal in
+brightness, but at their present distance are not within the reach of
+small telescopes.
+
+The brilliant star Sirius is also an interesting binary star. The
+companion, which is relatively very faint—about tenth magnitude,—was
+discovered by Alvan Clark in 1862. The existence of some such disturbing
+body was previously suspected by astronomers, owing to observed
+irregularities in the proper motion of Sirius. Several orbits, giving
+periods of about 50 years, have been computed. Some measures in recent
+years, however, seemed to show that this period was somewhat too short,
+but a period of about 58½ years, computed by the present writer in 1889,
+will probably prove too long. Some few years ago, Burnham found the
+companion an easy object with the 36-inch refractor of the Lick
+Observatory, but towards the end of the year 1890 it passed beyond the
+power of even this giant telescope. It will probably, however, emerge
+very soon now from the rays of its brilliant primary.[110] Burnham finds
+a period of about 52 years, but the German astronomer, Auwers, who has
+carefully investigated the observed irregularities in the proper motion
+of Sirius, adheres to a period of about 49½ years. The great brilliancy
+of Sirius, the brightest star in the heavens, naturally suggests a sun
+of great size. Recent investigations, however, do not favour this idea.
+Assuming a parallax of 0″·39 (about a mean of the results found by Elkin
+and Gill), Auwers finds the mass of the system to be about three times
+the mass of the sun, the mass of the companion being about equal to the
+sun’s mass. Placed at the distance of Sirius, the sun would, I find, be
+reduced to a star of about 1½ magnitude. As Sirius is about 1 magnitude
+brighter than the zero magnitude—that is, about 2 magnitudes brighter
+than a standard star of the first magnitude—it follows that it is about
+2½ magnitudes, or about ten times brighter than the sun would be in the
+same position. Its spectrum is, however, of the first type, and the star
+is therefore not comparable with the sun in brilliancy. The above result
+would indicate that stars of the first or Sirian type are intrinsically
+brighter than our sun.
+
+Sirius is about 11 magnitudes brighter than its faint companion. This
+makes the light of Sirius about 25,000 times the light of the small
+star. If, therefore, the two bodies were of the same intrinsic
+brilliancy, their diameters would be in the ratio of 158·5 to 1, and if
+of the same density, the mass of Sirius would be nearly five million
+times the mass of the companion! But, according to Auwers’ calculations,
+the companion’s mass is about one-half that of its primary. The two
+bodies must, therefore, be differently constituted, and, indeed, the
+companion must be nearly a dark body. It has been suggested that the
+companion may possibly shine by reflected light from Sirius; but this I
+have shown elsewhere to be quite impossible.[111] Even with a diameter
+equal to that of the sun, I find that with reflected light only it would
+be quite invisible in all parts of its orbit, even with the great Lick
+telescope. It must, therefore, shine with inherent light of its own, and
+it seems probable that it is a large body, cooling down and approaching
+the complete extinction of its light. If Sirius has any planets
+revolving round it—like those of our solar system—they must for ever
+remain invisible in our largest telescopes. This remark, of course,
+applies to all the fixed stars, single and double. They may possibly
+have attendant families of planets, like our sun, but if so, the fact
+can never be ascertained by direct observation. I find that the plane of
+the orbit of Sirius is at right angles to the general plane of the Milky
+Way.
+
+[Illustration:
+
+  FIG. 5.—_Apparent Orbit of the Companion of Sirius._
+
+  (From “Old and New Astronomy.”)
+]
+
+The star Zeta Cancri is a well-known triple star, the close pair
+revolving in a period of about 60 years. Nearly two revolutions have now
+been completed since its discovery by Sir William Herschel in 1781. All
+three stars probably form a connected system, but the motion of the
+third star round the binary pair is very slow and irregular. The motion
+of this interesting system has recently been investigated by Professor
+Seeliger, and he comes to the conclusion that, to make the observations
+agree with calculation, it is necessary to assume that the third star is
+in reality a very close double, the components of which revolve round
+their centre of gravity in about 17½ years, and both round the known
+binary pair. If this be so, we have here a remarkable quadruple pair;
+but it must be added that all efforts with large telescopes to see the
+companion star double have failed, and that the existence of the fourth
+star rests only on theory. Burnham, in 1889, using a power of 1500,
+failed to see any other component.
+
+Another interesting binary star is Xi Ursæ Majoris. As already stated,
+this was the first pair for which an orbit was computed. More than a
+complete revolution has now been performed since its discovery by Sir
+William Herschel in 1780. The period has, therefore, been well
+determined, and seems to be about 60 years. Although the components are
+not near their maximum distance at present, they are still within the
+reach of moderate telescopes, the distance being about 1¾ seconds, and
+the magnitudes of the components, not very unequal, about 4 and 5.
+
+The bright southern star, Alpha Centauri, the nearest of all the fixed
+stars to the earth, so far as is known at present, is also a remarkable
+binary star. It seems to have been first noticed as a double star by
+Richaud in 1690. Several orbits have been computed, ranging from about
+75 to 88½ years, but recent calculations by Mr. A. W. Roberts and Dr.
+See make the period about 81 years, which agrees closely with Dr.
+Elkin’s period of 80⅓ years. Combining Dr. Gill’s parallax of 0″·76 with
+Elkin’s elements, I find the sum of the masses nearly twice the mass of
+our sun, and the mean distance between the components about 23 times the
+earth’s distance from the sun, or somewhat greater than the distance
+between the sun and Uranus. Dr. Doberck finds a period of about 79
+years, and assuming a parallax of 0″·75, he finds the mean distance
+between the components 24·6 times the earth’s distance from the sun; and
+he points out that if we suppose that their diameter does not differ
+much from that of our sun, each component “would appear from the other
+as a mere star to unaided vision, the distance being too great to show a
+disc.”[112] From a recent investigation of the proper motion and
+position of Alpha Centauri, Mr. A. W. Roberts finds that the masses of
+the components are nearly equal, and the combined mass equal to twice
+the mass of our sun, a conclusion in close agreement with the result
+found above from the orbit. According to Dr. Gill, the difference in
+brightness of the two components is 1·25 magnitude, and Professor Bailey
+makes their photometric magnitudes 0·50 and 1·75. As this difference
+would make the brighter component over three times brighter than the
+companion, it follows that its surface must be much brighter, and Mr.
+Roberts concludes that the companion has proceeded “some distance on the
+down track from a sun to an ordinary planet.” Assuming my value of the
+sun’s stellar magnitude (about 27), I find that the sun, if placed at
+the distance of Alpha Centauri, would appear of about the same
+brightness as the star does to us. As, according to Professor Pickering,
+the spectrum of Alpha Centauri is of the second or solar type, it would
+seem that in mass, brightness, and physical condition, the star closely
+resembles our sun.
+
+We next come to another very interesting binary star, known to
+astronomers as 70 Ophiuchi. It is a very fine double star, the
+magnitudes of the components being about 4 and 6, and the colours yellow
+and orange. More than a complete revolution has now been described by
+the components since its discovery by Sir William Herschel in 1779.
+Numerous orbits have been computed with periods ranging from 73¾ to 98
+years. An orbit computed by the present writer, in 1888, gave a period
+of 87·84 years, and this was confirmed in 1894 by Burnham, who found a
+period of 87·85 years. A subsequent investigation by Schur gives a
+period of 88·356 years. My orbit, combined with Krüger’s parallax of
+0″·162, give for the combined mass of the components 2·777 times the
+mass of the sun, and the distance between them 27·777 times the earth’s
+distance from the sun, or somewhat less than the distance of Neptune
+from the sun. Schur has, however, recently found a parallax of 0″·286,
+which would reduce the mass of the system, and also the distance between
+the components. Recent observations show that the companion is now in
+advance of the theoretical position indicated by Schur’s orbit, and Dr.
+See thinks that the observed irregularities in the orbital motion of the
+pair indicate the existence of a third body, and that either the primary
+star or the companion, probably the latter, is a very close binary star.
+Careful search, however, for a third body, made with large telescopes,
+have failed to reveal its existence, and so the matter remains in
+suspense. Placed at the distance indicated by Krüger’s parallax, I find
+that our sun would be reduced to a star of about magnitude 3½, which
+shows that the sun and star are of about equal brightness. The spectrum
+is of the solar type, according to Vogel. I find that the plane of the
+orbit is at right angles to the plane of the Milky Way.
+
+The star Gamma, in Corona Borealis, is a close and difficult binary
+star. Dr. Doberck finds a period of 95½ years, and Celoria about 85¼. As
+in the case of 42 Comæ, the plane of the orbit nearly passes through the
+earth, and the apparent orbit is, consequently, nearly a straight line.
+I find that the plane of the orbit is at right angles to the plane of
+the Milky Way.
+
+The star Xi Scorpii is a remarkable triple star, like Zeta Cancri, the
+magnitudes of the components being about 4½, 5, and 7½. The components
+of the close pair have described a complete revolution since their
+discovery by Sir William Herschel in 1780. Dr. Doberck finds a period of
+about 96 years, and Schorr 105 years. The real orbit is nearly circular,
+but owing to its high inclination, about 70°, the apparent orbit is a
+very elongated ellipse. All three stars have a common proper motion
+through space, and, probably, form one system, but the motion of the
+third star is very slow, and its period of revolution must be several
+hundred years.
+
+[Illustration:
+
+  APPARENT ORBIT OF 70 OPHIUCHI, COMPUTED BY J. E. GORE (1888).
+
+  (_Showing positions of companion star in
+  different years._)
+
+  (From “The Scenery of the Heavens.”)
+]
+
+The star ο^2, or 40 Eridani, is another interesting object. It is a star
+of about 4½ magnitude, with a distant ninth magnitude companion, which
+is a double and binary star. It is sometimes stated that the bright star
+is the binary, but this is quite incorrect; the large star is single—at
+least, as far as is known at present. An orbit for the binary pair was
+computed, in 1886, by the present writer, who found a period of 139
+years; but Burnham, using later observations, finds a period of 180
+years. A physical connexion may possibly exist between the binary pair
+and the bright star, as both have the same common motion through space,
+but the angular motion, if any, is very slow. Professor Asaph Hall found
+a parallax of about one-fifth of a second of arc, and this, combined
+with Burnham’s orbit, gives the combined mass of the binary pair about
+two-thirds of the sun’s mass, a result which seems remarkable, for the
+sun, placed at the distance indicated by Hall’s parallax would, I find,
+shine as a star of about the third magnitude, or considerably brighter
+than the principal star of 40 Eridani. Owing to the faintness of the
+binary pair, the nature of its spectrum has not been determined.
+Computed by a well-known formula, its “relative brightness”—that is, its
+brightness compared with that of other binaries—is very small.
+
+A very famous binary star is that known to astronomers as Gamma
+Virginis. Its history is a very interesting one. It lies close to the
+celestial equator, about one degree to the south and about fifteen
+degrees to the north-west of the bright star Spica (Alpha of the same
+constellation), with which it forms the stem of a Y-shaped figure,
+formed by the brightest stars of the constellation Virgo, or the Virgin,
+Gamma being at the junction of the two upper branches. The brightness of
+Gamma Virginis is a little greater than an average star of the third
+magnitude. Photometric measures made at Oxford and Harvard Observatories
+agree closely, and make its brightness about 2·7 magnitude—that is to
+say, rather nearer the third than the second magnitude. Variation of
+light has, however, been suspected in one or both components, and this
+question of light variation will be considered further on. The Persian
+astronomer, Al-Sûfi, in his description of the heavens, written in the
+tenth century, rates it of the third magnitude, and describes it as “the
+third of the stars of _al-auvâ_, which is a mansion of the moon,” the
+first and second stars of this “mansion” being Beta and Eta Virginis,
+the fourth star Delta, and the fifth Epsilon, these five stars forming
+the two upper branches of the Y-shaped figure above referred to. Gamma
+was called _Zawiyah-al-auvâ_, “the corner of the barkers!” perhaps from
+its position in the figure, which formed the thirteenth Lunar Mansion of
+the old astrologers. It was also called _Porrima_ and _Postvarta_ in the
+old calendars. These ancient names of the stars are curious, and their
+origin doubtful.
+
+The fact that Gamma Virginis really consists of two stars very close
+together seems to have been discovered by the famous astronomer,
+Bradley, in 1718. He recorded the position of the components by stating
+that the line joining them was then exactly parallel to a line joining
+Alpha and Delta of the same constellation. This was, of course, only a
+rough method of measurement, and the position thus found by Bradley
+being probably more or less erroneous, has given much trouble to
+computers of the orbit described by the component stars round each
+other, or, rather, round their common centre of gravity. Bradley does
+not give the apparent distance between the component stars; but we may
+conclude from the orbit, which is now well determined, that they were
+then at nearly their greatest possible distance apart. It is curious
+that between Bradley’s time and 1794, the star was on several occasions
+occulted by the moon; but none of the observers refer to its duplicity.
+It was again measured by Cassini in 1720, by Tobias Mayer in 1756, and
+by Sir William Herschel in 1780. These measures showed that the distance
+between the components was steadily diminishing, and that the position
+angle of the two stars was also decreasing. This decrease in the
+position angle—measured from the north round by the east, south, and
+west, from 0 to 360°—shows that the apparent orbital motion is what is
+called retrograde, or in the direction of the hands of a clock, direct
+or “planetary motion” being in the opposite direction. The star was
+again measured by Sir John Herschel and South in the years 1822–38, by
+Struve in the same years, and by Dawes and other observers from 1831 to
+the present time. The recorded measures are very numerous, and have
+enabled computers to determine the orbit with considerable accuracy. The
+rapid decrease in the apparent distance from 1780–1834 indicated that
+the apparent orbit is very elongated, and that possibly the two stars
+might “close up” altogether, and appear as a single star even in
+telescopes of considerable power. This actually occurred in the year
+1836, or, at least, the stars were then so close together that the most
+powerful telescopes of that day failed to show Gamma Virginis as
+anything but a single star. Of course, it would not have been beyond the
+reach of the giant telescopes of our day. From the year 1836 the pair
+began to open out again, and at present the distance is again
+approaching a maximum. It is now within the reach of small telescopes,
+and forms a fine telescopic object with a moderate-sized instrument.
+
+The general character of the orbital motion may be described as
+follows:—In 1718, at the time of Bradley’s observation, the companion
+star was to the north-west of the primary star; it then gradually moved
+towards the west and south, and in 1836, when at its minimum distance,
+it was to the south-east. From that date it again turned towards the
+north, and at present it is north-west of the primary star, and not far
+from the position found by Bradley in 1718.
+
+The first to attempt a calculation of the orbit described by this
+remarkable pair of suns was Sir John Herschel, who in the year 1831
+found a period of about 513 years. In 1833, he re-calculated the orbit,
+and found nearly 629 years. We now know that both these periods are much
+too long; but the data then available were insufficient for the
+calculation of an accurate orbit. From these results Herschel predicted
+that “the latter end of the year 1833, or the beginning of the year
+1834, will witness one of the most striking phenomena which sidereal
+astronomy has yet afforded, _viz._, the perihelion passage of one star
+round another, with the immense angular velocity of between 60° and 70°
+per annum, that is to say, of a degree in five days. As the two stars
+will then, however, be within little more than half a second of each
+other, and as they are both large and nearly equal, none but the very
+finest telescopes will have any chance of showing this magnificent
+phenomenon. The prospect, however, of witnessing a visible and
+measurable change in the state of an object so remote, in a time so
+short, may reasonably be expected to call into action the most powerful
+instrumental means which can be brought to bear on it.” This prediction
+was not verified until the year 1836, when the pair “closed up out of
+all telescopic reach,” except at the Dorpat Observatory, where a
+magnifying power of 848 still showed an elongation in the telescopic
+disc of the star. The orbit found by Sir John Herschel was a tolerably
+elongated ellipse, with its longer axis lying north-east and south-west.
+This was not quite correct, for we now know that this axis lies
+north-west and south-east, and that the apparent orbit is much more
+elongated than Sir John Herschel at first supposed. This was soon
+recognised by Herschel himself, and he came to the conclusion that he
+and other computers had been misled by Bradley’s observation in 1718. He
+then rejected this early, and evidently faulty, observation, and using
+the measures up to 1845, he found a period of about 182 years, which we
+now know to be near the truth. The orbit was also computed by the famous
+German astronomer, Mädler, who found periods of 145, 157, and 169 years;
+by Hind, 141 years; by Henderson, 143 years; by Jacob, 133½, 157½ and
+171 years; by Adams, 174 years; by Flammarion, 175 years; and by Admiral
+Smyth, 148 and 178 years. All these periods, we now know, are too small.
+Fletcher found 184½ years, and Thiele 185 years. Two orbits were
+computed by Dr. Doberck, in recent years, with periods of 180½ and 179½
+years; but very recently (1895) the orbit has been re-computed by Dr.
+See, and he finds a period of 194 years. A comparison of the observed
+and computed positions shows, he thinks, that his elements are the most
+exact yet determined for any binary star.
+
+The apparent orbit of the pair is a very elongated ellipse, and as
+Admiral Smyth said, “more like a comet’s than a planet’s.” The real
+ellipse has a very high eccentricity, nearly 0·9—indeed, the greatest of
+all the known binary stars, and not much less than that of Halley’s
+comet
+
+As I said above, the variability of the light of one or both components
+of Gamma Virginis has been strongly suspected. So far back as 1851 and
+1852, O. Struve paid particular attention to this point. His
+observations in these years show that sometimes the component stars were
+exactly equal in brilliancy, and sometimes the southern star—the one
+generally taken as the primary—was from 0·2 to 0·7 magnitude brighter
+than the other. There seems to be little doubt that some variation
+really takes place in the relative brightness of the pair. This is
+clearly indicated by the measures of position angle. For example, in the
+year 1886, Professor Hall recorded the position as 154·9, evidently
+measuring from the northern star as the brightest of the two; while, in
+1887, Schiaparelli gives 334°·2—or about 180° more—thus indicating that
+he considered the _southern_ star as the primary, or brighter, of the
+pair. Burnham found 153°·4 in 1889, and Dr. See 332°·50 in 1891. This is
+also shown by earlier measures, for Otto Struve found the southern star
+half a magnitude brighter than the other on April 3, 1852, while on
+April 29 of the same year he found them “perfectly equal.” He thought
+the variation was about 0·7 of a magnitude, but that the climate of
+Poulkova, where he observed, was not suitable for such observations.
+This variation is very interesting, and the question should be
+thoroughly investigated with a good telescope.
+
+As the distance of Gamma Virginis from the earth has not been
+determined, it is not possible to calculate the actual dimensions of the
+orbit and the mass of the system. If we assume that the combined mass of
+the components is equal to the sun’s mass, I find from Dr. See’s orbit
+that the “hypothetical parallax” would be 0·119″, implying a distance of
+1,733,319 times the sun’s distance from the earth. If, however, we
+suppose that the mass of each of the components is equal to the sun’s
+mass, or the mass of the system double that of the sun—perhaps a more
+probable supposition—I find that the parallax would be about one-tenth
+of a second, denoting a distance of 2,062,650 times the sun’s distance
+from the earth. Placed at this last distance, the sun would, I find, be
+reduced to a star of about 4½ magnitude, or about 1¾ magnitudes fainter
+than Gamma Virginis appears to us. This difference implies that,
+supposing each of the component stars of the binary to have a mass equal
+to the sun’s mass, their combined light is about five times greater than
+the sun would emit if placed at the same distance, and as the components
+are nearly equal in brightness, each of them would be 2½ times brighter
+than the sun. According to Vogel, the star’s light gives a spectrum of
+the first or Sirian type, but according to the Draper “Catalogue of
+Stellar Spectra,” the spectrum is of the solar type. If the spectrum is
+of the first type, its brilliancy is easily explained; for, as I have
+shown elsewhere, the Sirian stars, are intrinsically much brighter in
+proportion to their mass than those of the solar type. But if its
+spectrum is of the solar type, it is not so easy to explain its
+brilliancy. Computing by a well-known formula, I find its relative
+brightness is nearly five times greater than that of Xi Ursæ Majoris,
+the spectrum of which is of the solar type. If, to account for its
+brilliancy, we assume that the star is nearer to the earth than the
+parallax assumed above would imply, then the mass of the system must be
+less than the mass of our sun. As we have seen above, doubling the
+supposed mass increased the distance; so, on the other hand, if we
+diminish the distance, we must diminish the mass also. Thus, if we
+reduce the distance to one-half, we must reduce the mass to one-eighth
+of the sun’s mass. A distance of one-third would give a mass of ¹⁄₂₇th,
+and a distance of one-fourth would imply a mass only ¹⁄₆₄th of the sun’s
+mass. To reduce the sun to the same brightness as Gamma Virginis, it
+should be removed to a distance indicated by a parallax of one-tenth of
+a second multiplied by the square root of five, or 0·223″. If, however,
+the star’s parallax were so much as this, it is probable that it would
+have been detected and measured long ago. In the case of the binary star
+Castor, I find from the orbit and a small parallax found by Johnson
+(about one-fifth of a second) that its mass is only ¹⁄₁₉th of the sun’s
+mass, but in this case the spectrum is of the Sirian type, and stars of
+this type are very bright in proportion to their mass. The colours of
+the components of Gamma Virginis, which are very similar to those of
+Castor—white or pale yellow—would suggest that they may belong to the
+same type.
+
+Another interesting binary star is Eta Cassiopeiæ. The components are
+about 4 and 7½ magnitude, and the pair have described a considerable
+portion of their orbit since its discovery in 1779 by Sir William
+Herschel, the distance diminishing from about 11 seconds to 4¾. Periods
+ranging from 149 to 222½ years have been found by different computers.
+The most recent computation makes it about 196 years. Assuming a
+parallax of 0·154″ found by Struve, the mass of the system will be from
+5¾ to 10¾ times the mass of the sun, according to the length of the
+period we assume. A much larger parallax of 0″·3743 was, however, found
+by Schweizer and Socoloff, which would considerably reduce the mass, and
+recently a still larger parallax of 0″·465 has been found by
+photography, which, with Grüber’s elements of the orbit, would reduce
+the mass of the system to ⅙th of that of the sun.
+
+The bright star Gamma Leonis, situated in the well-known “Sickle in
+Leo,” is also a binary star, but only a small portion of the orbit has
+been described since its discovery by Sir William Herschel in 1782. Dr.
+Doberck finds a period of 407 years. It is remarkable for its very high
+“relative brightness,” which is curious, as its spectrum is of the solar
+type. This pair forms a fine object for a small telescope.
+
+The star known as 12 Lyncis is a triple star, the components being 5, 6,
+and 7½ magnitude. The close pair form a binary system, for which an
+orbit has been computed by the present writer, who finds a period of
+about 486 years. Sir John Herschel predicted in 1823 that the angular
+motion of the pair would “bring the three stars into a straight line in
+57 years.” This prediction was fulfilled in 1887, when measures by
+Tarrant showed that the stars were then exactly in a straight line.
+
+[Illustration:
+
+  FIG. 7.—_Triple Stars._
+
+  (From “Scenery of the Heavens.”)
+]
+
+The bright star Castor is a famous double star, and has been known since
+the year 1718, when it was observed by Bradley and Pond. It was also
+observed by Maskelyne in 1759, and frequently by Sir William Herschel
+from 1799 to 1803. Numerous orbits have been computed, with periods
+ranging from 199 years by Mädler, and 1,001 years by Doberck. Wilson
+found a period of about 983 years, and Thiele about 997 years, so that
+the longest period would seem to be nearest the truth. According to a
+somewhat doubtful parallax found by Johnson, the distance of Castor from
+the earth is about double that of Sirius. With this distance, and
+Doberck’s elements of the orbit, I find that the mass of the system of
+Castor is only ¹⁄₁₉th of the sun’s mass, a result which would imply that
+the components are masses of glowing gas! The spectrum of Sirius is of
+the first, or Sirian, type, another example of the great brilliancy of
+stars of this type. Quite recently (1896), Dr. Bélopolsky has found,
+with the spectroscope, that the brighter component is a close binary
+star with a dark companion, like Algol. The period of revolution is
+about 3 days, and the relative orbital velocity about 20¾ miles a
+second. Dr. Bélopolsky’s observations show that the system is receding
+from the earth at the rate of about 4½ miles per second. Assuming the
+bright and dark companion to be of equal mass, and hence the absolute
+orbital velocity of each one half the relative velocity found by
+Bélopolsky, I find that, if the orbit is circular, the distance between
+the components is about 85,400 miles, or slightly less than the sun’s
+diameter, and their combined mass about ¹⁄₈₇th of the sun’s mass. This
+result would imply a still smaller mass for the whole system of Castor
+than that found from the orbit of the two bright components, but tends
+strongly to confirm the opinion already expressed, that the components
+of this remarkable system are merely masses of glowing gas. Assuming
+that all three components are of equal mass, the combined mass of the
+system would be ¹⁄₅₈th of the sun’s mass. From this result we can easily
+compute the stars’ parallax, which, from Dr. Doberck’s orbit, I find to
+be 0″·2873, a quantity which might be measured by the photographic
+method.
+
+With reference to the colours of the components of binary stars, the
+following relation between colour and relative brightness has been
+established[113]:—
+
+(1.) When the magnitudes of the components are equal, or approaching
+equality, the colours are generally the same, or similar.
+
+(2.) When the magnitudes of the components differ considerably, there is
+also a considerable difference in colour.
+
+A new class of binary stars has been discovered within the last few
+years by means of the spectroscope. These have been called
+“spectroscopic binaries,” and the brighter component of Castor, referred
+to above, is an example of the class. They are supposed to consist of
+two component stars, so close together that the highest powers of the
+largest telescopes fail to show them as anything but single stars.
+Indeed, the velocities indicated by the spectroscope show that they must
+be so close that the components must for ever remain invisible by the
+most powerful telescopes which could ever be constructed by man. In some
+of these remarkable objects, the doubling of the spectral lines
+indicates that the components are both bright bodies, but in others, as
+in Algol, the lines are merely shifted from their normal position, not
+doubled, thus denoting that one of the components is a dark body. In
+either case, the motion in the line of sight can be measured by the
+spectroscope, and we can, therefore, calculate the actual dimensions of
+the system in miles, and thence its mass in terms of the sun’s mass,
+although the star’s distance from the earth remains unknown. Judging,
+however, from the brightness of the star, and the character of its
+spectrum, we can make an estimate of its probable distance from the
+earth.
+
+Let us first take the case of Algol. This famous variable star has,
+according to the Draper catalogue, a spectrum of the Sirian type. It
+may, therefore, be comparable with that brilliant star in intrinsic
+brightness and density. Assuming the mass of Sirius at 2·20 times the
+mass of the sun, as found by Auwers, and that of the brighter component
+of Algol at four-ninths of the sun’s mass, as given by Vogel,[114] I
+find that for the _same distance_ Sirius would be about 2·8 times
+brighter than Algol. But photometric measures show that Sirius is about
+22 times brighter than Algol, from which it follows—since light varies
+inversely as the square of the distance—that Algol is 2·77 times further
+from the earth. Assuming the parallax of Sirius at 0·39″, this would
+give for the parallax of Algol O·14″, or a journey for light of about 23
+years. From the dimensions of the system, as given by Vogel—about
+3,230,000 miles from centre to centre of the components—this parallax
+would give an apparent distance between the components of less than
+¹⁄₂₀₀th of a second, a quantity much too small to be visible in our
+largest telescopes, or probably in any telescope which man can ever
+construct From a consideration of irregularities in the proper motion of
+Algol and in the period of its light changes, Dr. Chandler infers the
+existence of a third dark body and a parallax of 0·07″. As this is
+exactly one-half the parallax found above, it implies a distance just
+double of what I have found, and would, of course, indicate that Algol
+is intrinsically four times brighter than Sirius. This greater
+brilliancy would suggest greater heat, and would agree with its small
+density, which, from its diameter, as given by Vogel—1,061,000 miles—I
+find to be only one-third of that of water.
+
+Let us now consider the case of Beta Aurigæ, which spectroscopic
+observations show to be a close binary star with a period of about four
+days, and a distance between the components of about eight millions of
+miles. This period and distance imply that the mass of the system is
+about five times that of the sun. As in this case the spectral lines are
+doubled at regular intervals of two days, and not merely shifted, as in
+the case of Algol, we may conclude that both the components are bright
+bodies, and we may not be far wrong in supposing that they are of equal
+mass, each having 2½ times the mass of the sun. As the spectrum of Beta
+Aurigæ is of the same type as Sirius, we may compare it with that star,
+as we did in the case of Algol. Assuming the same density and intrinsic
+brightness for both Beta Aurigæ and Sirius, I find that Beta Aurigæ
+should be about twice as bright as Sirius. Now, according to the Oxford
+photometric measures, Sirius is 2·89 magnitudes, or 14·32 times brighter
+than Beta Aurigæ. Hence it follows that the distance of Beta Aurigæ
+should be about 5½ times greater than the distance of Sirius. Hence,
+assuming the parallax of Sirius at 0″·39, that of Beta Aurigæ should be
+about 0″·061. From actual measures of the parallax of Beta Aurigæ, made
+by the late Prof. Pritchard at Oxford, he found, from two companion
+stars, a mean parallax of 0″·062, a result in remarkably close agreement
+with that computed above from a consideration of the star’s mass and
+light, compared with that of Sirius. As the actual distance between the
+components of Beta Aurigæ is equal to the sun’s diameter divided by
+11·625, we have the maximum angular separation between the components
+equal to 0″·062 divided by 11·625, or about ¹⁄₂₀₀th of a second, or
+nearly the same as in the case of Algol.
+
+The bright star Spica has also been found by the spectroscope to be a
+close binary star. Vogel finds a period of four days with a distance
+between the components of about 6¼ millions of miles, and assuming that
+the components have equal mass and are moving in a circular orbit, he
+finds the mass of the system about 2·6 times the mass of our sun. This
+would give each of the components 1·3 times the mass of the sun, and it
+follows that the light of Spica—which gives a spectrum of the Sirian
+type—should, for equal distances, exceed that of Sirius about 1·4 times.
+Now, the photometric measures at Oxford show that Sirius is 1·91
+magnitude, or 5·8 times brighter than Spica. Hence it follows that the
+distance of Spica should be 2·85 times the distance of Sirius. This
+would make the parallax of Spica about 0″·137. So far as I know, a
+measurable parallax has not yet been found for this star. Brioschi, in
+1819–20, observing with a vertical circle of four inches aperture, found
+a negative parallax, which would imply that its parallax is too small to
+be measurable. Still, the above result would seem to indicate that its
+parallax might be measurable by the photographic method. The parallax
+found above would imply that the maximum distance between the components
+of Spica would not exceed ⅒th of a second, a quantity much too small to
+be detected by the most powerful telescopes. In addition to its orbital
+motion, Vogel finds that Spica is approaching the sun at the rate of
+over 9 miles per second.
+
+We now come to Zeta Ursæ Majoris (Mizar), which has also a spectrum of
+the Sirian type, and which the spectroscopic measures indicate is a
+close binary star with a period of about 104 days, and a combined mass
+equal to forty times the mass of the sun. Proceeding as before, we find
+that the light of Mizar should be about 8·7 times that of Sirius. But
+the photometric measures show that Sirius is about three magnitudes, or
+about sixteen times brighter than Mizar. Hence the distance of Mizar
+should be nearly twelve times the distance of Sirius. This gives for the
+parallax of Mizar about 0″·033. Klinkerfues found a parallax of 0″·0429
+to 0″·0477, which does not differ widely from the above result. As the
+velocity of the orbital motion shown by the spectroscope indicates a
+distance between the components of about 143 millions of miles, or about
+the distance of Mars from the sun, it follows that the maximum distance
+between the components would be 0″032, multiplied by 1½ or 0″·048, a
+quantity beyond the reach of our present telescopes.
+
+The well-known variable star, Delta Cephei, has recently been added to
+the list of “spectroscopic binaries.” From observations with the great
+30-inch refractor of the Pulkowa Observatory in the summer of 1894, M.
+Bélopolsky finds that the star is probably a very close double, the
+companion being a nearly, or wholly, dark body, as in the case of Algol,
+and the orbit a very eccentric one. The observed variation of light
+indicates, however, that there is no eclipse, as occurs in Algol, so
+that the fluctuations in the light of Delta Cephei are probably due to
+some other cause. The spectrum of the star is of the solar type, so that
+in this respect it differs from the other spectroscopic binaries
+referred to above. The observations show that the system is approaching
+the sun at the rate of about 15 miles a second. Spectroscopic
+observations also suggest that the well-known variable star Beta Lyræ
+may also consist of two close companions. Further details respecting
+these observations will be given in the next chapter.
+
+From a recent investigation of the proper motion of the star Tau
+Virginis, Dr. Fritz Cohen thinks it is probably a close binary, the
+companion star of which has not yet been detected.
+
+It should be mentioned that in the case of Beta Aurigæ, Spica, Zeta Ursæ
+Majoris, and Castor, as there is no variation of light, as in Algol, the
+plane of the orbit is probably inclined to the line of sight. This would
+have the effect of increasing the computed mass of the system, and thus
+diminishing the calculated parallax. As the above calculations have been
+made on the assumption that the plane of the orbit passes through the
+earth, it follows that the computed parallax is a maximum, and that
+these remarkable objects may be really further from the earth than even
+the minute parallaxes found above would indicate. As the parallaxes of
+the nearest stars, such as Alpha Centauri, 61 Cygni, Sirius, and some
+other stars, are considerably greater than those found above, it would
+seem that our solar system is not situated in a region of binary stars,
+and that these wonderful objects lie beyond our immediate neighbourhood.
+It is also remarkable that, with the exception of Delta Cephei, they
+have all spectra of the Sirian type, including those Algol variables
+whose spectra have been examined.
+
+By the aid of the parallaxes computed above, we can easily calculate the
+relative brightness of the sun compared with that of the spectroscopic
+binaries. Assuming that the sun is 27 magnitudes brighter than the Zero
+magnitude, or 28 magnitudes brighter than a standard star of the first
+magnitude, and taking the parallax of Algol as 0″·07, I find that the
+sun, placed at the distance indicated by this parallax, would be reduced
+to a star of 5·35 magnitude, or about three magnitudes fainter than
+Algol, which implies that Algol is about 15½ times brighter than our
+sun. In the case of Beta Aurigæ, if the sun were placed at the distance
+indicated by the parallax of 0″·061, it would be reduced to a star of
+5·65 magnitude, or about 3·7 magnitudes fainter than Beta Aurigæ, which
+would imply that Beta Aurigæ is about thirty times brighter than the
+sun. In the case of Spica we have the sun reduced to a star of about the
+fourth magnitude, or about three magnitudes fainter than Spica,
+indicating that Spica is, like Algol, about 15½ times brighter than the
+sun, although the mass of Spica is only 2·6 times the mass of the sun.
+Finally, in the case of Mizar, we have the sun reduced to a star of
+about the seventh, or about five magnitudes fainter than Mizar,
+indicating that Mizar is no less than one hundred times brighter than
+our sun. These results show the great relative brilliancy of stars with
+a Sirian spectrum, when compared with that of the sun, a consideration
+which has already been arrived at from other considerations.
+
+
+
+
+                               CHAPTER V.
+                     VARIABLE AND TEMPORARY STARS.
+
+
+To ordinary observers, the light of the stars seems to be constant. Even
+to those who are familiar with the constellations, the stars appear to
+maintain their relative brilliancy unchanged. To a great extent this is,
+of course, true; the great majority of the stars remaining of the same
+brightness from day to day, and from year to year. There are, however,
+numerous exceptions to this rule. Many of the stars, when carefully
+watched, are found to fluctuate in their light, being sometimes
+brighter, and sometimes fainter. These are known as “variable stars”—one
+of the most interesting class of objects in the heavens. Some of these
+have been known for a great number of years, and their variations having
+been carefully watched, the laws governing their light changes have been
+well determined.
+
+We will first consider the variable stars with long periods of
+variation, as these generally show the largest fluctuations of light.
+Among these, the first star in which variation of light seems to have
+been noticed is the extraordinary object, Omicron Ceti, popularly known
+as Mira, or the “wonderful” star. It appears to have been first noticed
+by David Fabricius in the year 1596. He observed that the star now
+called Omicron, in the constellation Cetus, was of the third magnitude
+on April 13 of that year, and that in the following year it had
+disappeared. Bayer saw it again in 1603, when forming his maps of the
+constellations, and assigned to it the Greek letter Omicron, but does
+not seem to have noticed the fact that it was the same star which had
+been observed by Fabricius seven years previously. No further attention
+seems to have been paid to it until 1638 and 1639, when it was observed
+at Francker by Professor Phocylides Holwarda to be of the third
+magnitude in December, 1638, invisible in the following summer, and
+again visible in October, 1639. From 1648 to 1662 it was carefully
+observed by Hevelius, and in subsequent years by several observers. Its
+variations are now regularly followed from year to year, and it forms
+one of the most interesting objects of its kind in the heavens. Its
+light varies from about the second magnitude to the ninth, but its
+brightness at maximum is variable to a considerable extent. Heis found
+its _average_ brightness at maximum in the years 1840–58 to be about the
+third magnitude, but on November 6, 1799, Sir William Herschel found it
+but little inferior to Aldebaran. On the other hand, at the maximum of
+1868, November 7, Heis found it only of the fifth magnitude, and fainter
+than he had seen it for twenty-seven years. Sawyer also observed a
+maximum of about the fifth magnitude (4·9) on November 10, 1887. M.
+Dumenel finds (1896) that in the last twelve periods the magnitude at
+maximum varied from 2·5 to 4·7.[115]
+
+It is stated in several books on astronomy, on the authority of
+Hevelius, that in the years 1672–76 Mira was invisible at the epoch of
+maximum. This is, however, quite a mistake, for it was long since (1837)
+pointed out by Bianchi that the supposed non-appearance of Mira in those
+years can be simply accounted for by the fact that the star was near the
+sun at the time of maxima, and could not be observed. If the star
+happens to be at a maximum in April or May, it will be too near the sun
+to be seen, and as the mean period is about 331 days, this occurs every
+ten years. For this reason the maxima seems to have passed unobserved in
+the years 1852, 1853, and 1854, and again in 1883. The star will be very
+favourably placed for observation in the year 1897, and some following
+years. It has also been stated that Mira wholly disappears at the
+maximum, but this is another error, for the star never becomes fainter
+than 9½ magnitude at any time, and always remains visible in a 3-inch
+telescope. The colour of the star is decidedly reddish, but this hue
+seems to be more marked at minimum than at maximum. The spectrum is a
+remarkable one of the third type, in which bright lines have been seen
+by Espin, Maunder, and Secchi. At the minimum of February, 1896, the
+spectrum was photographed by Professor Wilsing, and he found it very
+similar to a photograph taken by Professor Pickering some years
+previously. The recent photograph shows the lines of hydrogen broad and
+bright. There seems to be no other bright lines except those of
+hydrogen. The blue end of the spectrum is very similar to that of our
+sun, but towards the red end there are “dark flutings, fading towards
+the red.” The bright hydrogen lines have only been seen at maximum, but
+the instruments used by Professor Wilsing were not sufficiently powerful
+to show whether they are also visible at minimum.[116] Professor
+Pickering thinks that “probably most of the stars of long period give a
+spectrum resembling that of ο Ceti, and having the hydrogen lines G,
+_h_, α, β, γ, and δ, bright about the time of maximum. When the
+photographic spectrum is faint, only the brighter lines, G and _h_, are
+visible.” Within the last few years, Mrs. Fleming, while examining the
+photographs of stellar spectra taken for the Henry Draper Memorial, has
+detected a number of variable stars of long period by the presence of
+bright lines in their spectra. These are mostly telescopic stars.
+
+Although the average period of Mira is about 331 days, it is subject to
+marked irregularities, which Argelander has attempted to represent by an
+elaborate formula. In recent years, however, the epochs of maxima have
+deviated considerably from the dates computed from this formula, and at
+the maximum of February, 1896, the star did not reach its maximum light
+until nearly two months after the predicted time.
+
+Perhaps the long period variable star next in order of interest—at least
+to observers in the Northern Hemisphere—is that known as Chi Cygni. It
+was discovered by Kirch in 1686. A mistake is often made about the
+identity of this remarkable object It is sometimes confused with the
+neighbouring star, 17 Cygni of Flamsteed’s catalogue. At the time of
+Flamsteed’s observation, the variable star—which is the true Chi Cygni
+of Bayer’s map (made in 1603)—happened to be faint, and Flamsteed, not
+being able to find Bayer’s star, affixed the Greek letter χ to his No.
+17. It was proposed by Struve to call Flamsteed’s star χ^1, and the
+variable χ^2; but there seems to be no necessity to perpetuate
+Flamsteed’s error, which has been frequently pointed out. All
+authorities on the variable stars now give this variable its proper
+designation—χ Cygni. The star varies at maximum from 4 to 6½ magnitude,
+and at the minimum it sinks to below the thirteenth magnitude. At some
+maxima, therefore, it is easily visible to the naked eye, and at others
+it is just below the limit of ordinary vision. At the maximum of 1847,
+it was visible to the naked eye for a period of 97 days. The average
+period is about 406 days; but, according to Schönfeld—a well-known
+authority on the variables—observations indicate a small lengthening of
+the period. Observations in recent years show that the minimum occurs
+about 185 days before the maximum. This gives 221 days for the fall from
+maximum to minimum, and illustrates a feature common to many of the
+variable stars, namely, that the increase of light is more rapid than
+the decrease. This peculiarity is especially marked in the short period
+variables, which will be considered further on. Chi Cygni is said to be
+“strikingly variable in colour.” Espin’s observations in different years
+show it “sometimes quite red, at others only pale orange-red.” In the
+spectroscope, its light shows a splendid spectrum of the third type (or
+banded spectrum, very characteristic of these long period variables), in
+which bright lines were observed by Espin in May, 1889. One of these
+bright lines seems to be identical with the coronal line D_{3}, the
+characteristic line of helium.
+
+R Leonis is another remarkable variable star, which is sometimes visible
+to the naked eye at maximum. It lies closely south of the star known as
+19 Leonis. It was discovered by Koch in 1782. At the maximum, its
+brightness varies from 5·2 to 7 magnitude, and at minimum it fades to
+about the tenth magnitude. The mean period is about 313 days; but this
+is subject to some irregularities, and Chandler finds “good evidence of
+cyclical variation of period, with a long term.” The star is red in all
+phases of its light, and forms a fine telescopic object. Close to it are
+two small stars, which form, with the variable, an isosceles triangle.
+The spectrum is a fine one of the third type, a type very characteristic
+of these long period variables. Espin finds that the bright bands of the
+spectrum are brighter when the star is increasing in light, and fainter
+when decreasing. At the maximum of 1889, he found bright lines in its
+spectrum.
+
+Another long period variable star which is visible to the naked eye at
+maximum is R Hydræ—the Upsilon Hydræ of Bayer—but it is rather too far
+south to be well observed in this country. Its variability was discerned
+by Maraldi in 1704; but the star was also observed by Hevelius in 1672.
+Its light at maximum varies from 3½ to 5½ magnitude, and at minimum it
+fades to nearly the tenth magnitude. The period has diminished
+considerably since the year 1708, when it was about 500 days. This had
+decreased to about 487 days in 1785, to 461 days in 1825, and to 437
+days in 1870, and it seems to be still diminishing. Formulæ have been
+computed by Gould and Chandler, but do not agree. Schmidt found that the
+minimum occurs about 200 days before the maximum. The star is very
+reddish, and the spectrum is a fine one of the third type, which Dunér
+describes as of “extraordinary beauty,” the typical bands of this type
+of spectrum being very large, and perfectly black. At the maximum of
+1889, Espin observed a bright line in its spectrum, and finds—as in R
+Leonis—that the bright bands are brighter when the star is increasing in
+light, and fainter as it decreases.
+
+There is a very remarkable variable star in the Southern Hemisphere
+known as Eta Argûs. It lies in the midst of the great nebula in Argo,
+and the history of its fluctuations in light is very interesting.
+Observed by Halley in 1677 as a star of the fourth magnitude, it was
+seen of the second magnitude by Lacaille in 1751. After this, it must
+have again faded, for Burchell found it of only the fourth magnitude
+from 1811 to 1815. From 1822 to 1826, it was again of the second
+magnitude, as observed by Fallows and Brisbane; but on Feb. 1, 1827, it
+was estimated of the first magnitude by Burchell. It then faded again,
+for on Feb. 29, 1828, Burchell found it of the second magnitude. From
+1829 to 1833, Johnson and Taylor rated it of the second magnitude; and
+it was still of this magnitude, or a little brighter, when Sir John
+Herschel commenced his observations at the Cape of Good Hope in 1834. It
+does not seem to have varied much in brightness from that time until
+December, 1837, when Herschel was astonished to find its light “nearly
+tripled.” He says:[117] “It very decidedly surpassed Procyon, which was
+about the same altitude, and was far superior to Aldebaran. It exceeded
+α Orionis, and the only star (Sirius and Canopus excepted) which could
+at all be compared with it was Rigel, which, as I have already stated,
+it somewhat surpassed.”
+
+From this time its light continued to increase. On the 28th December it
+was far superior to Rigel, and could only be compared with α Centauri,
+which it equalled, having the advantage of altitude, but fell somewhat
+short of it as the altitudes approached equality. The maximum of
+brightness seems to have been obtained about the 2nd January, 1838, on
+which night, both stars being high and the sky clear and pure, it was
+judged to be very nearly matched, indeed, with α Centauri, sometimes the
+one, sometimes the other, being judged brighter; but, on the whole, a
+was considered to have some little superiority. After this, the light
+began to fade. Already on the 7th and 15th January, α Centauri was
+unhesitatingly placed above, and Rigel as unhesitatingly below, it. On
+the 20th, it was “visibly diminished—now much less than α Centauri, and
+not _much_ greater than Rigel. The change is palpable.” And on the 22nd,
+Arcturus (the nearest star in light and colour to α Centauri which the
+heavens afford), when only 10° high, surpassed η, the latter being on
+the meridian; η was still, however, superior to β Centauri, α Crucis,
+and Spica, and continued so (and even superior to Rigel) during the
+whole of February, nor was it until the 14th April, 1838, that it had so
+far faded as to bear comparison with Aldebaran, though still somewhat
+brighter than that star. In 1843, it again increased in brightness, and
+in April of that year it was observed by Maclear to be brighter than
+Canopus, and nearly equal to Sirius! It then faded slightly, but seems
+to have remained nearly as bright as Canopus until February, 1850, since
+which time its brilliancy gradually decreased. It was still of the first
+magnitude in 1856, according to Abbott, but was rated a little below the
+second magnitude by Powell in 1858. Tebbutt found it of the third
+magnitude in 1860; Abbott a little below the fourth in 1861. Ellery
+rated it fifth magnitude in 1863, and Tebbutt sixth magnitude in 1867.
+In 1874 it was estimated 6·8 magnitude at Cordoba, and only 7·4 in
+November, 1878. Tebbutt’s observations from 1877–86 show that it did not
+rise above the seventh magnitude in those years, and in March, 1886, it
+was rated 7·6 magnitude by Finlay at the Cape of Good Hope. This seems
+to have been the minimum of light, for in May, 1888, Tebbutt found that
+it “had increased fully half a magnitude” since April, 1887, and might
+“be rated as a star of 7·0 magnitude.” From photometric measures made
+with the meridian photometer in Peru in the years 1889–91, Professor
+Bailey found its mean magnitude to be 6·32, so that probably the star is
+now slowly rising to another maximum. Bailey found the hydrogen lines
+Hβ, Hγ, and Hδ, bright in the spectrum of its light. Wolf suggested a
+period of 46 years, and Loomis, 67; but Schönfeld thought that a regular
+period is very improbable. The star is very reddish in colour.
+
+There are many other variables of long period, but they are too numerous
+to be described in detail in a work of this character. Particulars
+respecting some of them will be found in “The Scenery of the Heavens,”
+by the present writer.
+
+We will now consider the variables of short period, which are
+particularly interesting objects, owing to the comparative rapidity of
+their light changes. The periods vary in length from about 17¼ days down
+to a few hours. Perhaps the most interesting of these short period
+variables, at least to the amateur observer, is the star Beta Lyræ,
+which is easily visible to the naked eye in all phases of its light. It
+can be readily identified, as it is the nearest bright star to the south
+of the brilliant Vega, and one of two stars of nearly the same
+magnitude, the second being Gamma Lyræ. The variability of Beta Lyræ was
+discovered by Goodricke in the year 1784. The period is about 12 days,
+21 hours, 46 minutes, 58 seconds. At maximum the star is about 3·4
+magnitude, and there are two minima, one of magnitude 3·9, and the
+other—the chief minima—of 4·5 magnitude. That is, the star has at
+maximum 2¾ times the light of the chief minimum, and 1·6 times the light
+of the secondary minimum. In other words, if we represent the light of
+the star at maximum by 27 candles, placed at a suitable distance from
+the eye, the secondary minimum will be represented by 17 candles, and
+the chief minimum by 10 candles. These fluctuations, although not very
+great, can be easily recognised with the naked eye by comparison with
+the neighbouring star Gamma Lyræ. Professor Pickering thought that this
+variation in the light of Beta might be explained by supposing that the
+star rotated on its axis in the period indicated by the variation, that
+the ratio of the axis of the rotating spheroid is as 5 to 3, and that
+there is a darker portion at one of the ends, which is “symmetrically
+situated as regards the longer axis.” Recent observations with the
+spectroscope, however, render this explanation doubtful, and indicate
+rather that the star is a very close double or “spectroscopic binary,”
+although it does not seem certain that an actual eclipse of one
+component by the other takes place, as in the case of Algol. Bright
+lines were detected in the star’s spectrum by Secchi so far back as
+1866. In 1883, M. Von Gothard noticed that the appearance of these
+bright lines varied in appearance, and from an examination of
+photographs taken at Harvard Observatory in 1891, Mrs. Fleming found
+displacements of bright and dark lines in a double spectrum, the period
+of which agreed fairly well with that of the star’s light changes.
+Professor Pickering thence concluded that the star consists of two
+components, one stellar and the other gaseous, but this conclusion has
+been somewhat modified by subsequent investigations. M. Bélopolsky, from
+photographs taken with the great 30-inch telescope at the Pulkowa
+Observatory, confirms the periodical displacement in the bright spectral
+lines “in a period identical with that of the star’s usual double
+fluctuation,” but Keeler and Vogel agree that the observed displacements
+are incompatible with the supposed occurrence of eclipses. Vogel,
+however, is “convinced that Beta Lyræ represents a binary or multiple
+system, the fundamental revolutions of which, in 12 days 22 hours, in
+some way control the light change, while the spectral variations,
+although intimately associated with the star’s phases, are subject,
+besides, to complicated disturbances running through a cycle perhaps
+measured by years.”[118] The helium line, D_{3}, is visible in the
+spectrum.
+
+Another interesting star of short period is Delta Cephei, which is one
+of three stars forming an isosceles triangle a little to the west of
+Cassiopeia’s Chair, the variable being at the vertex of the triangle,
+and the nearest of the three to Cassiopeia. Its variability was also
+discovered by Goodricke in 1784. It varies from 3·7 to 4·9 magnitude,
+with a period of 5 days, 8 hours, 47 minutes, 40 seconds. The amount of
+the variation is, therefore, the same as in the case of Algol, the
+star’s light at maximum being about three times its light at minimum.
+The period and light curve, however, show, according to Schönfeld, some
+irregularities, the computed times of maxima and minima being sometimes
+in error to the extent of over an hour. These are, however, small, and,
+on the whole, the star seems to be very uniform in its fluctuations.
+From seven years’ observations, Argelander found no deviation from
+perfect uniformity. The curve representing the light variations is not,
+however, very smooth, particularly during the decrease of light, when a
+nearly stationary period seems to occur from 16 to 24 hours after the
+maximum. The rise from minimum to maximum occupies about one-third of
+the period, another example of the feature so characteristic of variable
+stars, namely, that the increase of light is quicker than the decrease.
+As already stated (Chapter IV.), observations of the spectrum recently
+made by M. Bélopolsky, with the great Pulkowa telescope, show that, like
+Beta Lyræ, the star is probably a close binary, the period of the
+observed fluctuations in the positions of the spectral lines agreeing
+with that of the star’s light changes. In this case, however, the lines
+are not doubled, as in Beta Lyræ, but merely displaced from their normal
+position, indicating that, as in the case of Algol, one of the
+components is a dark body. There are, however, no indications that any
+eclipse of the bright star by its dark companion takes place. Indeed,
+the nature of the light changes, which are continuous and not confined
+to a few hours, as in Algol, are inconsistent with the occurrence of an
+eclipse. We must, therefore, conclude that the fluctuations of light are
+caused in some way by physical disturbances produced by the approach and
+recession of the two component bodies in an elliptic orbit round their
+centre of gravity. The observations indicate that the component stars,
+when furthest apart in their orbital revolution, are separated by a
+distance three times as great as when at their point of nearest
+approach. The observations also show that Delta Cephei is approaching
+the earth at the rate of about 8¾ miles a second. Its spectrum is of the
+second or solar type, differing in this respect from the other
+spectroscopic binaries, which show a spectrum of the first or Sirian
+type. The colour of the star is yellow, and it has a distant bluish
+companion of about the fifth magnitude, which may possibly have some
+physical connexion with the brighter star, as both stars have a common
+proper motion through space.
+
+Another remarkable star of short period is Eta Aquilæ, the variability
+of which was discovered by Pigott in 1784. It varies from magnitude 3·5
+to 4·7, with a period of 7 days, 4 hours, 14 minutes, but Schönfeld
+found marked deviations from a uniform period. It will be seen that the
+amount of the light change, 1·2 magnitude, is the same as that of Delta
+Cephei. Its colour is yellow, and its spectrum, like that of Delta
+Cephei, of the second or solar type. The minimum takes place about three
+days before the maximum.
+
+Zeta Geminorum is another variable star with a comparatively short
+period. It varies from about 3·7 to 4·5 magnitude, with a period of 10
+days, 3 hours, 41½ minutes. Here the variation of light is only 0·8 of a
+magnitude, or, in other words, the light at maximum is about double the
+light of minimum, as in the case of the Algol type variable, Lambda
+Tauri. Its light curve, unlike that of Delta Cephei and Eta Aquilæ, is
+nearly symmetrical; that is, the period occupied in the increase of
+light is about the same as that of the decrease. Prof. Pickering thinks
+that Zeta Geminorum is possibly a “surface of revolution,” one side of
+the rotating star being about four-fifths of the brightness of the
+other; but Prof. Lockyer finds it to be a “spectroscopic binary,” like
+Beta Lyræ and Delta Cephei.
+
+Among variables with very short periods may be mentioned the southern
+star R Muscæ, which is close to Alpha Muscæ. It varies from 6·6 to 7·4,
+and goes through all its changes in the short period of 21 hours 20
+minutes. The minimum takes place about nine hours before the maximum. It
+was discovered at the Cordoba Observatory, and Dr. Gould remarks that
+“its average brightness is so near the limit of ordinary visibility in a
+clear sky at Cordoba, that the small regular fluctuations of light place
+it every few hours alternately within or beyond this limit.”
+
+A remarkable variable star of short period was discovered in 1888 by Mr.
+Paul in the southern constellation Antlia. It varies from magnitude 6·7
+to 7·3, with the wonderfully short period of 7 hours, 46 minutes, 48
+seconds, all the light changes being gone through no less than three
+times in twenty-four hours! It was for some years believed that the
+variation was of the Algol type, but recent measures made at the Harvard
+College Observatory show that it belongs to the same class as Delta
+Cephei and Eta Aquilæ.
+
+A telescopic variable with a wonderfully short period was discovered by
+Chandler in 1894. It lies a little to the west of the star Gamma Pegasi,
+and has been designated U Pegasi. It varies from magnitude 8·9 to 9·7,
+and was first supposed to be of the Algol type with a period of about
+two days, but further observations showed that the period was much
+shorter, and only 5 hours, 31 minutes, 9 seconds. The light curve is
+quite different from the Algol type, and also from that of Delta Cephei
+and other short period variables, the times of increase and decrease of
+light being about equal, as in the case of Zeta Geminorum. This fact,
+combined with the remarkable rapidity of its light changes, which are
+gone through four times in less than twenty-four hours, makes this
+remarkable star a most interesting object. Possibly there may be other
+stars in the heavens with a similar rapidity of variation which have
+hitherto escaped detection.
+
+Several southern variables of short period have been discovered in
+recent years by Mr. A. W. Roberts at Lovedale in South Africa.
+
+Unlike the variable stars of long period which seem scattered
+indifferently over the surface of the heavens, the great majority of the
+short period variables are found in a zone which nearly coincides with
+the course of the Milky Way. The most notable exceptions to this rule
+are W Virginis with the comparatively long period of 17¼ days, and U
+Pegasi, above described, which has the shortest known period of all the
+variable stars. Another peculiarity is that most of them are situated in
+what may be called the following hemisphere, that is between 12 hours
+and 24 hours of right ascension. The most remarkable exception to this
+rule is Zeta Geminorum. The above rules do not apply to variables of the
+Algol type, which we will now proceed to consider.
+
+Algol, or Beta Persei, is a famous variable star, and the typical star
+of the class to which it belongs. Its name, Algol, is derived from a
+Persian word, meaning the “demon,” which suggests that the ancient
+astronomers may have detected some peculiarity in its behaviour. The
+real discovery of its variation was, however, made by Montanari in 1667,
+and his observations were confirmed by Maraldi in 1692. Its fluctuations
+of light were also noticed by Kirch and Palitzsch, but the true
+character of its variations was first determined by the English
+astronomer, Goodricke, in 1782. Its fluctuations of light are very
+curious and interesting. Shining with a constant, or nearly constant,
+brightness for a period of about 59 hours as a star of a little less
+than the second magnitude, it suddenly begins to diminish in brightness,
+and in about 4½ hours it is reduced to a star of about magnitude 3½. In
+other words, its light is reduced to about one-third of its normal
+brightness. If we suppose three candles placed side by side at such a
+distance that their combined light is merged into one, and equal to the
+usual brightness of Algol, then if two of these candles are
+extinguished, the remaining candle will represent the light of Algol at
+its minimum brilliancy. It is stated in several books on astronomy that
+Algol varies to the extent of two magnitudes, but this is quite
+incorrect, as a change of two magnitudes would imply that the light at
+maximum is over six times the light at minimum, which is more than
+double the star’s real variation. The star remains at its minimum, or
+faintest, for only about 15 minutes. It then begins to increase, and in
+about 5 hours recovers its normal brightness, all the light changes
+being gone through in a period of about 10 hours out of nearly 69 hours,
+which elapse between successive minima. These curious changes take place
+with great regularity, and the exact hour at which a minimum of light
+may be expected can be predicted with as much certainty as an eclipse of
+the sun.
+
+Goodricke, comparing his own observations with one made by Flamsteed in
+the year 1696, found the period from minimum to minimum to be 2 days, 20
+hours, 48 minutes, 59½ seconds, and he came to the conclusion that the
+diminution in the light of the star is probably due to a partial eclipse
+by “a large body revolving round Algol.” This hypothesis was fully
+confirmed in the years 1888–89 by Professor Vogel with the spectroscope.
+As no close companion to Algol is visible in the largest telescopes, we
+must conclude that either the satellite is a dark body, or else so close
+to the primary that no telescope could show it. As has been stated in
+Chapter III., the motion of a star in the line of sight can be
+ascertained by measuring displacements in the positions of the spectral
+lines. Now, if the diminution in Algol’s light is due to a dark body
+revolving round it, and periodically coming between us and the bright
+star, it follows that both components will be in motion, and both will
+revolve round the common centre of gravity of the pair. A little before
+a minimum of light takes place, the dark companion should therefore be
+approaching the eye, and, consequently, the bright companion will be
+receding. During the minimum there will be no apparent motion in the
+line of sight, as the motion of both bodies will be at right angles to
+the visual ray. After the minimum is over, the motion of the two bodies
+will be reversed, the bright one approaching the eye, and the dark one
+receding. Now, this is exactly what Vogel found. Before the diminution
+in the light of Algol begins, the spectroscope showed that the star is
+receding from the earth, and after the minimum, that it is approaching
+the eye. That the companion is dark and not bright, like the primary, is
+evident from the fact that the spectral lines are merely shifted from
+their normal position and not doubled, as would be the case were both
+components bright, as in the case of some of the “spectroscopic
+binaries”—for example, Beta Aurigæ—which has been considered in the
+chapter on binary stars (Chapter IV.). Vogel found that before the
+minimum of light, Algol is receding from the earth with the velocity of
+24½ miles a second, and after the minimum it is approaching at the rate
+of 28½ miles a second. The difference between the observed velocities
+indicates that the system is approaching the earth with a velocity of
+about 2 miles a second. Knowing, then, the orbital velocity, which is
+evidently about 26½ miles a second, and assuming the orbit to be
+circular, it is easy, with the observed period of revolution, or the
+period of light variation, to calculate the diameter of the orbit in
+miles, although the star’s distance from the earth remains unknown.
+Further, comparing its period of revolution and the dimensions of the
+orbit with that of the earth round the sun, it is easy to calculate, by
+Kepler’s third law of motion, the mass of the system in terms of the
+sun’s mass, and the probable size of the component bodies. Calculating
+in this way, Vogel computes that the diameter of Algol is about
+1,061,000 miles, and that of the dark companion 830,300 miles, with a
+distance between their centres of 3,230,000 miles, and a combined mass
+equal to two-thirds of the sun’s mass, the mass of Algol being
+four-ninths, and that of the companion two-ninths, of the mass of the
+sun. Taking the diameter of the sun as 866,000 miles, and its density as
+1·44 (water being unity), I find that the above dimensions give a mean
+density for the components of Algol of about one-third that of water, so
+that the components are probably gaseous bodies, as Hall has already
+concluded.
+
+From the recorded observations of minima in past years, it has been
+found that the period of variation of Algol’s light has been slowly
+diminishing since Goodricke’s time, and Dr. Chandler finds the present
+period is about 2 days, 20 hours, 48 minutes, 51 seconds, or about 8½
+seconds less than Goodricke made it. Chandler thinks that this variation
+in the length of the period is cyclical, and that it has now about
+reached its smallest value, and will soon begin to increase again. He
+believes that this variation is probably due to the orbital revolution
+of the pair round a third body in a period of about 130 years. M.
+Tisserand, however, explains the irregularities by supposing an
+elliptical orbit, and a slight flattening or polar compression in the
+primary star. Professor Boss is inclined to favour Chandler’s
+hypothesis.
+
+It is a curious fact that Al-Sûfi, the Persian astronomer, in his
+“Description of the Heavens,” written in the tenth century, speaks
+distinctly of Algol as a red star (_étoile, brillant; d’un éclat,
+rouge_), while at present it is white, or at the most, of a yellow
+colour. A similar change of colour is supposed to have taken place in
+the case of Sirius, but the change in Algol seems more certain, as
+Al-Sûfi’s descriptions are generally most accurate and reliable.
+
+Stars of the Algol type of variable are very rare objects, only a dozen
+or so having been hitherto discovered in the whole heavens. Those
+visible to the naked eye, when at their normal brightness, are: Algol,
+Lambda Tauri, Delta Libræ, R Canis Majoris, and U Ophiuchi. The
+variation of Lambda Tauri was discovered by Baxendell in 1848. It varies
+from magnitude 3·4 to 4·2, and its period from minimum to minimum of
+light is about 3 days, 22 hours, 52 minutes, 12 seconds. Its
+fluctuations have not been so well studied as those of Algol, but it is
+known that the “period is subject to marked inequalities,” sometimes
+amounting to 3 hours. The variation of light is less than that of Algol,
+the light at maximum being only twice the light at minimum. Two candles
+at a suitable distance would therefore represent the maximum light, and
+one candle the minimum brightness. All the light changes take place in a
+period of about 10 hours. The star is white like Algol.
+
+The variability of Delta Libræ was discovered by Schmidt in 1859. It
+varies from magnitude 4·9 to 6·1, with a period of 2 days, 7 hours, 51
+minutes, 22·8 seconds. The period is, however, according to Schönfeld,
+subject to some irregularities. The variation of light is about the same
+as that of Algol, the light at maximum being about three times the light
+at minimum. The variation takes about 12 hours, of which the decrease
+occupies 5½ hours. The star is white like Algol.
+
+The variability of R Canis Majoris was detected by Sawyer in 1887. The
+variation is from 5·9 to 6·7 magnitude, or about equal in amount to that
+of Lambda Tauri, and the period 1 day, 3 hours, 15 minutes, 55 seconds.
+
+U Ophiuchi was also discovered by Sawyer in 1881. Its variation is from
+magnitude 6·0 to 6·7, or slightly less than that of Lambda Tauri, and
+the period 20 hours, 7 minutes, 41·6 seconds, but subject to an apparent
+diminution. The maximum brightness lasts for about 16 hours, and all the
+fluctuations of light take place in the short period of 4 hours. Its
+colour is white, like most stars of the Algol type.
+
+U Cephei is a very interesting variable of the Algol type, discovered by
+Ceraski in 1880. It varies from 7·1 to 9·5, with a period of 2 days, 11
+hours, 49 minutes, 45 seconds. Here the variation of light is greater
+than that of Algol, the light at maximum being nearly seven times the
+light at minimum. Its rapidity of variation is very great, sometimes
+exceeding a magnitude in an hour. The light variations occupy about 6
+hours, and the minimum lasts for about an hour and a half, Professor
+Pickering thinks that the variation of light is, as in the case of
+Algol, caused by an eclipsing satellite, but that in this case the
+eclipse may possibly be total, the light at minimum being that due to
+the satellite, which may have some inherent light of its own. Lord
+Crawford examined the star with the spectroscope, and found that at the
+minimum the blue end of the spectroscope faded, and the red was
+intensified, which seems to suggest that the light of the star in that
+phase shines through a gaseous medium, and that the eclipsing body may
+be surrounded with an atmosphere.
+
+Another interesting Algol variable is that known as Y Cygni, which was
+discovered by Chandler in 1886, while using it as a comparison star for
+the short period variable X Cygni. It varies from 7·1 to 7·9 magnitude,
+or about the same amount as Lambda Tauri, with a period of 1 day, 11
+hours, 56 minutes, 48 seconds. It has alternate bright and faint minima,
+which suggest, according to Dunér, that the star consists of two
+_bright_ components, one of them being brighter than the other, and both
+revolving round their common centre of gravity in an elliptic orbit,
+with a period double that of the light variation. Yendell, who has
+carefully observed the star’s fluctuations, fully concurs in Dunér’s
+views, and says “the substantial corrections of his fundamental
+assumption appears to be proved beyond the possibility of a cavil.”
+
+The variability of the star known as S Cancri was discovered by Hind in
+1848. It varies from 8·2 to 9·8, or it is said, at some minima, to 11·7,
+with the comparatively long period of 9 days, 11 hours, 37 minutes, 45
+seconds. The variations of light occupy about 21½ hours. If the minimum
+of 11·7 is correct, we have a variation of no less than 3½ magnitudes,
+which implies that the normal light of the star is 25 times its light at
+a faint minimum. If this be so, the eclipse must be nearly total.
+Argelander found that after the minimum the light increases very
+rapidly, and he thinks that the descent from the maximum is even more
+rapid.
+
+Some interesting examples of the Algol type of variable have been
+discovered in recent years. One detected by Chandler, in 1894, and now
+known as Z Herculis, varies from about the seventh to the eighth
+magnitude, and has a period of 3 days, 23 hours, 48½ minutes. Faint and
+very bright minima alternate in periods of 47 and 49 hours, the ratios
+of the light at maximum and minima being 3, 2, and 1. These Professor
+Dunér considers, indicate that the star consists of two revolving
+components of equal size, one of which is twice as bright as the other,
+and he computes that the components revolve round their common centre of
+gravity in an elliptic orbit, the plane of which is in the line of
+sight, and the semi-axis major about six times the diameter of the
+stars. If we assume that the diameter of each component is equal to the
+diameter of our sun, I find, from the above data, that the combined mass
+of the system is about 1½ times the mass of the sun.
+
+Another remarkable example of the Algol type was discovered by Miss
+Wells in 1895. The star lies a little north of the “Dolphin’s rhomb,”
+and at its normal brightness is about magnitude 9½. The period of
+variation is about four days. The variation somewhat resembles that of U
+Cephei. Professor Pickering says: “For nearly two hours before and after
+the minimum it is fainter than the twelfth magnitude. It is impossible
+at present to say how much fainter it becomes, or whether it disappears
+entirely. It increases at first very rapidly, and then more slowly,
+attaining its full brightness, magnitude 9·5, about five hours after the
+minimum. One hundred and thirty photographs indicate that, during the
+four days between the successive minima, it does not vary more than a
+few hundredths of a magnitude. The variation may be explained by
+assuming that the star revolves round a comparatively dark body, and is
+totally eclipsed by it for two or three hours, the light at minimum, if
+any, being entirely that of the dark body.”[119] This seems to be an
+unique object, and it should be carefully followed through its minimum
+with a large telescope.[120]
+
+With reference to the Algol type of variable stars, Chandler finds that
+“the shorter the period of the star, the higher the ratio which the time
+of oscillation bears to the entire period.” Thus, in U Ophiuchi, with a
+period of about 20 hours, the light changes occupy five hours, or
+one-fourth of the period, while in S Cancri, which has a period of 227½
+hours, the fluctuations of light take up 21½ hours, or only about
+one-tenth of the period. In all cases in which the Algol type variables
+have been examined with the spectroscope, the spectrum has been found to
+be of the first or Sirian type, and they seem to be the only stars with
+spectra of the Sirian type whose light is variable. It should be noted,
+however, that, on the eclipse theory, the variation of light in these
+stars is due merely to an occultation of one star by another, and not to
+any physical change in the star itself. The bright star Spica, although
+shown by the spectroscope to be a close binary star, like Algol, is not
+variable, because, in this case, the plane of its orbit is inclined to
+the line of sight, and hence the comparison star does not transit the
+disc of its primary. Seen from some other point in space, it would
+probably be an Algol variable.
+
+A remarkable peculiarity about the variable stars in general is that
+none of them have any considerable proper motion. As a large proper
+motion is generally considered to indicate proximity to the earth, we
+may conclude, with great probability, that the variable stars, as a
+rule, lie at a great distance from our system. In other words, it
+appears that the sun does not lie in a region of variable stars, and,
+with the exception of Alpha Cassiopeiæ and Alpha Herculis, a measurable
+parallax has not yet been found, so far as I know, for any known
+variable star.
+
+Plotting the known variables on star charts, I find a marked tendency to
+cluster into groups. Thus, in and near the constellation, Corona
+Borealis, there are five; near Cassiopeia’s Chair, five. In Cancer there
+are four in a limited area. Near Eta Argûs there are several, and in a
+comparatively small region in the northern portion of Scorpio there are
+no less than fifteen variable stars.
+
+We now come to the interesting and mysterious class of objects known as
+“new” or “temporary” stars. These phenomena are of very rare occurrence,
+and but few undoubted examples of the class are recorded in the annals
+of astronomy. Possibly in some cases they have been merely variable
+stars, of irregular period and fitful variability; but others may have
+been due to a real catastrophe, such as the collision of two dark bodies
+in space, or, possibly, the passage of a bright or dark body through a
+gaseous nebula.
+
+The earliest temporary star of which we have any reliable information
+seems to be one which is recorded in the Chinese annals of Ma-tuan-lin,
+as having appeared in the year 134 B.C. in the constellation Scorpio.
+Its position seems to have been somewhere between the stars Beta and Rho
+of Scorpio. Pliny informs us that it was the sudden appearance of a new
+star which induced the famous astronomer Hipparchus to form his
+catalogue of stars, the first ever constructed. As the date of
+Hipparchus’ catalogue is 125 B.C., it seems highly probable that the new
+star referred to by Pliny was the same as that recorded by the Chinese
+astronomer as having appeared nine years previously.
+
+A new star is said to have appeared in the year 76 B.C. between the
+stars Alpha and Delta in the Plough, but the accounts are vague.
+
+In 101 A.D., a small “yellowish-blue” star is said to have appeared in
+the “sickle” in Leo, but its exact position is not known. In 107 A.D., a
+new star is mentioned near Delta, Epsilon and Eta in Canis Major, three
+bright stars south-east of Sirius. In 123 A.D., another new star is
+recorded by Ma-tuan-lin to have appeared between Alpha Herculis and
+Alpha Ophiuchi.
+
+The Chinese annals record that on Dec. 10, 173 A.D., a brilliant star
+appeared between Alpha and Beta Centauri in the Southern Hemisphere. It
+remained visible for eight months, and is described as resembling “a
+large bamboo mat!”—a curious description. There is at present close to
+the spot indicated, a known variable star—R Centauri—of which the period
+seems to be long and the variation of light irregular. Possibly an
+unusually bright maximum of this variable star formed the star of the
+Chinese annals, or perhaps the variable star is the remnant of the
+outburst which took place in the first century. The variable is a very
+reddish star, and at present varies from about the sixth to the tenth
+magnitude
+
+A new star is recorded in the year 386 A.D. as having appeared between
+Lambda and Phi Sagittarii. Near the position indicated, Flamsteed
+observed a star, No. 65 of his catalogue, which is now missing; and it
+has been conjectured that the star seen by Flamsteed may possibly have
+been a return of the star mentioned in the Chinese annals.
+
+Cuspianus relates that a star as bright as Venus appeared near Altair in
+389 A.D., during the reign of the Emperor Honorius, and that he had
+himself seen it. There is some doubt, however, about the exact date, as
+other accounts give the year 388 or 398. The star seems to have
+disappeared in about three weeks.
+
+In the year 393 A.D., another strange star is recorded in the tail of
+Scorpio. An extraordinary star is said to have been seen near Alpha
+Crateris in 561 A.D. Here again a known variable and red star—R
+Crateris—is close to the position indicated by the ancient records.
+
+The Chinese annals record a new star in 829 A.D., somewhere in the
+vicinity of the bright star Procyon, and in this locality there are
+several known variable stars.
+
+The Bohemian astronomer, Cyprianus Leoviticus, mentions the appearance
+of new stars in Cassiopeia in the years 945 A.D. and 1264, and it has
+been conjectured that perhaps these were apparitions of Tycho Brahé’s
+famous star of 1572 (to be presently described), forming a variable star
+with a period of over 300 years. Lynn and Sadler, however, have shown
+that the supposed stars of 945 and 1264 were, in all probability,
+comets.
+
+Extraordinary stars are recorded near Zeta Sagittarii in 1011 A.D., near
+Mu Scorpii in 1203, and near Pi Scorpii on July 1, 1584. It is
+remarkable how many of these objects seem to have appeared in this
+portion of the heavens.
+
+A very brilliant star is mentioned by Hepidannus as having appeared in
+Aries in May, 1012. He describes it as “dazzling the eye.” Other
+temporary stars are mentioned in 1054 A.D., near Zeta Tauri, and in
+1139, near Kappa Virginis; but the accounts of these are very vague, and
+it seems by no means certain that they were really new stars.
+
+No possible doubt, however, can be entertained with reference to the
+appearance of the object which suddenly blazed out in Cassiopeia’s Chair
+in November, 1572. It was called the “Pilgrim Star,” and was observed by
+the famous astronomer, Tycho Brahé, who has left us a very elaborate
+account of its appearance, position, etc. Although usually spoken of as
+Tycho Brahé’s star, it seems to have been really discovered by Cornelius
+Gemma on the evening of November 9. That its appearance was very sudden
+may be inferred from Cornelius Gemma’s statement, that it was not
+visible on the preceding night in a clear sky. Tycho Brahé’s attention
+was first attracted to it on November 11. His description of the new
+star is as follows—as quoted by Humboldt:[121]—“On my return to the
+Danish islands from my travels in Germany, I resided for some time with
+my uncle, Steno Bille, in the old and pleasantly situated monastery of
+Herritzwadt, and here I made it a practice not to leave my chemical
+laboratory until the evening. Raising my eyes, as usual, during one of
+my walks, to the well-known vault of heaven, I observed with
+indescribable astonishment, near the zenith in Cassiopeia, a radiant
+fixed star of a magnitude never before seen. In my amazement, I doubted
+the evidence of my senses. However, to convince myself that it was no
+illusion, and to have the testimony of others, I summoned my assistants
+from the laboratory, and inquired of them, and of all the country people
+that passed by, if they also observed the star that had thus suddenly
+burst forth. I subsequently heard that in Germany, waggoners and other
+common people first called the attention of astronomers to this great
+phenomenon in the heavens—a circumstance which, as in the case of
+non-predicted comets, furnished fresh occasion for the usual raillery at
+the expense of the learned. This new star I found to be without a tail,
+not surrounded by any nebula, and perfectly like all other fixed stars,
+with the exception that it scintillated more strongly than stars of the
+first magnitude. Its brightness was greater than that of Sirius, α Lyræ,
+or Jupiter. For splendour, it was only comparable to Venus when nearest
+to the earth (that is, when only a quarter of her disc is illuminated).
+Those gifted with keen sight could, when the air was clear, discern the
+new star in the day-time, and even at noon. At night, when the sky was
+overcast, so that all other stars were hidden, it was often visible
+through the clouds, if they were not very dense (_nubes non admodum
+densas_). Its distances from the nearest stars of Cassiopeia, which
+throughout the whole of the following year I measured with great care,
+convinced me of its perfect immobility. Already, in December, 1572, its
+brilliancy began to diminish, and the star gradually resembled Jupiter,
+but by January, 1573, it had become less bright than that planet.
+Successive photometric estimates gave the following results: for
+February and March, equality with stars of the first magnitude
+(_stellarum affixarum primi honoris_—for Tycho Brahé seems to have
+disliked Manilius’ expression of _stellæ fixæ_); for April and May, with
+stars of the second magnitude; for July and August, with those of the
+third; for October and November, those of the fourth magnitude. Towards
+the month of November, the new star was not brighter than the eleventh
+in the lower part of Cassiopeia’s Chair. The transition to the fifth and
+sixth magnitude took place between December, 1573, and February, 1574.
+In the following month the new star disappeared, and, after having shone
+seventeen months, was no longer discernible to the naked eye.” (The
+telescope was not invented until thirty-seven years afterwards.)
+Humboldt adds:—“At its first appearance, as long as it had the
+brilliancy of Venus and Jupiter, it was for two months white, and then
+passed through yellow into red. In the spring of 1573, Tycho Brahé
+compared it to Mars; afterwards he thought it nearly resembled
+Betelgeuse, the star in the right shoulder of Orion. The colour for the
+most part was like the red tint of Aldebaran. In the spring of 1573, and
+especially in May, its white colour returned (_albedinam quandam
+sublividam induebat, qualis Saturni stellæ subesse videtur_). So it
+remained in January, 1574; being, up to the time of its entire
+disappearance in the month of March, 1574, of the fifth magnitude, and
+white, but of a duller whiteness, and exhibiting a remarkably strong
+scintillation in proportion to its faintness.”
+
+[Illustration:
+
+  FIG. 8.—_The Temporary Star of 1572._
+
+  (From “Planetary and Stellar Studies.”)
+]
+
+According to a sketch of the position given in Tycho Brahé’s work,
+referred to above, the star was situated a little to the north of Kappa
+Cassiopeiæ, the faintest star in the Chair. This position is confirmed
+by Argelander’s examination of Tycho Brahé’s observations: The spot is a
+rather blank one to the naked eye, and even with an opera-glass, only a
+few faint stars are visible. Quite close to the place fixed by
+Argelander, d’Arrest observed in 1865 a star of the eleventh magnitude,
+which seems to have escaped Argelander’s notice. Hind and Plummer
+observed this small star in 1873, and thought they could detect
+fluctuations in its light to the extent of about one magnitude. Espin
+has also observed it, and the region has been photographed by Dr.
+Roberts. Some have thought that Tycho Brahé’s star might possibly be
+identical with the Star of Bethlehem, and this idea has been supported
+by Cardanus, Chladni, and Klinkerfues, but Lynn and Sadler have shown
+that the theory is quite untenable, and it has now been rejected by all
+astronomers.
+
+Ma-tuan-lin speaks of a star in 1578 “as large as the sun”(!) but does
+not state its position.
+
+The star known as P (34) Cygni is sometimes spoken of as a “Nova,” or
+new star; but it is still visible to the naked eye as a star of the
+fifth magnitude. It was observed of the third magnitude by Jansen in
+1600 and by Kepler in 1602. After the year 1619, it appears to have
+diminished in brightness, and is said to have vanished in 1621; but it
+may merely have become too faint to be seen with the naked eye. It was
+again observed of the third magnitude by Dominique Cassini in 1655, and
+it afterwards disappeared. It was again seen by Hevelius in November,
+1665. In 1667, 1682, and 1715, it is recorded as of the sixth magnitude,
+and there is no further record of any marked increase in its light. A
+period of about 18 years was assumed by Pigott; but this is now
+disproved, and it seems probable that the star is a variable of
+irregular period and fitful variability, and not, properly speaking, a
+temporary star. Its present colour is yellow, and bright lines have been
+seen in its spectrum.
+
+Another remarkable object of the temporary class was observed by Kepler
+in 1604 in Ophiuchus, and is described by him in his work, “De Stella
+Nova in pede Serpentarii.” He and his assistants were observing the
+planets Mars, Jupiter, and Saturn, which were then near each other in
+this region of the heavens, a few degrees to the south-east of the star
+Eta Ophiuchi, and on the evening of October 10, Brunowski, a pupil of
+Kepler’s, noticed that a new and very brilliant star was added to the
+group[122]. When first seen, it was white, and exceeded in brightness
+Mars and Jupiter, but seems not to have quite equalled Venus in
+brilliancy. It slowly diminished, and in January, 1605, it was brighter
+than Antares but less than Arcturus. At the end of March, 1605, it had
+faded to the third magnitude. Its proximity to the sun then prevented
+further observations for several months. In March, 1605, it had
+disappeared to the naked eye. It was also observed by Galileo and by
+David Fabricius, whose observations place it about midway between the
+fifth magnitude star Xi and 58 Ophiuchi. Its exact position, however,
+does not seem to be known with such accuracy as that of Tycho Brahé’s
+star, nor is there any known star very close to the spot indicated by
+Schönfeld from an examination of Fabricius’ observations. It seems
+possible that Kepler’s star may have been seen previously by Ptolemy,
+for in his catalogue he gives a star of the fourth magnitude close to
+the position of Kepler’s star; but there is some doubt about the exact
+position indicated by Ptolemy. The Chinese annals mention a “ball-like”
+star as having appeared near Pi Scorpii on September 30, 1604, and
+remaining visible until March, 1606, which may possibly be identical
+with Kepler’s star.
+
+A new star of the third magnitude was observed near Beta Cygni by the
+Carthusian monk Anthelmus in 1670. It remained visible for about two
+years, and is said to have increased and diminished several times before
+its final disappearance. Schönfeld computed its exact position from
+observations made by Hevelius and Picard. Quite close to the spot
+indicated, a star of the eleventh magnitude has been observed at the
+Greenwich Observatory, and fluctuations of light were suspected in this
+small star by Hind and others. Hind says that, to his eye, “there is a
+hazy, ill-defined appearance about it which is not perceptible in other
+stars in the same field of view. Mr. Talmage received the same
+impression; and I may add that Mr. Baxendell, who has examined it with
+Mr. Worthington’s reflector, observed that no adjustment of focus would
+bring the star up to a sharp focus.” This hazy appearance is very
+suggestive, as it indicates that the “Nova” may possibly have faded into
+a small planetary nebula, as in the case of the new star in Cygnus,
+observed by Schmidt in 1876, and the new star in Auriga, found by Dr.
+Anderson in 1892. Near the position of Anthelm’s new star is a known
+variable star, S Vulpeculæ, discovered by Hind in 1861, which might be
+suspected to be identical with Anthelm’s star; but Hind has shown that
+the variable has no proper motion which would account for the difference
+of position since 1670, and he concludes that, “from the fixity of its
+position during eight years, it may be inferred that the variable is
+distinct from Anthelm’s.” It has been supposed that the star 11
+Vulpeculæ in Flamsteed’s catalogue is identical with Anthelm’s star; but
+Baily could not find any evidence to show that Flamsteed’s star ever
+really existed, and he says: “Under the presumption, however, that it
+may be a variable and not a _lost_ star, I have preserved its recorded
+position with a view of inducing astronomers to look out for it from
+time to time.”
+
+On the evening of April 28, 1848, Hind, observing at Mr. Bishop’s
+private observatory, in Regent’s Park, London, noticed a new star of
+about the fifth magnitude, between Zeta and Eta Ophiuchi. Its colour was
+reddish-yellow, and it seems to have subsequently increased in
+brightness to nearly the fourth magnitude, but it soon faded to the
+tenth or eleventh magnitude. This curious object has become very faint
+in recent years. In 1866, it was of the twelfth magnitude, and in 1874
+and 1875, not above the thirteenth.
+
+On May 28, 1860, Pogson discovered a new star in the globular cluster,
+80 Messier, which lies between Antares and Beta Scorpii. When first
+noticed, it was about the seventh magnitude, and its brightness was
+sufficient to obscure the cluster. In other words, the cluster was
+apparently replaced by a star. On June 10, the star had nearly
+disappeared, and the cluster again shone with great brilliancy, and with
+a condensed centre. The observations of Auwers and Luther confirm those
+of Pogson. Pogson states that he examined the cluster on May 9, but
+noticed nothing peculiar; and, according to Schönfeld, the cluster
+presented its usual appearance on May 18, when examined at the
+Königsberg Observatory. The apparition of the temporary star was,
+therefore, probably sudden, as in the case of other “new” stars. The
+phenomenon was possibly caused by a collision between two of the stars
+composing the cluster, which is, at least, apparently very condensed.
+
+A very remarkable star, sometimes called the “Blaze Star,” suddenly
+appeared in Corona Borealis, in May, 1866. It was first seen by the late
+Mr. Birmingham, at Tuam, Ireland, about midnight, on the evening of May
+12, when it was of the second magnitude, and equal to Alphecca, “the gem
+of the coronet.” Its appearance must have been very sudden, for Schmidt,
+the Director of the Athens Observatory, stated that he was observing the
+constellation on the same evening, about 2½ hours previous to
+Birmingham’s discovery, and observed nothing unusual. He was certain
+that no star, of even the fifth magnitude, could possibly have escaped
+his notice. On the following night it was seen by several observers in
+different parts of the world. M. Faye, the French astronomer, in his
+work—“L’Origine du Monde”—attributes the discovery to M. Courbebaisse, a
+French engineer, and does not mention Mr. Birmingham! He says M.
+Courbebaisse first saw it on the evening of May 13. This may be true; he
+was not the only observer who saw it on that evening; but it was,
+undoubtedly, _first_ seen by Mr. Birmingham on the _preceding_ night,
+and to Mr. Birmingham alone is certainly due the credit of the
+discovery. The star rapidly diminished in brightness, and on May 24 of
+the same year, had faded to 8½ magnitude. It afterwards increased to
+about 7·8 magnitude, but soon diminished again. Soon after its discovery
+it was found that the star was not really a new one, as it had been
+previously observed at Bonn by Schönfeld, in May, 1855, and March, 1856,
+while making the observations for Argelander’s _Durchmusterung_, in
+which it appears as No. 2765, in degree 26. On both occasions it was
+rated as 9½ magnitude, and no suspicion of variable light seems to have
+arisen. When viewed with the naked eye at the time of its greatest
+brilliancy, it was remarked by some observers that it twinkled decidedly
+more than other stars in the vicinity, and that this peculiarity made it
+very difficult to form a correct estimation of its relative brilliancy
+During the years 1866 to 1876, fluctuations in its light were observed
+by Schmidt, and he deduced a probable period of about 94 days, with a
+variation from the seventh to the ninth magnitude. This conclusion was
+confirmed by Schönfeld, and the star would therefore seem to be an
+irregular variable, and not a true temporary star.
+
+A very remarkable and interesting variable star was discovered by
+Schmidt at Athens, near Rho Cygni, on the evening of November 24, 1876,
+when it was about the third magnitude, and somewhat brighter than Eta
+Pegasi. Schmidt stated that he had observed the vicinity on several
+occasions between November 1 and 20, and was certain that no star of
+even the fifth magnitude could possibly have escaped his notice, so that
+the star probably blazed out very suddenly, as most of these
+extraordinary objects have done. Between November 20 and 24, the sky was
+overcast, so the exact time of its appearance is unknown. The star would
+seem to be quite new, as there is no star in any of the catalogues in
+the position of the “Nova,” the nearest being one of the ninth
+magnitude, which occurs in the Bonn observations. The new star rapidly
+faded, and on November 30 had descended to the fifth magnitude. On the
+night of its discovery it was remarked that its brightness was such as
+to render its near neighbour, 75 Cygni (a sixth magnitude star),
+invisible; while on December 14 and 15, 75 Cygni, in its turn, nearly
+obliterated the light of the stranger. In the 48 hours following the
+night of November 27, the star diminished in light to the extent of
+nearly 1½ magnitude! It afterwards faded very regularly to August, 1877,
+and showed no oscillations of brightness as have been observed in other
+temporary stars. On the evening of its discovery, Schmidt considered the
+star to be of a strong golden-yellow, and that it afterwards remained of
+a deep golden-yellow, but at no time was it as ruddy as 75 Cygni. I
+could see no trace of colour in the star with a 3-inch telescope in the
+Punjab on January 12, 1877, but it had then faded to the eighth
+magnitude. On February 7, 1877, I estimated it ninth magnitude. A few
+days after its discovery, it was examined with the spectroscope, and its
+spectrum showed bright lines similar to the “Blaze Star” in Corona,
+which appeared in May, 1866. One of the bright lines was thought to be
+identical with the line numbered 1474 by Kirchoff, visible in the
+spectrum of the solar Corona during total eclipses of the sun. The other
+bright lines were identified by M. Cornu of the Paris Observatory with
+some of the lines of hydrogen, sodium, and magnesium. In September,
+1877, the star was examined with a 15-inch refractor by Lord Lindsay
+(now Lord Crawford), who found “the light coming from it almost entirely
+monochromatic, that is, of only one colour, the star appearing exactly
+the same as when looked at without the spectroscope, the direct prism
+having no effect on it,” and he considers that “there is little doubt
+that the star has changed into a planetary nebula of small angular
+diameter!” On September 3, the star’s magnitude was 10½; “faint blue,
+near another star of same size rather red.” Lord Crawford remarks that
+no observer, discovering the object in its present state, would, after
+viewing it through a prism, hesitate to pronounce as to its nebulous
+character,[123] but no disc was detected with powers ranging up to 1000
+diameters. Ward found the star only sixteenth magnitude in October,
+1881, and it was estimated fifteenth magnitude at Mr. Wigglesworth’s
+Observatory in September, 1885. At Lord Crawford’s Observatory the exact
+position of the star, with reference to above fifty closely adjacent
+stars, was carefully determined with the micrometer. The vicinity was
+photographed by Dr. Roberts on September 27, 1891, with an exposure of
+two hours, and “the _Nova_ appears as a star of about the thirteenth
+magnitude.” Observations in 1894 and 1895, made its magnitude about
+14·8, with an apparently continuous spectrum.[124]
+
+In August, 1885, a star of about the seventh magnitude made its
+appearance close to the nucleus of the Great Nebula in Andromeda
+(Messier 31), a remarkable nebula, which will be described in the next
+chapter. The new star was independently discovered by several observers
+towards the end of August. It was not visible to Tempel at the Florence
+Observatory on August 15 and 16, but is said to have been seen by M.
+Ludovic Gully on August 17. It was, however, certainly seen by Mr. I. W.
+Ward at Belfast on August 19, at 11 P.M., when he estimated it 9½
+magnitude, and it was independently detected by the Baroness Podmaniczky
+on August 22, by M. Lajoye on August 30, by Dr. Hartwig, at Dorpat, on
+August 31, and by Mr. G. T. Davis, at Theale, near Reading, on September
+1. On September 3, the star was estimated 7½ magnitude by Lord Crawford
+and Dr. Copeland, and its spectrum was found to be “fairly continuous.”
+On September 4, Mr. Maunder, at the Greenwich Observatory, found the
+spectrum “of precisely the same character as that of the nebula, _i.e._,
+it was perfectly continuous, no lines, either bright or dark, being
+visible, and the red end was wanting.” Dr. Huggins, however, on
+September 9, thought he could see a few bright lines in its spectrum, a
+continuous spectrum being visible from the line D to F. The star
+gradually faded away. On December 10, 1885, it was estimated of the
+fourteenth magnitude at the Radcliffe Observatory, Oxford, and on
+February 7, 1886, it was rated only sixteenth magnitude with the 26-inch
+refractor of the Washington Observatory. A series of measures by
+Professor Hall, from September 29, 1885, to February 9, 1886, showed “no
+certain indications of any parallax,” so that the star and the nebula,
+in which it probably lies, are evidently situated at a vast distance
+from the earth. Seeliger has investigated the decrease in the light of
+the star on the hypothesis that it was a cooling body, which had been
+suddenly raised to an intense heat by the shock of a collision, and
+finds a fair agreement between theory and observation. Auwers points out
+the similarity between this outburst and the new star of 1860, in the
+cluster 80 Messier (already described), and thinks it probable that both
+phenomena were caused by physical changes in the nebulæ in which they
+occurred. Proctor considered that the evidence of the spectroscope shows
+that the new star was situated _in_ the nebula, and in this opinion I
+fully concur.
+
+Several temporary stars have been detected in recent years by Mrs.
+Fleming, from an examination of photographs of stellar spectra, taken at
+the Harvard Observatory, for the Draper Memorial. Plates of the
+constellation Perseus show the existence of a star in 1887, the spectrum
+of which shows the bright lines of hydrogen, and it was on this account
+assumed to be a long period variable. During the following eight years,
+however, 81 photographs of the same region show no trace of the star,
+and it has been frequently looked for with a telescope, but without
+success. It would, therefore, seem probable that the star was a
+temporary one. Its magnitude was about the ninth.
+
+A remarkable and very interesting temporary star was discovered in 1892
+in the constellation Auriga. On February 1, of that year, an anonymous
+post-card was received by Dr. Copeland at the Royal Observatory,
+Edinburgh, with the following announcement:
+
+“Nova in Auriga. In Milky Way, about two degrees south of χ Aurigæ,
+preceding 26 Aurigæ. Fifth magnitude, slightly brighter than χ.”
+
+Such an announcement evidently required immediate attention, and on that
+evening, Dr. Copeland and his assistants looked for the new star, and
+easily found it with an opera-glass at 6 hours 8 minutes. They estimated
+it of the sixth magnitude, and equal to 26 Aurigæ. It was of a yellow
+colour. When examined with a prism placed before the eye-piece of a
+24-inch reflector, its spectrum was seen to resemble the “Blaze Star” of
+1866 in Corona. “The C line was intensely bright, a yellow line about D
+fairly visible; four bright lines, or bands, were conspicuous in the
+green; and, lastly, a bright line in the violet (probably Hγ) was easily
+seen.” Notice of the discovery was at once telegraphed to Greenwich and
+Keil Observatories, and the star was photographed at Greenwich on the
+same night. It is not in the Bonn star charts, which show stars to
+nearly the tenth magnitude. In _Nature_ of February 18, 1892, a letter
+appeared, signed Thomas D. Anderson, in which the writer stated that the
+post-card was sent by him, and he gives the following details respecting
+the discovery:
+
+“Prof. Copeland has suggested to me that as I am the writer of the
+anonymous post-card mentioned by you a fortnight ago (p. 325), I should
+tell your readers what I know about the Nova.
+
+“It was visible as a star of the fifth magnitude certainly for two or
+three days, very probably even for a week, before Prof. Copeland
+received my post-card. I am almost certain that at two o’clock on the
+morning of Sunday, the 24th ult., I saw a fifth magnitude star making a
+very large obtuse angle with β Tauri and χ Aurigæ, and I am positive
+that I saw it, at least, twice subsequently during that week.
+Unfortunately, I mistook it on each occasion for 26 Aurigæ, merely
+remarking to myself that 26 was a much brighter star than I used to
+think it. It was only on the morning of Sunday, the 31st ult., that I
+satisfied myself that it was a strange body. On each occasion of my
+seeing it, it was slightly brighter than χ. How long before the 24th
+ult. it was visible to the naked eye I cannot tell, as it was many
+months since I had looked minutely at that region of the heavens.
+
+“You might also allow me to state, for the benefit of your readers, that
+my case is one that can afford encouragement to even the humblest of
+amateurs. My knowledge of the technicalities of astronomy is,
+unfortunately, of the most meagre description; and all the means at my
+disposal on the morning of the 31st ult., when I made sure that a
+strange body was present in the sky, were Klein’s ‘Star Atlas’ and a
+small pocket-telescope, which magnifies ten times.”
+
+Soon after the discovery of the new star, an examination was made by
+Professor Pickering of photographs taken of the region at Harvard
+Observatory, previous to Dr. Anderson’s discovery. It was found that on
+eighteen photographs taken between the dates November 3, 1885, and
+November 2, 1891, there is no trace of the new star; but in those taken
+from December 16, 1891, to January 31, 1892, a star of the fifth
+magnitude is shown in the position of the new star. “In another series
+of plates taken with the transit photometer, no record of the new star
+up to December 1, 1891, was obtained, although χ Aurigæ (magnitude 5·0)
+was always visible, but the plates taken on the nights of December 10,
+1891, and ending January 20, 1892, indicated clearly the position of the
+new star.” Professor Pickering says: “It appears that the star was
+fainter than the eleventh magnitude on November 2, 1891, than the sixth
+magnitude on December 1, and that it was increasing rapidly on December
+10. A graphical construction indicates that it had probably attained the
+seventh magnitude within a day or two of December 2, and the sixth
+magnitude on December 7. The brightness increased rapidly until December
+18, attaining its maximum about December 20, when its magnitude was 4·4.
+It then began to decrease slowly, with slight fluctuations, until
+January 20, when it was slightly below the fifth magnitude. All these
+changes took place before its discovery, so that it escaped observation
+nearly two months. During half of this time it was probably brighter
+than the fifth magnitude.”
+
+It would seem from the above remarks that the star did not—like some
+other temporary stars—attain its full brilliancy at once, but increased
+gradually in brightness. After the decrease of light in January, 1892,
+it seems to have again risen to another maximum, for photographs taken
+at the Greenwich Observatory after its discovery show that the star rose
+to a magnitude of 3·5 (photographic) on February 3, and then began to
+fade again slowly during February, but rapidly during the month of
+March. Owing to cloudy weather in the west of Ireland, I could not
+observe the new star until February 14. The following are my
+observations, made with a binocular field-glass, the comparison stars
+being Chi Aurigæ, 26 Aurigæ, and D M + 30°, 898:—February 14, 4·55
+magnitude; February 15, 5·56; February 16, 5·84; February 18, 5·51;
+February 21, 5·56; February 24, 5·66; February 28, 5·44; March 1, 5·68;
+March 5, 5·66; March 10, 7·3; March 11, 7¾; March 16, 8½, or fainter;
+March 18, 9 magnitude, or less, “only _very_ faint stars seem near the
+place of the Nova; clear sky, no moon.” The general accuracy of the
+above observations were confirmed by the photographic estimates of the
+star’s light made at Greenwich,[125] and also by Schaeberle’s
+observations of its brightness.
+
+After March 18, the light of the star steadily and rapidly decreased,
+and on April 1, it had faded to nearly the fifteenth magnitude, and
+afterwards to about the sixteenth. In August, 1892, it brightened again,
+as it was found by Corder of about the ninth magnitude on August 21. Dr.
+J. Holetschek of the Vienna Observatory observed it from August 24 to
+September 2, 1892, and estimated it about 9½ magnitude. In October,
+1892, most observers rated it between 10 and 10½ magnitude. Observations
+by Mr. C. E. Peck, “from October 3, 1893, to May 4, 1894, only vary from
+10·1 to 11·0 magnitude, and observations up to the end of 1894 give the
+same results.”[126] In 1895 Professor Barnard found that it “is still
+visible as a small star, and has not changed in physical appearance
+since the autumn of 1892. It remains perfectly fixed with reference to
+the comparison stars.”[127]
+
+Examined with the spectroscope soon after its discovery, many bright
+lines were seen in its spectrum, and it was found that “the bright lines
+in the spectrum of the new star were accompanied by dark ones on their
+more refrangible sides,” that is, the dark lines were on the blue side
+of the bright ones. This suggested the idea that the outburst was
+probably due to a collision between two bodies, one of which, having a
+spectrum of dark lines, was rushing towards the earth, and the other,
+with a bright-line spectrum, was receding. Lockyer supposed the outburst
+to be due to a collision between two swarms of meteorites. Dr. Huggins
+advanced the view that the phenomenon was due to the near approach of
+two gaseous bodies. “But,” he says, “a casual near approach of two
+bodies of great size would be a greatly less improbable event than an
+actual collision. The phenomena of the new star scarcely permits us to
+suppose even a partial collision, though, if the bodies were diffused
+enough, or the approach close enough, there may have been, possibly,
+some interpenetration and mingling, of the rare gases near the
+boundaries.” But Maunder and Seeliger consider this hypothesis to be
+untenable. Mr. Monck suggested that a star or swarm of meteorites
+rushing through a gaseous nebula might explain the phenomena. Seeliger
+advocates a similar theory. Maunder also favours a collision theory.
+
+A photograph of the spectrum taken by Maunder on February 22, 1892 (when
+the photographic magnitude was 4·78, and visual magnitude about 5·7),
+showed a displacement of the dark lines, which implied a relative motion
+of the two supposed colliding bodies of about 820 miles a second! Vogel
+found that the bright lines showed a double maxima, and he thought that
+these were due to “two different bodies moving with different
+velocities, so that the spectrum of the Nova consists of, at least,
+three spectra superposed. The measurement of the photograph gives the
+body showing the dark line spectrum as approaching the earth with a
+speed of nearly 420 miles per second, one of the two bright line bodies
+as approaching with a speed of 22 miles, whilst the other is receding
+with a speed of 300 miles a second.”[128]
+
+At the time of its increase of brightness, in August, 1892, Professor
+Barnard, observing it with the great 36-inch Lick telescope, says, the
+“Nova appeared as a small, bright nebula, with a star-like nucleus of
+the tenth magnitude. The nebulosity was pretty bright and dense, and was
+3″ in diameter. Surrounding this was a fainter glow, perhaps half a
+minute in diameter.” At this time, Professor Campbell of the Lick
+Observatory found that its spectrum showed the characteristic nebular
+lines. This observation was confirmed by Dr. Copeland on August 25 and
+26, and by Herr Gothard, who photographed the spectra of a number of
+nebulæ, and compared them with his photograph of the spectrum of the new
+star. He says, “Each new photograph increased the probability, which may
+be considered as a proved fact, that the _spectrum not only resembles,
+but that the aspect and position of the lines show it to be identical
+with the spectra of the planetary nebula_. In other words, the new star
+has changed into a planetary nebula.”
+
+A nebulous spectrum was also found by Espin. From observations of the
+spectrum in November, 1894, Professor Campbell finds that “the spectrum
+is not only nebular, but it is approaching the average type of nebular
+spectrum,” and he adds, “We may say that only five ‘new stars’ have been
+discovered since the application of the spectroscope to astronomical
+investigations, and that three of these had substantially identical
+spectroscopic histories.” Espin found the star distinctly nebulous on
+December 9, 1895, and its magnitude about 10½.
+
+Another new star was discovered by Mrs. Fleming by the photographic
+method in the southern constellation, Norma, in the year 1893. When at
+its brightest, it seems to have been about the seventh magnitude. It was
+situated in the Milky Way, a little to the east of the pair of stars
+known as Gamma one and Gamma two Normæ. Its spectrum was similar to that
+of the new star in Auriga, when it first appeared, and, like that
+object, the spectrum has now, according to Professor Campbell, “become
+distinctly nebular.”
+
+Another temporary star of about the eighth magnitude was also discovered
+by Mrs. Fleming in 1895, in that portion of the southern constellation
+Argo, known as Carina. It was in or close to the Milky Way—like so many
+of these new stars—between the variable star Eta Argûs and the star
+Lambda Centauri, near the Southern Cross, and close to a star of
+magnitude 5½. The photographic plates on which the discovery was made
+were taken at the Arequipa Station, in Peru. An examination of 62
+photographs of the region showed no trace of the star on May 17, 1889,
+and March 5, 1895, although stars so faint as the fourteenth magnitude
+are visible on some of the plates. On nine plates, however, taken
+between April 8, 1895, and July 1, 1895, the star is visible, and during
+this interval the brightness diminished from the eighth to the eleventh
+magnitude. The spectrum showed the bright lines of hydrogen “accompanied
+by dark lines of slightly shorter wave-length,” and in all its
+“essential features” was “apparently identical” with the spectra of the
+temporary stars in Auriga and Norma.
+
+With reference to this outburst, and the similarity of the star’s
+spectrum to that of the new star in Auriga, Professor William H.
+Pickering points out “the improbability of two successive collisions
+between stars, occurring nearly in the line of sight, in both cases a
+bright and a dark line star being involved, and in each case the
+bright-line star being the one to recede from us. The same remark
+applies to the theory of a collision of a star and a nebula. As a
+substitute I offered an explosion hypothesis, in which a dark sun
+suddenly gave out in all directions large quantities of hydrogen in an
+incandescent state. This would, of course, merely produce a spectrum
+with bright lines. But if the expulsion of hydrogen continued, the outer
+layers of gas would cool, producing absorption lines in the spectrum of
+the approaching hydrogen, but still leaving the spectrum lines of the
+receding hydrogen bright. Finally, when the expulsion ceased, we should
+find a heated spherical mass of gas, similar to a planetary nebula. It
+was shown that the velocities which were observed in the cases of these
+two _novæ_ were less than fifty per cent. greater than had been observed
+in our own sun. The discovery of this third _nova_, with a spectrum
+identical with that of the two others, increases many times the
+improbability of the collision theories, and thereby strengthens the
+explosion hypothesis. If this latter is correct, we must look upon the
+phenomena presented by a _nova_ not as indicating the birth of a new
+star, but rather as a cataclysm testifying to the death and final
+disrupture of an old one.”[129]
+
+Another apparently new star was detected by Mrs. Fleming in 1895, in the
+constellation Centaurus. It was situated about three degrees north-west
+of the double star 3 Centauri, and when at its brightest, seems to have
+been about the seventh magnitude. Mrs. Fleming’s attention was first
+directed to it by its peculiar spectrum, as shown on a photographic
+plate taken at Arequipa in July, 1895. No trace of the star is visible
+on 55 plates taken from May 21, 1889, to June 14, 1895, but on plates
+taken on July 8 and 10, 1895, it appears of about the seventh magnitude.
+A photograph taken on December 16, 1895, shows it as a star of about the
+eleventh magnitude. On that date, and on December 19, it was seen about
+the same magnitude by Mr. O. C. Wendell, with a 15-inch telescope. The
+spectrum at first resembled that of the nebula 30 Doradus, and was
+unlike the spectra of the temporary stars in Auriga, Norma, and Carina.
+When it had faded to the eleventh magnitude, its spectrum seemed to be
+monochromatic, and very similar to that of a neighbouring nebula, N G C
+5253, so that, like the new stars in Cygnus, Auriga, and Norma, “it
+appears to have changed into a gaseous nebula.”
+
+It is a remarkable fact that the great majority of the temporary stars
+appeared in or near the Milky Way. The chief exceptions to this rule
+are:—the star of 76 B. C., in the Plough, the star recorded by
+Hepidannus in Aries, 1012, A.D., and the “Blaze Star” of 1866 in Corona
+Borealis.
+
+
+
+
+                              CHAPTER VI.
+                          CLUSTERS AND NEBULÆ.
+
+
+Clusters of stars and nebulæ are frequently classed together in one
+group. But this is incorrect. The term nebulæ should be restricted to
+those objects which the spectroscope shows to consist of gaseous matter,
+while the term cluster should be applied to those groups of stars in
+which the components are individually visible as distinct star-like
+points. There may be, of course, intermediate forms, like the Great
+Nebula in Andromeda, which, although not resolvable into stars with
+powerful telescopes, the spectroscope shows to be not gaseous. We will
+begin with clusters of stars, many of which can be seen with telescopes
+of moderate power, and some, like the Pleiades, even with the naked eye.
+
+The Pleiades form perhaps the most remarkable group of stars in the
+heavens, and are probably familiar to most people, even to those whose
+knowledge of the constellations is limited to a few of the brighter
+stars. The cluster is a very remarkable and brilliant one, and forms a
+striking object in a clear sky. There is no other group visible to the
+naked eye in either hemisphere similar to it in the brightness and
+closeness of the component stars. It seems to have attracted the
+attention of observers since the earliest ages. Job says: “Can’st thou
+bind the sweet influences of Pleiades, or loose the bands of Orion?”
+
+Hesiod, writing nearly 1,000 years B.C., speaks of the Pleiades in words
+thus translated by Cooke:—
+
+            “There is a time when forty days they lie,
+            And forty nights conceal’d from human eye;
+            But in the course of the revolving year,
+            When the swain sharps the scythe, again appear.”
+
+This passage refers to the disappearance of the group in the sun’s rays
+in summer, and their reappearance in the evening sky in the east at
+harvest time. Hesiod also speaks of them as the seven sisters, and in
+Cicero’s “Aratus,” they are represented as female heads, bearing the
+names Merope, Alcyone, Celæno, Electra, Taygeta, Asterope, and Maia,
+names by which they are still known to astronomers. The origin of the
+name Pleiades is somewhat doubtful. Some think that it is derived from
+the Greek word _pleia_, to sail. Others from the words _pleios_, full, a
+name perhaps suggested by the appearance of the cluster. Although seven
+stars are mentioned by Hipparchus and Aratus, Homer only speaks of six,
+and this is the number now visible to average eyesight. A larger number
+has, however, been seen with the naked eye by those gifted with
+exceptionally keen eyesight. Möstlin, Kepler’s tutor, is said to have
+seen fourteen, and he actually measured and recorded the position of
+eleven, with wonderful accuracy, without the aid of a telescope! In
+recent years, Miss Airy, daughter of the late astronomer-royal, has seen
+twelve, and Carrington and Denning fourteen. But to most eyes probably
+six only are visible with any certainty. There is a tradition that,
+although seven stars were originally visible, one disappeared at the
+taking of Troy. Professor Pickering has recently discovered that the
+spectrum of Pleione, which forms a wide pair with Atlas, bears a
+striking resemblance to that of P Cygni, the so-called “temporary star”
+of 1600. This similarity of spectra suggests the idea that Pleione may
+possibly—like the star in Cygnus—be subject to occasional fluctuations
+of light, which might perhaps account for its visibility to the naked
+eye in ancient times.
+
+The grouping of even six stars visible to the naked eye in so small a
+space is very remarkable. Considering the total number of stars visible
+without optical aid, Mitchell—writing in 1767—calculated by the
+mathematical theory of probability that the chances are 500,000 to one
+against the close arrangement of six stars in the Pleiades being merely
+the result of accident. He therefore concludes “that this distribution
+was the result of design, or that there is reason or cause for such an
+assemblage.”
+
+Although to a casual observer the component stars may appear of merely
+equal magnitude, there is considerable difference in their relative
+brilliancy. Measures with a photometer show that Alcyone—the brightest
+of the group—is of the third magnitude, Maia, Electra, and Atlas of the
+fourth, Merope about 4⅓, Taygeta 4½, Celæno about 5⅓, and Asterope about
+the sixth. Pleione is about 5½, according to the photometric measures
+made at Oxford, but it lies so close to Atlas that to most eyes the two
+will probably appear as one star. About thirty more range from the sixth
+to the ninth magnitude, and this is about the number visible with an
+opera-glass. Galileo counted thirty-six stars with his small telescopes,
+but with modern instruments the number is largely increased. Some years
+since, M. Wolf, the distinguished French astronomer, published a chart
+of the Pleiades, showing about 500 stars made from his own observations.
+Photography has further added to the number of stars visible in this
+interesting group. On a photograph taken at the Paris Observatory in
+1887, with an exposure of three hours, no less than 2,326 stars can be
+distinctly counted on a space of about three square degrees. The fainter
+stars on this photograph are supposed to be of the seventeenth
+magnitude. Now, as Alcyone, the brightest star of the group, is of the
+third magnitude, we have a difference of fourteen magnitudes between the
+brightest and the faintest. This implies that Alcyone is 398,100 times
+brighter than the faintest stars visible on the photographic plate. If
+we could conclude that the fainter stars really belonged to the cluster,
+they would be at practically the same distance from the earth, and the
+great difference of brightness would be very remarkable, and would
+suggest that Alcyone is a vastly larger body than the smallest stars of
+the group. The difference of brilliancy given above would indicate that
+the diameter of Alcyone is 631 times greater than that of the faintest
+stars revealed by photography. This is of course on the assumption that
+all the stars of the cluster are, surface for surface, of the same
+intrinsic brilliancy, and that this apparent brightness to the eye
+depends simply on their diameter. As spheres vary in volume as the cubes
+of their diameters, we have the volume of Alcyone equal to the cube of
+631, or over 250 million times the volume of the faintest stars of the
+group. This startling result was very difficult to explain, for either
+we must assume that Alcyone is an enormously vast body, or else that the
+faint stars of the group are exceedingly small. If we take the diameter
+of Alcyone as 1,400,000 miles, then the diameter of the faintest stars
+in the group would be only 2,200 miles, or about the size of our moon,
+and it seems highly improbable, if not impossible, that such small
+bodies should shine with inherent light of their own. They would indeed
+be “miniature suns.” On the other hand, if we assume that the faintest
+stars are of about the same size as the planet Jupiter, or about 87,000
+miles, the diameter of Alcyone would be nearly 55 millions of miles, a
+result which is also highly improbable. The difficulty has, I think,
+been satisfactorily cleared up by some photographs recently taken by
+Professor Barnard at the Lick Observatory. A photograph taken with a
+lens of six inches aperture, and 31 inches focal length, and an exposure
+of 10 hours 15 minutes, shows that the sky surrounding the Pleiades is,
+on all sides, as thickly studded with small stars as the cluster itself.
+It seems clear, therefore, that the faint stars in the Pleiades are
+merely some of the “hosts of heaven” which happen to lie in that
+direction, and have probably no connexion with the cluster, which is
+merely projected on a starry background of faint and distant stars.
+
+The brilliancy of the Pleiades cluster would naturally suggest a
+comparative proximity to the earth. Attempts to determine their distance
+have, however, hitherto proved unsuccessful. This would indicate that
+the distance is very great, and would, of course, lead to the conclusion
+that the group is of vast dimensions. An effort has been made to
+determine the distance indirectly by a consideration of the “proper
+motion” of the principal stars. Professor Newcomb finds a proper motion
+for Alcyone of about 5·8 seconds of arc per century. This motion is in a
+direction nearly opposite to that of the sun’s motion in space, and may
+possibly be due to that cause. If we assume that this apparent motion of
+Alcyone is wholly due to the effect of the sun’s real motion at the rate
+of, say, fourteen miles a second, the distance of Alcyone would
+correspond to a “light journey” of about 267 years! Our sun, placed at
+this vast distance, would, I find, be reduced in brilliancy to a star of
+about the ninth magnitude, or six magnitudes fainter than Alcyone. This
+would imply that Alcyone is about 250 times brighter than the sun! As,
+however, the spectrum of Alcyone is of the first or Sirian type, it
+cannot properly be compared with the sun.
+
+There are six other small stars in the Pleiades having proper motions
+similar in amount and direction to that of Alcyone. As the other bright
+stars of the group have much smaller motions, it has been suggested that
+the seven stars with comparatively large, proper motions do not really
+belong to the group, but are only optically associated with it. This
+would imply that the real cluster lies much farther from us than
+Alcyone, and the comparative brilliancy of some of its component stars
+would still denote enormous size.
+
+In the year 1859, the well-known astronomer, Tempel, announced his
+discovery of a faint nebulosity extending in a southerly direction from
+Merope, the nearest bright star to Alcyone. This interesting discovery
+was practically confirmed by other astronomers; but from its visibility
+to some observers with small telescopes, and the failure of others to
+detect it with much larger instruments, the variability of its light was
+strongly suspected. The question remained in doubt for many years, but
+has now been finally set at rest by photography, which shows not only a
+mass of nebulous light surrounding Merope, but other nebulous spots
+involving Alcyone, Maia, and Electra. Indeed, a photograph taken by Dr.
+Roberts in 1889 shows that all the brighter stars of the group are more
+or less surrounded by nebulosity. The nebula surrounding Maia is of a
+somewhat spiral form, and its existence was not even suspected until it
+was revealed by photography. It was afterwards seen with the great
+30-inch refractor of the Pulkowa Observatory. Had, however, its
+existence been unknown, it would probably have escaped detection, even
+with this large telescope, as it is one thing to see a faint object
+known to exist and another to discover it independently. Maia is
+surrounded by several faint stars of the twelfth to the fourteenth
+magnitude; and the Russian observers believe that one of these is
+variable in light, as it was seen distinctly on February 5, 1886, when
+its magnitude was carefully determined with reference to the
+neighbouring stars; but on February 24 of the same year, it could not be
+seen with a telescope of 15 inches aperture. Some of the other stars in
+the group seem to be connected by nebulous rays with the principal
+nebulous centres, and in looking at this wonderful Paris chart it seems
+impossible to avoid the conclusion that the stars and nebulous masses
+are actually mixed up together, and not merely placed accidentally in
+the same direction. Indeed, Professor Barnard’s photograph referred to
+above shows the whole group involved in dense nebulosity.
+
+Other well-known clusters or groups of stars are the Hyades, marked by
+the bright, reddish star, Aldebaran, the Præsepe, or Beehive, in Cancer,
+and Comæ Berenices, but these are larger and more scattered.
+
+[Illustration:
+
+  FIG. 9.—_The Double Star Cluster in Perseus._
+
+  (From “Scenery of the Heavens.”)
+]
+
+Of other irregular clusters, somewhat similar to the Pleiades, but not
+so bright, may be mentioned the double cluster in Perseus, which is
+visible to the naked eye on a clear night as a hazy spot of light in the
+midst of the Milky Way. Admiral Smyth says they form “one of the most
+brilliant telescopic objects in the heavens.” They may be seen with a
+binocular field-glass, but, of course, a good telescope is necessary to
+see them well. They have been beautifully photographed at the Paris
+Observatory, the photograph showing no trace of nebulosity. They have
+also been photographed by Dr. Roberts, who says, “The photograph
+presents to the eye the stars in the two clusters, and in the
+surrounding parts of the sky, with a completeness and accuracy of detail
+never before seen. The stars are shown in their true relative positions
+and magnitudes to about the sixteenth, and among them are many apparent
+double, triple, and multiple stars. They also appear to be arranged in
+clusters, curves, festoons, and patterns that are suggestive of some
+physical connexion existing between the groups; but it is premature to
+assert that these appearances are not due to perspective effect by the
+eye arranging numerous close points of light into various patterns.
+Similar photographs to this, taken at intervals of several years between
+them, will determine the reality, or otherwise, of these remarkable
+groupings of the stars.”
+
+[Illustration:
+
+  FIG. 10.—_Star Cluster in Gemini._
+
+  (From “Scenery of the Heavens.”)
+]
+
+A little north of the star Eta Geminorum is a pretty cluster of small
+stars known as 35 Messier, which is just visible to the naked eye. The
+component stars may be well seen with a telescope of moderate power.
+This cluster has been also photographed at the Paris Observatory, and
+shows a well-marked clustering tendency in the component stars. Admiral
+Smyth says: “It presents a gorgeous field of stars from the ninth to the
+sixteenth magnitude, but with the centre of the mass less rich than the
+rest. From the small stars being inclined to form curves of three or
+four, and often with a large one at the root of the curve, it somewhat
+reminds one of the bursting of a sky rocket.”
+
+[Illustration:
+
+  FIG. 11.—_37 Messier._
+
+  (From “Worlds of Space.”)
+]
+
+About ten degrees to the north of the cluster just described is another
+fine cluster known as 37 Messier. The accompanying photograph will show
+its telescopic appearance.
+
+In the Southern Hemisphere there is a magnificent cluster of small stars
+surrounding the star Kappa Crucis, a reddish star of the seventh
+magnitude. It was thus described by Sir John Herschel: “A most vivid and
+beautiful cluster of 50 to 100 stars. Among the larger there are one or
+two evidently greenish. South of the red star is one, 13 minutes, also
+red, and near it one, 12 minutes, bluish ... though neither a large nor
+a rich one, is yet an extremely brilliant and beautiful object when
+viewed through an instrument of sufficient aperture to show distinctly
+the very different colours of its constituent stars, which gives it the
+effect of a superb piece of fancy jewellery.” He gives the positions of
+110 stars, from the seventh to the sixteenth magnitude. It lies near the
+northern edge of the well-known “coal sack,” and Dr. Gould says of it:
+“The exquisitely beautiful cluster, κ _Crucis_, contains a large number
+of stars of various tints and hues, contrasting wonderfully with each
+other, when viewed with a telescope of large aperture.” Mr. Russell’s
+drawing of this cluster, made at Sydney (N.S.W.) in 1872, shows several
+changes in the relative positions of the stars as laid down by Sir John
+Herschel, probably the result of proper motion.
+
+About 2½° north of the star M Velorum, Sir John Herschel describes “an
+enormous cluster, of a degree and a half in diameter, very rich in stars
+of all magnitudes, from 8 minutes downwards, a sort of telescopic
+Præsepe.”
+
+Another fine cluster is that known as 11 Messier. It lies a little to
+the west of the star Lambda Aquilæ, and is just visible to the naked eye
+on a clear night. It consists of stars of about the eleventh magnitude,
+and Admiral Smyth compared it to a “flight of wild ducks.” It has been
+beautifully photographed by Dr. Roberts, who says: “The negative shows
+the stars individually, though the print, owing to their closeness, does
+not separate them.... It is entirely free from nebulosity.”
+
+There are many other similar objects in both hemispheres too numerous to
+mention here, but those described are interesting objects of their
+class.
+
+[Illustration:
+
+  FIG. 12.—_Star Cluster in Hercules._
+
+  (From “Scenery of the Heavens.”)
+]
+
+We now come to the “globular clusters.” This term has been applied to
+those clusters of stars which evidently occupy a space of more or less
+spherical form. Some of these “balls of stars,” as they have been
+called, are truly wonderful, and are among the most interesting objects
+visible in the sidereal heavens. Good specimens of the class are,
+however, rather rare objects, and there are not many in the Northern
+Hemisphere. The most remarkable, perhaps, is that called “the Hercules
+cluster,” but known to astronomers as 13 Messier, it being No. 13 in the
+first catalogue of remarkable “nebulæ” formed by Messier, the famous
+discoverer of comets. It was discovered by Halley in 1714. This
+wonderful object lies between the stars Zeta and Eta in Hercules, nearer
+to the latter star. It may be seen with a binocular or good opera-glass
+as a hazy star of the sixth magnitude. Messier was certain that it
+contained no stars; but when examined with a good telescope it is at
+once resolved into a multitude of small stars, which can be individually
+seen, and even counted, with large telescopes. According to Admiral
+Smyth, “No plate can give a fitting representation of this magnificent
+cluster. It is indeed truly glorious, and enlarges on the eye by
+studious gazing.” And Dr. Nichol says: “Perhaps no one ever saw it for
+the first time through a telescope without uttering a shout of wonder.”
+The number of stars included in the cluster was estimated by Sir William
+Herschel at 14,000; but the real number is probably much smaller. Were
+the number so great as Herschel supposed, I find that the cluster would
+form a much brighter object than it does. Assuming the average magnitude
+of the component stars at 12½, I find that an aggregation of 14,000
+stars would shine as a star of about the second magnitude. But the
+cluster is only as bright as a star of about the sixth magnitude, and,
+with this magnitude, I find that the total number would be about 400.
+Examining it with his giant telescope, Lord Rosse observed three dark
+rifts radiating from the centre. These were afterwards seen by Buffham
+with a 9-inch reflector, and also by Webb. They were also observed at
+Ann Arbor Observatory (U.S.A.), in April, 1887, by Professor Harrington
+and Mr. Schaeberle, using telescopes of six and twelve inches aperture.
+It has been well photographed at the Paris Observatory, and also by Dr.
+Roberts and Mr. Wilson. In some of these photographs the dark rifts are
+perceptible to some extent, but owing to the over exposure of the
+central portion of the cluster, they are not so distinct as in drawings
+made at the telescope. Dr. Huggins, examining it with the spectroscope,
+finds that the spectrum is not gaseous; but spectroscopic evidence is
+not necessary to prove that the cluster consists of small stars, as
+these are distinctly seen as points of light with telescopes of moderate
+power, and with the great Lick telescope the component stars are visible
+even in the central portion of the cluster. Its globular shape is
+evident at a glance, and we cannot doubt that the stars composing it
+form a gigantic system, probably isolated in space. Many people might
+think that this cluster was a mass of double and multiple stars; but
+this is not so. The components, close as they are, are too far apart to
+constitute true double stars. Mr. Burnham, the famous double star
+observer, finds _one_ close double star near the centre, and notes the
+remarkable absence of close double stars in bright and apparently
+compressed clusters.
+
+In the same constellation, Hercules, between the stars Eta and Iota, but
+nearer the latter, will be found another object of the globular class,
+but not so bright or so easily resolvable into stars as the cluster
+described above. It is known as 92 Messier. Buffham, with a 9-inch
+mirror, thought the component stars brighter and more compressed than in
+13 Messier. Sir William Herschel found it seven or eight minutes of arc
+in diameter. The brighter components are easily visible in telescopes of
+moderate power, but even Lord Rosse’s giant telescope failed to resolve
+the central blaze. This object was photographed by Dr. Roberts in May,
+1891, with a 20-inch reflecting telescope, and an exposure of one hour.
+He says: “The photograph shows the cluster to be involved in dense
+nebulosity, which, on the negative, almost prevents the stars being seen
+through it, and on the print quite obscures the stars. The stars in
+this, as in all other globular clusters, are arranged in various
+patterns, and many of them appear to be nebulous.”
+
+About three degrees north preceding the star 9 Boötis, is another fine
+globular cluster, known as 3 Messier. Smyth describes it as “a brilliant
+and beautiful globular congregation of not less than 1,000 stars,
+between the southern Hound and the knee of Boötis; it blazes splendidly
+towards the centre, and has outliers.... This mass is one of those balls
+of compact and wedged stars, whose laws of aggregation it is so
+impossible to assign.” The idea of the component stars being “compact
+and wedged” is, however, a mistake, as I have shown elsewhere.[130] Sir
+John Herschel described it as a remarkable object, exceedingly bright
+and very large, with stars of the eleventh magnitude. Buffham found it
+resolved even in the centre with a 9-inch mirror. It was photographed by
+Dr. Roberts in May, 1891, with an exposure of two hours, and the
+photograph confirms the general descriptions given of the cluster,
+though “the print fails to show the stars that on the negative crowd the
+space covered by the dense nebulosity.” Dr. Roberts remarks that
+“nebulosity seems invariably to be present in globular clusters.” From
+photographs of this cluster, taken at Arequipa in Peru, Professor Bailey
+finds 87 stars of the cluster to be variable in light, the variability
+amounting in some cases to two magnitudes, with usually short periods.
+
+Another fine globular cluster is that known as 5 Messier. It lies
+closely north of the fifth magnitude star, 5 Serpentis. It was
+discovered by Kirch in 1702, and was observed in 1764 by Messier, who
+found he could see it with a telescope of one foot in length, but could
+not resolve it into stars. Smyth says: “This superb object is a noble
+mass, refreshing to the senses after searching for faint objects, with
+outliers in all directions, and a bright central blaze, which even
+exceeds 3 Messier in concentration.” Sir William Herschel, with his
+40-foot telescope, could count about 200 stars, but could not
+distinguish the stars near the central blaze. Sir John Herschel
+describes it as an excessively compressed cluster of a globular form,
+with stars from the eleventh to the fifteenth magnitude, condensed into
+a blaze at the centre. Lord Rosse found it more than seven or eight
+minutes of arc in diameter, with a nebulous appearance in the centre.
+This cluster was photographed by Dr. Roberts in April, 1892. “The
+photograph shows the stars to about the fifteenth magnitude, and the
+cluster is involved in dense nebulosity about the centre. The nebulosity
+hides the stars even on the negative.” With reference to this latter
+remark, however, Dr. Common says[131] that, in photographs of this
+cluster taken with a larger instrument, “the stars are quite distinct,
+though the exposure was much longer, a result that might fairly be
+expected.” From photographs of this cluster taken at Arequipa, Peru, by
+Professor Bailey, he finds that the cluster contains about 750 stars, of
+which 46 are variable in light, or about 6 per cent. of the whole. This
+is remarkable, for, of the stars visible to the naked eye, less than 1
+per cent. are variable, so far as is at present known. A further
+examination of the photographs made by Miss Leland shows that the
+periods of these variables are in general very short, not exceeding a
+few hours.[132] One star, situated about eight minutes of arc from the
+centre of the cluster, has a probable period of 11 hours, 7 minutes, 52
+seconds, and varies from about magnitudes 13·50 to 14·73. The star
+remains at the minimum light for about half the period, and the maximum
+brightness is of comparatively short duration. The rate of increase is
+more rapid than the decrease—as in most short period variables—but in
+other respects the character of the light fluctuations does not seem to
+be similar to that of any other known variable star.
+
+Another fine object of this class is that known as 15 Messier in
+Pegasus, discovered by Maraldi in 1745. Sir John Herschel describes it
+as a remarkable globular cluster, very bright and large, and blazing in
+the centre. Webb found it a glorious object with a nine and one-third
+inch mirror. It was photographed by Dr. Roberts in November, 1890, with
+an exposure of two hours. He says: “The photograph confirms the general
+descriptions, and the negative shows, separately, the stars of which the
+cluster is composed distinctly through the nebulosity in the centre.
+Many of the stars have a nebulous appearance, and they are arranged in
+curves, lines, and patterns of various forms, with lanes or spaces
+between them.”
+
+We may also mention the globular cluster known as 2 Messier, which is
+situated about five degrees north of the star Beta Aquarii. It was
+discovered by Maraldi in 1746 while looking for Cheseaux’s comet. Sir
+William Herschel, with his forty-foot telescope, could “actually see and
+distinguish the stars even in the central blaze.” Sir John Herschel
+compared it to a mass of luminous sand, and estimated the stars to be of
+the fifteenth magnitude. It is about five or six minutes of arc in
+diameter, and Smyth says: “This magnificent ball of stars condenses to
+the centre, and presents so fine a spherical figure that imagination
+cannot but figure the inconceivable brilliancy of the visible heavens to
+its animated myriads.” Taking Sir John Herschel’s estimate of the
+component stars at fifteenth magnitude, and the total light of the
+cluster at sixth magnitude, I find that the total number of stars it
+contains would be about 4,000.
+
+[Illustration:
+
+  FIG. 13.—_The Star Cluster, Omega Centauri._
+
+  (From “Worlds of Space.”)
+]
+
+In the Southern Hemisphere there are some magnificent examples of
+globular clusters, and indeed, this hemisphere seems to be richer in
+these objects than the northern sky. Among these southern clusters is
+the truly marvellous object known as Omega Centauri. Its apparent size
+is very large—about two-thirds of the moon’s diameter—and it is
+distinctly visible to the naked eye as a hazy star of the fourth
+magnitude, and I have often so seen it in the Punjab sky. Sir John
+Herschel, observing it with a large telescope at the Cape of Good Hope,
+describes it as “beyond all comparison, the richest and largest object
+of its kind in the heavens. The stars are literally innumerable.... All
+clearly resolved into stars of two sizes, _viz._, 13 and 15; the larger
+lying in lines and ridges over the smaller.... The larger form rings
+like lace-work on it. One of these rings, 1½″ diameter, is so marked as
+to give the appearance of comparative darkness, like a hole in the
+centre.... On further attention, the hole is double, or an oval space
+crossed by a bridge of stars.... Altogether, this object is truly
+astonishing.” This wonderful object has recently been photographed by
+Dr. Gill, at the Royal Observatory, Cape of Good Hope, and also at
+Arequipa, Peru, with a telescope of thirteen inches aperture. On the
+latter photograph, the individual stars can be distinctly seen and
+counted. The enumeration has been made by Professor and Mrs. Bailey, and
+a mean of their counts gives 6,389 for the number of stars in the
+cluster, but they consider that the real number is considerably greater.
+
+Another wonderful object is that known as 41 Toucani, which lies near
+the smaller “Magellanic Cloud” in the Southern Hemisphere. Humboldt
+found it very visible to the naked eye in Peru, and mistook it for a
+comet.[133] Sir John Herschel describes it as “a most magnificent
+globular cluster. It fills the field with its outskirts; but within its
+more compressed part I can insulate a tolerably defined circular space
+of 90″ diameter, wherein the compression is much more decided, and the
+stars seem to run together, and this part has, I think, a pale pinkish
+or rose colour, ... which contrasts evidently with the white light of
+the rest.... The stars are equal, fourteen magnitude, immensely
+numerous, and compressed.... It is _completely insulated_. After it has
+passed, the ground of the sky is perfectly black throughout the whole
+breadth of the sweep. There is a double star of eleventh magnitude
+preceding the centre, ... condensation in three distinct stages.... A
+stupendous object.” Dr. Gould calls it one of the most impressive, and
+perhaps the grandest, of its kind in either hemisphere, and he estimated
+its apparent magnitude at 4½, as seen with the naked eye.
+
+Another remarkable globular cluster is that known as 22 Messier, which
+lies about midway between Mu and Sigma Sagittarii. Sir John Herschel
+says: “The stars are of two sizes, _viz._, 15 ... 16 and 12m; and, what
+is very remarkable, the largest of these latter are visibly reddish, one
+in particular, the largest of all (12–11m) south following the middle,
+is decidedly a ruddy star, and so, I think, are all the other larger
+ones ... very rich, very much compressed, gradually much brighter in the
+middle, but not to a nucleus ... consists of stars of two sizes ... with
+none intermediate, as if consisting of two layers, or one shell over
+another. A noble object” I saw the larger stars well with a 3-inch
+refractor in the Punjab.
+
+Sir John Herschel remarks “the frequent association of nebulæ in pairs
+forming double nebulæ,” and in his “Cape Observations” he figures
+several examples of this class. One of these is evidently a globular
+cluster, with two centres of condensation, one nucleus being much
+brighter than the other. Two others, much smaller, show two distinct
+nuclei. Another drawing shows apparently two globular clusters in
+contact. There are other examples in the Northern Hemisphere. Dr. See
+considers that some of these double nebulæ represent an early stage in
+the evolution of binary or revolving double stars, and certainly some of
+the drawings of these nebulæ are very remarkable and suggestive.
+
+The actual dimensions of the globular clusters is an interesting
+question. Are they composed of stars comparable in size and mass with
+our sun? or are the component stars really small and comparatively close
+together? This is a difficult question to answer satisfactorily, as the
+distance of these objects from the earth has not yet been determined.
+They may, on the one hand, be collections of suns similar to ours in
+size and brightness, and situated at vast distances from the earth; or,
+on the other hand, the stars composing them may be comparatively small
+objects, lying at a distance from the earth not exceeding that of some
+stars visible to the naked eye. Perhaps the latter hypothesis may be
+considered the more probable of the two. But there is really no reason
+to suppose that these collections of suns are comparatively near our
+system. The probability seems to be in favour of their great distance
+from the earth. The question of the absolute size of the component stars
+is one which, I think, has not been hitherto sufficiently considered.
+Let us examine both alternatives, and let us take the cluster Omega
+Centauri as one in which the number of the component stars has been
+_actually counted_. Assuming that the real number of stars in this
+cluster is 10,000, and that they are individually equal, on an average,
+to our sun in mass and volume, we may estimate the probable distance and
+dimensions of the cluster. Taking the stellar magnitude of Omega
+Centauri as four (as estimated at the Cordoba Observatory), I find that,
+with the number 10,000, the average magnitude of the component stars
+would be fourteen. This agrees with Sir John Herschel’s estimate of
+thirteenth to fifteenth magnitude. Now, to reduce the sun to a star of
+the fourteenth magnitude, I find that, assuming the sun to be 28
+magnitudes brighter than an average star of the first magnitude, it
+would be necessary to remove it to a distance of about 158,500,000 times
+the sun’s distance from the earth—a distance so great that light would
+take no less than 2,500 years to reach us from the cluster! Taking the
+apparent diameter of the cluster at twenty minutes of arc, I find that
+its real diameter would be 922,000 times the sun’s distance from the
+earth—a distance so great that light would take over 14 years to pass
+across the cluster. These results are certainly very startling, and
+might lead us to suspect that these globular clusters are external
+universes. Judging, however, from the average distance recently found
+for stars of the first and second magnitude (see p. 423), the distance
+of ordinary stars of the first magnitude—on the supposition that they
+are of the same size and brightness as the sun, and that their light is
+simply reduced by distance—would be about five times greater than that
+found above for Omega Centauri. If, then, we increase the distance of
+the cluster five times, it would be necessary to increase the diameters
+of the component stars to five times that of the sun. This would give
+them a volume 125 times that of our sun—a result which seems improbable.
+If, on the other hand, we do not like to admit that each of the faint
+points of light composing the cluster is equal in volume to our sun, let
+us diminish the distance ten times. If we do so, we must also diminish
+the diameter of the component stars ten times. This would make them
+about the size of the planet Jupiter, and it seems improbable that such
+comparatively small bodies could retain their solar heat for any great
+length of time. They would probably have cooled down, as Jupiter has
+done—at least to a great extent—ages ago, and would not now be visible
+as a cluster of stars. Even this reduction of the distance to one-tenth
+of the value first found would still leave the cluster at an immense
+distance from the earth, a distance represented by 250 years of light
+travel! A reduction of the distance to one-tenth of this again, or 25
+years of light travel, would make the components about the size of the
+earth, and that bodies of this small size could shine with stellar light
+seems to be an untenable hypothesis. We seem, therefore, forced to
+conclude that these globular star clusters lie at an immense distance
+from the earth.
+
+There is, however, another point to be considered with reference to the
+size of the bodies composing a globular cluster. This is the character
+of their light. I am not aware that the spectrum of a globular cluster
+has yet been thoroughly examined, but if that of Omega Centauri is of
+the first or Sirian type, it would modify the above conclusions to some
+extent. It now seems probable that stars having a spectrum of the Sirian
+type are intrinsically brighter than our sun, and I have shown already
+that Sirius is considerably brighter than the sun would be if placed at
+the same distance, although the mass of Sirius is but little more than
+twice the sun’s mass. The components of a star cluster, therefore—if of
+the Sirian type of stars—might be as bright as the sun, and at the same
+time have a smaller mass and volume. This, however, would not make a
+very great difference in the computed vast distance of the cluster, and
+the calculations given above seem to point to the conclusion that these
+globular clusters are probably composed of stars of average size and
+mass, and that the faintness of the component stars is simply due to
+their immense distance from the earth.
+
+We will now consider the nebulæ, properly so-called, that is to say,
+objects which the spectroscope shows to consist of glowing gas. These
+are sometimes large and irregular in form, like the great nebula in the
+“Sword” of Orion, sometimes with spiral convolutions, and sometimes of a
+definite shape, like the planetary and annular nebulæ.
+
+Of the large and irregular nebulæ, one of the most remarkable is that
+known as “the great nebula in Orion.” It surrounds the multiple star,
+Theta Orionis, which has been already referred to in a preceding
+chapter. It is a curious fact that it escaped the searching eye of
+Galileo, although he gave special attention to the constellation of
+Orion, for even with a good opera-glass a nebulous gleam is distinctly
+visible round the central star of the “Sword.” The nebula seems to have
+been discovered by Cysat, a Swiss astronomer, in the year 1618, and it
+was sketched by Huygens in 1656. Huygens says: “While I was observing
+with a refractor of twenty-five feet focal length, the variable belts of
+Jupiter, a dark central belt in Mars, and some phases of this planet, my
+attention was attracted by an appearance among the fixed stars, which,
+as far as I know, has not been observed by anyone else, and which,
+indeed, could not be recognised, except by such powerful instruments as
+I employ. Astronomers enumerate three stars in the Sword of Orion, lying
+very near one another. On one occasion when, in 1656, I was accidentally
+observing the middle one of these stars through my telescope, I saw
+twelve stars instead of a single one, which, indeed, not unfrequently
+happens in using the telescope. Three of this number were almost in
+contact with one another, and _four_ of them shone as if through a mist,
+so that the space around them, having the form drawn in the appended
+figure, appeared much brighter than the rest of the sky, which was
+perfectly clear, and looked almost black. This appearance looked,
+therefore, almost as if there were a _hiatus_ or interruption. I have
+frequently observed this phenomenon, and up to the present time, as
+always unchanged in form; whence it would appear that this marvellous
+object, be its nature what it may be, is very probably permanently
+situated at this spot. I never observed anything similar to this
+appearance in the other fixed stars.”[134] It has been called the
+“fish-mouth” nebula, from the fancied resemblance of the centre portion
+to the mouth of a fish. A number of small stars are visible over the
+surface of the nebula, and at one time, Lord Rosse thought it showed
+indications of resolution into stars when examined with his giant
+telescope; but this is now known to have been a mistake, for Dr. Huggins
+finds, with the spectroscope, that it consists of nothing but glowing
+gas, of which hydrogen is certainly one constituent, and he has
+succeeded in photographing the complete series of lines of this gas in
+the spectrum of the nebula.
+
+Referring to his earlier observations, Dr. Huggins says:—“The light from
+the brightest parts of the nebula near the trapezium was resolved by the
+prisms into three bright lines, in all respects similar to those of the
+gaseous nebulæ. The whole of this great nebula, as far as lies within
+the power of my instrument, emits light which is identical in character.
+The light from one part differs from the light of another in intensity
+alone.” The brightest line in the nebular spectrum—the “chief nebular
+line,” as it is called—has not yet been identified with that of any
+terrestrial substance. It was at first supposed to be identical with a
+line of nitrogen, but this was afterwards disproved. It was then
+incorrectly identified with a line of lead, and more recently by Lockyer
+with the edge of a “fluting” in the magnesium spectrum. Dr. Huggins and
+Professor Keeler, however, have shown conclusively that the nebular line
+does not coincide with the magnesium fluting, although very close to it.
+Observations by Dr. Copeland in 1886 showed the existence of the yellow
+line, know as D_{3}, which is visible in the solar spectrum during total
+eclipses of the sun, and indicates the existence of a gas in the sun’s
+surroundings, to which the name “helium” has been given. Dr. Copeland
+says:—“The recurrence of this line in the spectrum of a nebula is of
+great interest, as affording another connecting link between gaseous
+nebula and the sun and stars with bright line spectra, especially with
+that remarkable class of stars of which the finest examples were
+detected by M. M. Wolf and Rayet in the constellation of Cygnus.”[135]
+As has been already mentioned in the chapter on variable and new stars,
+the bright lines of hydrogen and helium have also been observed in the
+spectra of these remarkable objects. The gas, giving the line D_{3} in
+its spectrum, has quite recently been discovered by Professor Ramsay in
+gases obtained by heating certain terrestrial minerals, so that the
+objective existence of the gaseous element “helium”—previously only
+suspected—is now definitely established. From recent spectroscopic
+observations of the Orion nebula, Dr. Huggins thinks that “the stars of
+the ‘trapezium’ are not merely optically connected with the nebula, but
+are physically bound up with it, and are very probably condensed out of
+the gaseous matter of the nebula.” With reference to this point,
+Professor Keeler, who has carefully examined the spectra of the nebula
+and the associated stars, says:—“The trapezium stars have spectra marked
+by strong absorption bands; they have not the direct connexion with the
+nebula that would be indicated by a bright line spectrum, but are, in
+fact, on precisely the same footing (spectroscopically) as other stars
+in the constellation of Orion. While their relation to the nebula is
+more certain than ever, they can no longer be regarded as necessarily
+situated _in_ the nebula, but within indefinite limits they may be
+placed anywhere in the line of sight.” These results were confirmed by
+Professor Campbell. He finds, “that of the twenty-five bright lines
+known to exist in the spectrum of the Orion nebula, at least nineteen
+are definitely matched by dark lines in the Orion stars, and at least
+fifteen by dark lines in the six faint stars situated in the dense parts
+of the nebula.”
+
+Numerous drawings of this wonderful nebula have been made. Of these, the
+best are those by Sir John Herschel, made at the Cape of Good Hope in
+the years 1834–38, by Bond in America, and by Lassell at Malta. The
+difficulty of accurately delineating so difficult and delicate an object
+has given rise to discrepancies in the drawings, which have led to the
+idea that changes of form have occurred, but this seems improbable. The
+nebula has been very successfully photographed by Dr. Common and Dr.
+Roberts, and these photographs confirm the general accuracy of the later
+drawings.
+
+From a consideration of the apparent size of the Orion nebula and its
+probable mass and distance from the earth, the late Mr. Ranyard came to
+the conclusion that its average density “cannot exceed one ten thousand
+millionth of the density of atmospheric air at the sea-level.”[136]
+
+Mr. W. H. Pickering and Dr. Max Wolf have photographed another nebula
+surrounding the star Zeta Orionis—the southern star of the “Belt,” which
+seems to be connected with the nebula in the “Sword”; and, Prof.
+Barnard, using the “lens of a cheap oil lantern” of 1½ inch aperture,
+and 3½ inches focal length, has photographed “an enormous curved
+nebulosity” stretching over nearly the whole of the constellation of
+Orion, and involving the “great nebula.”
+
+[Illustration:
+
+  FIG. 14.—_The Orion Nebulæ._
+
+  (From “Worlds of Space.”)
+]
+
+Prof. Keeler has recently found, with the spectroscope, that the Orion
+nebula is apparently receding from the earth at the rate of nearly
+eleven miles a second, but this motion may be, in part at least, due to
+the sun’s motion in space in the opposite direction. Prof. Pickering
+considers that the parallax of the nebula is probably not more than
+0·″003, which corresponds to a thousand years’ journey for light!
+
+In the southern constellation, Argo is a magnificent nebula, somewhat
+similar in appearance to the great nebula in Orion. It surrounds the
+famous variable star Eta Argûs, whose remarkable fluctuations in light
+have been already described in the chapter on variable stars. It is
+sometimes spoken of as the “key-hole” nebula, owing to a curious opening
+of that shape near its centre. It was carefully drawn by Sir John
+Herschel at the Cape of Good Hope in the years 1834–38. It lies in a
+very brilliant portion of the Milky Way, and Sir John Herschel thus
+describes it: “It is not easy for language to convey a full impression
+of the beauty and sublimity of the spectacle which the nebula offers as
+it enters the field of view of a telescope, fixed in right ascension, by
+the diurnal motion, ushered in as it is by so glorious and innumerable a
+procession of stars, to which it forms a sort of climax, and in a part
+of the heavens otherwise full of interest,” and he adds: “In no part of
+its extent does this nebula show any appearance of resolvability into
+stars, being, in this respect, analogous to the nebula of Orion. It has,
+therefore, nothing in common with the Milky Way, on the ground of which
+we see it projected, and may therefore be, and not improbably is, placed
+at an immeasurable distance behind that stratum.” Sir John Herschel’s
+conclusion as to its physical constitution has been fully confirmed by
+the spectroscope, which shows it to consist of luminous gas. As in the
+Orion nebula, there are numerous stars scattered over it. Some of these
+may possibly have a physical connexion with the nebula, while others may
+belong to the Milky Way. The nebula is of great extent, covering an
+apparent space about five times the area of the full moon, and its real
+dimensions must be enormous. It was photographed by Mr. Russell,
+director of the Sydney Observatory, in July, 1890, and the photograph
+shows that “one of the brightest and most conspicuous parts of the
+nebula”—the swan-shaped form near the centre of Herschel’s drawing—has
+“wholly disappeared,” and its place is now occupied by “a great, dark
+oval.” Mr. Russell first missed the vanished portion of the nebula in
+the year 1871, while examining it with a telescope of 11½ inches
+aperture, and the photograph now confirms the disappearance, which is
+very remarkable, and shows that changes are actually in progress in
+these wonderful nebulæ, changes which may be detected after a
+comparatively short interval of time.
+
+[Illustration:
+
+  FIG. 15.—_Sir John Herschel’s drawing of the Nebula round Eta Argus._
+
+  (From Flammarion’s “Popular Astronomy.”)
+]
+
+Smaller than the nebula in Argo, but somewhat similar in general
+appearance, is that known as 30 Doradus, which forms one of the numerous
+and diverse objects which together constitute the greater Magellanic
+Cloud. Sir John Herschel drew it carefully at the Cape of Good Hope, and
+describes it as “one of the most singular and extraordinary objects
+which the heavens present,” and he says “it is unique even in the system
+to which it belongs, there being no other object in either nubecula to
+which it bears the least resemblance.” It is sometimes called the
+“looped nebula,” from the curious openings it contains. One of these is
+somewhat similar to the “key-hole” opening in the Argo nebula. Near its
+centre is a small cluster of stars, and scattered over the nebula are
+many faint stars, of which Sir John Herschel gives a catalogue of 105
+ranging from the ninth to the seventeenth magnitude. I do not know
+whether this nebula has been examined with the spectroscope, but its
+appearance would suggest that it is gaseous. It is remarkable as being
+the only object of its class which is found outside the zone of the
+Milky Way.
+
+Among the nebula of irregular shape, although its spectrum is said to be
+not gaseous, may be mentioned that known as the “trifid nebula,” or 20
+Messier. It lies closely north of the star 4 Sagittarii in a magnificent
+region of the heavens. As will be seen in the drawing made by Sir John
+Herschel at the Cape of Good Hope, the principal portion consists of
+three masses of nebulous matter separated by dark “lanes” or “rifts.”
+Near the junction of the three “rifts” is a triple star. A beautiful
+drawing of this nebula has also been made by Trouvelot. It agrees fairly
+well with that of Sir John Herschel, but shows more detail.
+
+[Illustration:
+
+  FIG. 16.—_The Trifid Nebula, Sagittarius._
+
+  (From “Scenery of the Heavens.”)
+]
+
+Among other gaseous nebula may be mentioned that called by Sir John
+Herschel the “dumb-bell” nebula. It lies a little south of the sixth
+magnitude star 14 Vulpeculæ, and was discovered by Messier in 1779,
+while observing Bode’s comet of that year. In small telescopes it has
+the appearance of a dumb-bell, or hour-glass, but in larger telescopes
+the outline is filled in with fainter nebulous light, giving to the
+whole an elliptical form. Several faint stars have been seen in it, but
+these probably belong to the Milky Way, as Dr. Huggins finds the
+spectrum gaseous. Dr. Roberts has photographed it, and he thinks that
+“the nebula is probably a globular mass of nebular matter, which is
+undergoing the process of condensation into stars, and the faint
+protrusions of nebulosity in the _south following_ and _north preceding_
+ends are the projections of a broad ring of nebulosity which surrounds
+the globular mass. This ring, not being sufficiently dense to obscure
+the light of the central region of the globular mass, is dense enough to
+obscure those parts of it that are hidden by the increased thickness of
+the nebulosity, thus producing the ‘dumb-bell’ appearance. If these
+inferences are true, we may proceed yet a step, or a series of steps,
+farther, and predict that the consummation of the life-history of this
+nebula will be its reduction to a globular cluster of stars.”
+
+Among the gaseous nebula may also be included those known as “annular
+nebulæ.” These are very rare objects, only a few being known in the
+whole heavens. The most remarkable is that known as 57 Messier, which
+lies between the stars Beta and Gamma Lyræ, south of the bright star
+Vega. It was discovered by Darquier, at Toulouse, in 1779, while
+following Bode’s comet of that year. Lord Rosse thought it resolvable
+into stars, and so did Chacornac and Secchi, but no stars are
+perceptible with the great American telescopes, and Dr. Huggins finds it
+to be gaseous. The central portion is not absolutely dark, but contains
+some faint nebulous light. Examined with the great telescope of the Lick
+Observatory, Professor Barnard finds that the opening of the ring is
+filled in with fainter light “about midway in brightness between the
+brightness of the ring and the darkness of the adjacent sky.”[137] “The
+aperture was more nearly circular than the outer boundary of the nebula,
+so that the ends of the ring were thicker than the sides.” The entire
+nebula was of a milky colour. A central star, noticed by some observers,
+was usually seen by Professor Barnard, but was never a conspicuous
+object. He found the extreme dimensions of the nebula about 81″ in
+length by about 59″ in width, or more than double the apparent area of
+Jupiter’s disc. It has been beautifully photographed by Dr. Roberts, and
+he says “the photograph shows the nebula and the interior of the ring
+more elliptical than the drawings and descriptions indicate; and the
+star of the _following_ side is nearer to the ring than the distance
+given. The nebulosity on the _preceding_ and _following_ ends of the
+ring protrudes a little, and is less dense than on the _north_ and
+_south_ sides. This probably suggested the filamentous appearance which
+Lord Rosse shows. Some photographs of the nebula have been taken between
+1887 and 1891, and the central star is strongly shown on some of them,
+but on others it is scarcely visible, which points to the star being
+variable.” On a photograph taken by MM. Androyer and Montaugerand of the
+Toulouse Observatory, with an exposure of nine hours (in multiple
+exposures), about 4,800 stars are visible on and near the nebula in an
+area of three square degrees.
+
+Another object of the annular class will be found a little to the
+south-west of the star Lambda Scorpii. It is thus described by Sir John
+Herschel: “A delicate, extremely faint, but perfectly well defined,
+annulus. The field crowded with stars, two of which are on the nebula. A
+beautiful, delicate ring, of a faint, ghost-like appearance, about 40″
+in diameter in a field of about 150 stars, eleven and twelve magnitude
+and under.”
+
+Near the stars 44 and 51 Ophiuchi is another object of the annular
+class, which Sir John Herschel describes as “exactly round, pretty
+faint, 12″ diameter, well terminated, but a little cottony at the edge,
+and with a decided darkness in the middle, equal to a tenth magnitude
+star at the most. Few stars in the field, a beautiful specimen of the
+planetary annular class of nebula.”
+
+The Planetary Nebulæ form an interesting class. They were so named by
+Sir William Herschel from their resemblance to the discs of the planets,
+but, of course, much fainter. They are generally of uniform brightness,
+without any nucleus or brighter part in the centre. There are numerous
+examples of this class, one of the most remarkable being that known as
+97 Messier, which is situated about two degrees south-east of Beta Ursæ
+Majoris—the southern of the two “pointers” in the Plough. It is of
+considerable apparent size, and even supposing its distance to be not
+greater than that of 61 Cygni, its real dimensions must be enormous.
+Lord Rosse observed two openings in the centre with a star in each
+opening, and from this appearance he called it the “owl nebula.” One of
+the stars seems to have disappeared since 1850, and a photograph
+recently taken by Dr. Roberts confirms the disappearance.
+
+Another fine object of the planetary class is one which lies close to
+the pole of the ecliptic. Webb saw it “like a considerable star out of
+focus.” Smyth found it pale blue in colour. Dr. Huggins finds a gaseous
+spectrum, the first discovery of the kind made. Professor Holden,
+observing it with the great Lick telescope, finds its structure
+extraordinary. He says it “is apparently composed of rings overlying
+each other, and it is difficult to resist the conviction that these are
+arranged in space in the form of a true helix,” and he ranks it in a new
+class which he calls “helical nebulæ.”
+
+A somewhat similar nebula lies a little to the west of the star Nu
+Aquarii. Secchi believed it to be in reality a cluster of small stars,
+but Dr. Huggins finds its spectrum gaseous. A small nebula on each side
+gives it an appearance somewhat similar to the planet Saturn, with the
+rings seen edgeways. The great Lick telescope shows it as a wonderful
+object—“a central ring lies upon an oval of much fainter nebulosity.”
+Professor Holden says “the colour is a pale blue,” and he compares the
+appearance of the central ring “to that of a footprint left in the wet
+sand on a sea beach.”
+
+About two degrees south of the star Mu Hydræ is another planetary
+nebula, which Smyth describes as resembling the planet Jupiter in “size,
+equable light and colour.” Webb saw it of “a steady, pale blue light,”
+and Sir John Herschel, at the Cape of Good Hope, speaks of its colour as
+“a decided blue—at all events, a good sky-blue,” a colour which seems
+characteristic of these curious objects. Although Sir William Herschel,
+with his large telescopes, failed to resolve it into stars, Secchi
+thought he saw it breaking up into stars with a “sparkling ring.” Dr.
+Huggins, however, finds the spectrum to be gaseous, so that the luminous
+points seen by Secchi could not have been stellar.
+
+Sir John Herschel, in his “Cape Observations,” describes a planetary
+nebula which lies between the stars Pi Centauri and Delta Crucis. He
+says it is “perfectly round, very planetary, colour fine blue ... very
+like Uranus, only about half as large again, and blue.... It is of the
+most decided independent blue colour when in the field by itself, and
+with no lamplight and no bright star. About 10′ north of it is an
+orange-coloured star, eighth magnitude. When this is brought into view,
+the blue colour of the nebula becomes intense ... colour, a beautiful
+rich blue, between Prussian blue and verditer green.”
+
+There are some rare objects called “nebulous stars.” The star Epsilon
+Orionis—the centre star of Orion’s Belt—is involved in a great nebulous
+atmosphere. The triple star Iota Orionis is surrounded by a nebulous
+haze. The star Beta in Canes Venatici is a 4½ magnitude star surrounded
+by a nebulous atmosphere.
+
+The term elliptical nebulæ has been applied to those of an elliptical or
+elongated shape. This form is probably due in many cases to the effect
+of perspective, their real shape being circular, or nearly so. Perhaps
+the most remarkable object of this class is the well-known “nebula in
+Andromeda,” known to astronomers as 31 Messier. It can be just seen with
+the naked eye, on a clear moonless night, as a hazy spot of light near
+the star Nu Andromedæ, and it is curious that it is not mentioned by the
+ancients, although it must have been very visible to their keen eyesight
+in the clear Eastern skies. It was, however, certainly seen so far back
+as 905 A.D., and it Is referred to as a familiar object by the Persian
+astronomer, Al-Sûfi, who wrote a description of the heavens about the
+middle of the tenth century. Tycho Brahé and Bayer failed to notice it,
+but Simon Marius saw it in December, 1612, and described it “as a light
+seen from a great distance through half-transparent horn plates.” It was
+also observed by Bullialdus, in 1664, while following the comet of that
+year. It has frequently been mistaken for a comet by amateur observers
+in recent years. Closely north-west of the great nebula is a smaller one
+discovered by Le Gentil in 1749, and another to the south, detected by
+Miss Caroline Herschel in 1783. The great nebula is of an elliptical
+shape and considerable apparent size. The American astronomer, Bond,
+using a telescope of 15 inches aperture, traced it to a length of about
+four degrees, and a width of two and a half degrees. A beautiful
+photograph taken by Dr. Roberts in December, 1888 (see p. 398), shows an
+extension of nearly two degrees in length, and about half a degree in
+width, or considerably larger than the apparent size of the full moon.
+Bond could not see any symptom of resolution into stars, but noticed two
+dark rifts or channels running nearly parallel to the length of the
+nebula. In Dr. Roberts’ photograph these rifts are seen to be really
+dark intervals between consecutive nebulous rings into which the nebula
+is divided. Dr. Roberts says: “A photograph which I took with the
+20-inch reflector on October 10, 1887, revealed for the first time the
+true character of the great nebula, and one of the features exhibited
+was that the dark bands, referred to by Bond, formed parts of divisions
+between symmetrical rings of nebulous matter surrounding the large
+diffuse centre of the nebula. Other photographs were taken in 1887,
+November 15; 1888, October 1; 1888, October 2; 1888, December 29;
+besides several others taken since, upon all of which the rings of
+nebulosity are identically shown, and thus the photographs confirm the
+accuracy of each other, and the objective reality of the details shown
+of the structure of the nebula.” Dr. Roberts adds: “These photographs
+throw a strong light on the probable truth of the _Nebular Hypothesis_,
+for they show what appears to be the progressive evolution of a gigantic
+stellar system.”
+
+The largest telescopes have hitherto completely failed to resolve this
+wonderful object into stars. Dr. Huggins, however, finds that the
+spectrum is _not_ gaseous, so that if the nebula really consists of
+stellar points, they must be of very small dimensions. Assuming a
+parallax of one-fiftieth of a second of arc—corresponding to 163 years
+of “light travel”—I find that our sun, placed at this distance, would be
+reduced in brightness to a star of about the eighth magnitude. If we
+assume the components to have only one-hundredth of the sun’s diameter,
+they would shine as stars of only the eighteenth magnitude, which no
+telescope yet constructed would show as separate points of light. A more
+probable explanation, however, seems to be that the nebula may consist
+of masses of nebulous matter partially condensed into the solid form,
+but not yet arrived at the stage in which our sun is at present. In
+other words, the whole nebulous mass may be in a fluid or viscous state,
+which might perhaps account for the continuous spectrum found by Dr.
+Huggins.
+
+The question may be asked, What is the probable size and distance of
+this wonderful nebula? and could it be an external universe? Possibly
+its distance from the earth may be even greater than that indicated by
+the small parallax I have assumed above, but taking this parallax and
+the apparent dimensions of the nebula as shown by Dr. Huggins’
+photograph, I find that its real distance would be no less than 330,000
+times the sun’s diameter from the earth, a diameter so great that light
+would take over five years to pass from one side of the nebula to the
+other! This result might lead us to imagine that the nebula may be
+really an external universe. But let us consider the matter a little
+further. The diameter found above is not very much greater than the
+distance of the _nearest_ fixed star, Alpha Centauri, from the earth,
+and the limits of _our_ universe are certainly far beyond Alpha
+Centauri. If we diminish the parallax to, say ¹⁄₂₀₀th of a second, or a
+“light journey” of 652 years, the diameter of the nebula would be
+increased to 1,320,000 times the sun’s distance from the earth, or about
+five times the distance of Alpha Centauri, and there are probably many
+faint stars belonging to our system much farther from the earth than
+this.
+
+The temporary star which appeared near the nucleus of the nebula in
+August, 1885—already referred to in the chapter on variable stars—was of
+the seventh magnitude. I find that our sun, if placed at the distance
+indicated by a parallax of ¹⁄₂₀₀th of a second, would be reduced to a
+star of about the eleventh magnitude, or four magnitudes fainter than
+the temporary star appeared to us. That is to say, the star would have
+been—with the assumed distance—about forty times brighter than the sun.
+With any greater distance, the star would have been proportionately
+brighter, compared with the sun. This seems improbable, and tends to the
+conclusion that the nebula is _not_ an external galaxy, but a member of
+our own sidereal system, a system which probably includes all the stars
+and nebulæ visible in our largest telescopes. Dr. Common, indeed,
+suggests that it may be comparatively near our system. He says: “It is
+difficult to imagine that such an enormous object, as the Andromeda
+nebula must be, is not very near to us; perhaps it may be found to be
+the nearest celestial object of all beyond the solar system. It is one
+that offers the best chance of the detection of parallax, as it seems to
+be projected on a crowd of stars, and there are well defined points that
+might be taken as fiducial points for measurement,” and he adds: “Apart
+from the great promise this nebula seems to give of determining
+parallax, there is a fair presumption that in the course of time, the
+rotation of the outer portion may perhaps be detected by observation of
+the positions of the two outer detached portions in relation to the
+neighbouring stars.”[138] Prof. Hall’s failure to detect any parallax in
+the temporary star, as mentioned in the last chapter, is, of course,
+against Dr. Common’s idea of its proximity to the earth. Referring to
+the latter portion of Dr. Common’s remarks, Mr. C. Easton points
+out[139] that a comparison of a drawing by Trouvelot, in 1874, with Dr.
+Roberts’ photograph, suggests that the small elongated nebula—_h_
+44—which lies to the north of the great nebula, “has turned about 15°
+from left to right. The globular nebula (M 32), to the other side of M
+31, seems to have slightly shifted its position.”
+
+[Illustration:
+
+  FIG. 17.—_Spiral Nebula, 51 Messier._
+
+  (From “The Visible Universe.”)
+]
+
+The spiral nebulæ are wonderful objects, and were discovered by the late
+Lord Rosse, with his great six-foot telescope. Their character has been
+fully confirmed by photographs taken by Dr. Roberts. One of the most
+remarkable of these extraordinary objects is that known as 51 Messier.
+It lies about three degrees south-west of the bright star Eta Ursæ
+Majoris—the star at the end of the Great Bear’s tail. It was discovered
+by Messier while comet-hunting on October 13, 1773. Telescopes of
+moderate power merely show two nebulæ nearly in contact, but Lord Rosse
+saw it as a wonderful spiral, and his drawing agrees fairly well with a
+photograph taken by Dr. Roberts in April, 1889. The nebula has also been
+photographed by Dr. Common. Dr. Roberts says: “The photograph shows both
+nuclei of the nebula to be stellar, surrounded by dense nebulosity, and
+the convolutions of the spiral in this as in other spiral nebulæ are
+broken up into star-like condensations with nebulosity around them.
+Those stars that do not conform to the trends of the spiral have
+nebulous trails attached to them, and seem as if they had broken away
+from the spirals.” A tendency to a spiral structure in the smaller
+nebula is also visible on the original negative. Dr. Huggins finds that
+the spectrum is _not_ gaseous.
+
+The nebulæ known as 99 Messier is of the spiral form. It lies on the
+borders of Virgo and Coma Berenices, near the star 6 Comæ. In large
+telescopes it somewhat resembles a “Catherine wheel.” D’Arrest and Key
+thought it resolvable into stars. It has been photographed by M. Von
+Gothard.
+
+Among the clusters and nebulæ, we may class the Magellanic Clouds, or
+Nubeculæ in the Southern Hemisphere, as they consist of stars, clusters,
+and nebulæ. These very remarkable objects form two bright spots of milky
+light, which, at first sight, look like luminous patches of the Milky
+Way, but are in no way connected with the Galaxy. Sir John Herschel,
+speaking of the larger cloud, says: “The immediate neighbourhood of the
+Nubecula Major is somewhat less barren of stars than that of the Minor,
+but it is by no means rich, nor does any branch of the Milky Way
+whatever form any certain or conspicuous junction with, or include, it,”
+and again he says, with reference to the smaller cloud: “Neither with
+the naked eye, nor with a telescope, is any connexion to be traced
+either with the greater Nubecula, or with the Milky Way.” The Nubeculæ
+are roughly circular in form, and, viewed with the naked eye, they very
+much resemble irresolvable nebulæ as seen in a telescope. The larger
+cloud, or Nubecula Major, as it is called, is of considerable extent,
+and covers about 42 square degrees, or over two hundred times the
+apparent size of the full moon. It was called by the Arabs _el-baker_,
+or “the White Ox,” and is referred to by Al-Sûfi in his “Description of
+the Heavens,” written in the tenth century. When examined with a good
+telescope, it is found to consist of about six hundred stars of the
+sixth to the tenth magnitude, with many fainter ones, and about three
+hundred clusters and nebulæ. Sir John Herschel, in his “Cape
+Observations,” says: “The Nubeculæ Major, like the Minor, consists
+partly of large tracts and ill-defined patches of irresolvable nebula,
+and of nebulosity in every stage of resolution, up to perfectly resolved
+stars like the Milky Way, as also of regular and irregular nebulæ
+properly so-called, of globular clusters in every stage of
+resolvability, and of clustering groups sufficiently insulated and
+condensed to come under the designation of ‘clusters of stars.’... It is
+evident, from the intermixture of stars and unresolved nebulosity, which
+probably might be resolved with a higher optical power, that the
+nubeculæ are to be regarded as systems _sui generis_, and which have no
+analogues in our hemisphere.”
+
+The smaller Magellanic Cloud, or Nubecula Minor, is fainter to the eye,
+and not so rich in the telescope. It covers about 10 square degrees, or
+about fifty times the area of the full moon. Sir John Herschel, in his
+“Cape Observations,” describes it as “a fine large cluster of very small
+stars, 12 ... 18 magnitude, which fills more than many fields, and is
+broken into many knots, groups, and straggling branches, but _the whole_
+(_i.e._, the whole of the clustering part) is clearly resolved.” It is
+surrounded by a barren region remarkably devoid of stars. Sir John
+Herschel says: “The access to the Nubecula Minor is on all sides through
+a desert.”... “It is preceded at a few minutes in R. A. by the
+magnificent globular cluster, 47 Toucani (Bode), but is completely cut
+off from all connexion with it; and with this exception, its situation
+is in one of the most barren regions in the heavens.” Herschel found the
+middle of the cloud clearly resolved into stars, while its edges
+remained irresolvable with his large reflector. He says: “The edge of
+the smaller _cloud_ comes on as a mere nebula.... We are now _in the
+cloud_. The field begins to be full of a faint light perfectly
+irresolvable.... I should consider about this place to be the body of
+the cloud which is here fairly resolved into excessively minute
+stars.... It is not like the stippled ground of the sky. The borders
+fade away, quite insensibly, and are less or not at all resolved.”
+Herschel gives a catalogue of 244 objects in the Nubecula Minor. Of
+these about 200 are stars, and the remainder nebula and clusters. From
+this it appears that the smaller nubecula contains a much larger
+proportion of stars than the larger cloud.
+
+Judging from their roughly globular form, the dimensions of the
+Magellanic Clouds are probably small compared with their distance from
+the earth, so that in these remarkable objects—particularly in the
+larger cloud—we see stars of the seventh, eighth, ninth, and tenth
+magnitude, apparently mixed up with fainter stars, and “clusters of all
+degrees of resolvability,” and Sir John Herschel says: “It must
+therefore be taken as a demonstrated fact, that stars of the seventh or
+eighth magnitude, and irresolvable nebulæ, may co-exist within limits of
+distance not differing in proportion more than as 9 to 10.”[140] It
+should be remembered, however, that possibly some of the fainter stars
+may—as in the Pleiades—lie far out in space beyond the greater
+Magellanic Cloud.
+
+The Magellanic Clouds have recently been photographed by Mr. Russell at
+the Sydney Observatory. He finds the larger cloud—the Nubecula Major—to
+be of a most complex form, with evidence of a spiral structure, a
+feature also traceable, but not so clearly, in a photograph of the
+Nubecula Minor, or smaller cloud.
+
+Dr. Dreyer’s new index catalogue of recent discoveries of nebulæ,
+together with the general catalogue previously published, gives the
+position of 9,369 nebulæ.[141] A very small proportion of the new
+discoveries have been made by photography, and more than half of them
+were found by M. Javelle with the great refractor of the Nice
+Observatory. Most of the new objects are very small and faint, and form
+probably “only a small portion of the number visible in large
+telescopes.”
+
+[Illustration:
+
+  FIG. 18.—_Magellanic Clouds._
+
+  (From “Worlds of Space.”)
+]
+
+Several nebulæ have been suspected of variation in light. One discovered
+by Dr. Hind in 1852 near the variable star T Tauri was found to be an
+easy object with the great Lick telescope in February, 1895, but in
+September of the same year it had “entirely vanished.” In the same
+instrument, “T Tauri was involved in a small hazy nebulosity, but the
+definite nebula in which it shone in 1890 did not exist in September,
+1895.”[142]
+
+
+
+
+                              CHAPTER VII.
+                    THE CONSTRUCTION OF THE HEAVENS.
+
+
+The construction of the visible universe is one of great interest, but
+of considerable difficulty. If we reflect that in viewing the starry
+heavens we are placed at the centre of a hollow sphere of indefinite
+extent, and that the distance of only a few of the stars from the earth
+has hitherto been ascertained with any approach to accuracy, the great
+difficulty of framing a satisfactory theory of the construction of the
+heavens will be easily understood.
+
+In considering the subject, let us first inquire as to the probable
+number of stars visible in our largest telescopes. Are the visible stars
+infinite or limited in number? The reply to this question is easy. As
+the number of stars visible to the naked eye is limited, so the number
+of stars visible in the largest telescopes is limited also. Those who do
+not give the subject sufficient consideration seem to think that the
+number of the stars is practically infinite, or at least that the number
+is so great that it cannot be estimated. But this idea is totally
+incorrect, and due to complete ignorance of telescopic revelations. It
+is certainly true that, to a certain extent, the larger the telescope
+used in the examination of the heavens, the more the number of the stars
+seems to increase; but we now know that there is a limit to this
+increase of telescopic vision. And the evidence clearly shows that we
+are rapidly approaching this limit. Although the number of stars visible
+in the Pleiades rapidly increases at first with increase in the size of
+the telescope used, and although photography has still further increased
+the number of stars in this remarkable cluster, it has recently been
+found that an increased length of exposure—beyond three hours—adds very
+few stars to the number visible on the photograph taken at the Paris
+Observatory in 1885, on which over 2,000 stars can be counted. Even with
+this great number on so small an area of the heavens, comparatively
+large vacant spaces are visible between the stars, and a glance at the
+original photograph is sufficient to show that there would be ample room
+for many times the number actually visible. I find that, if the whole
+heavens were as rich in stars as the Pleiades, there would be only 33
+millions in both hemispheres.
+
+On a photograph of the region surrounding Gamma Cassiopeiæ, taken by Dr.
+Roberts in December, 1895, with a reflecting telescope of 20 inches
+aperture, and an exposure of two hours and twelve minutes, he finds
+17,100 stars on an area of four square degrees. This would give for the
+whole area of the heavens—if equally rich in stars—a total of about 176
+millions; but Gamma Cassiopeiæ lies in a rich region of the Milky Way,
+and probably the great majority of the stars shown on Dr. Roberts’
+photograph belong to the Galaxy, which we know to be especially rich in
+stars. One thing is certain, that the heavens as a whole are not nearly
+so rich as this particular spot. There may, perhaps, be richer spots
+elsewhere in the Milky Way, but in other parts of the sky there are many
+regions considerably poorer.
+
+Let us consider a still more extreme case of stellar richness. On a
+photograph of the great globular cluster, Omega Centauri, recently taken
+in Peru, a count of the stars has been carefully made by Professor and
+Mrs. Bailey, and, as stated in the last chapter, the number of stars
+contained in the cluster may be taken as 10,000. Now, if the whole sky
+were as thickly studded with stars as in this cluster, the total number
+visible in the whole heavens would be 1,650 millions, a very large
+number, of course, but not much in excess of the present population of
+the earth, and I am not aware that the number of the earth’s inhabitants
+has ever been described as “infinite.”
+
+Clusters, such as the Pleiades and Omega Centauri, are, of course,
+remarkable, and rare exceptions to the general rule of stellar
+distribution, and the heavens in general are not—even in the richest
+portions of the Milky Way—nearly so rich in stars as the globular
+clusters. The fact of these clusters being remarkable objects, proves
+that they are unusually rich in stars, and there is strong
+evidence—evidence amounting to absolute proof in the case of the
+globular clusters—that these collections of stars are really, and not
+apparently, close, and that they are actually systems of suns, and
+occupy a comparatively limited volume in space. We cannot, then,
+estimate the probable number of the visible stars by counting those
+visible in one of the globular clusters.
+
+That the number of the visible stars will not probably be largely
+increased by any increase in telescopic power, is indicated by the fact
+that Celoria, using a small telescope, of power barely sufficient to
+show stars to the eleventh magnitude, found that he could see almost
+exactly the same number of stars near the north pole of the Milky Way as
+were visible in Sir William Herschel’s great telescope! thus indicating
+that, here at least, no increase of optical power will materially
+increase the number of stars visible in that direction; for Herschel’s
+large telescope certainly showed far fainter stars than those of the
+eleventh magnitude in other portions of the heavens. It should therefore
+have shown fainter stars at the pole of the Milky Way also, if such
+stars existed in that region of space. Their absence, therefore, seems
+certain proof that very faint stars do _not_ exist in that direction,
+and that, here at least, our sidereal universe is limited in extent A
+photograph, taken by Dr. Roberts not very far from the spot in question,
+shows only 178 stars to the square degree. This rate of distribution
+would give a total of only 7,343,000 stars for both hemispheres!
+
+An examination by Miss Clerke of Professor Pickering’s catalogue of
+stars surrounding the north pole of the heavens shows that “the small
+stars are overwhelmingly too few for the space they must occupy, if of
+average brightness; and they are too few in a constantly increasing
+ratio.”[143] Here again, a “thinning out” of the stellar hosts seems
+clearly indicated, and suggests that a limit will soon be reached,
+beyond which our most powerful telescopes and photographic plates will
+fail to reveal any further stars.
+
+Let us now consider the number of stars actually visible. Maps of the
+northern portion of the heavens have been published by Argelander and
+Heis, and charts of the southern sky by Behrmann and Gould. Heis shows
+stars to about magnitude 6⅓, and Behrmann to about the same brightness.
+I find that the total number shown by both observers, as visible to the
+naked eye, is 7,249. The total number, to the sixth magnitude inclusive,
+shown by both observers, is 4,181. Argelander gives 5,000 stars to the
+sixth magnitude inclusive, and for stars to the ninth magnitude, the
+following numbers in each magnitude:—First magnitude, 20; second
+magnitude, 65; third magnitude, 190; fourth magnitude, 425; fifth
+magnitude, 1,100; sixth magnitude, 3,200; seventh magnitude, 13,000;
+eighth magnitude, 40,000; and ninth magnitude, 142,000, or a total of
+“200,000 for the entire number of stars from the first to the ninth
+magnitude inclusive.”[144] This result agrees closely with an estimate
+previously made by Struve. From a formula given by Dr. Gould, deduced
+from observations in the Southern Hemisphere, I find the number of stars
+to the ninth magnitude inclusive would be 215,674, so that Argelanders
+estimate of 200,000 stars to the ninth magnitude inclusive cannot be far
+from the truth. It will be seen from Argelanders figures that the number
+of stars in each class of magnitude is roughly three times that in the
+class one magnitude brighter. Supposing this progressive increase
+continued to the seventeenth magnitude—the faintest visible in the great
+Lick telescope—I find that the total number of stars would be nearly
+1,400 millions, or less than the number found from a consideration of
+the cluster Omega Centauri. But it is evident from Celoria’s
+observation, referred to above, and from Professor Pickering’s
+photographs of stars near the North Pole, that the fainter stars do
+_not_ increase in the ratio assumed above. We must therefore conclude
+that there is a “thinning out” of the fainter stars at some point below
+the ninth magnitude. Taking into consideration the rich regions of the
+Milky Way, and the comparatively poor portions of the sky, it is now
+generally admitted by astronomers, who have studied this particular
+question, that the probable number of stars visible in our largest
+telescopes does not exceed 100 millions, a number which, large as it
+absolutely is, may be considered as relatively very small, and even
+utterly insignificant, when compared with an “infinite number.”
+
+Let us see what richness of stellar distribution is implied by this
+number of 100 millions of visible stars. It may be easily shown that the
+area of the whole sky, in both hemispheres, is 41,253 square degrees, or
+about 200,000 times the area of the full moon. This gives 2,424 stars to
+the square degree. The moon’s apparent diameter being slightly over half
+a degree (31′ 5″), the area of its disc is about one-fifth of a square
+degree. Hence, for 100 millions of stars in the whole star sphere, we
+have 485 stars to each space of sky, equal in area to the full moon.
+This seems a large number, but stars scattered even as thickly as this
+would appear at a considerable distance apart when viewed with a large
+telescope and a high power. As the area of the moon’s disc contains
+about 760 square minutes of arc, there would not be an average of even
+one star to each square minute. A pair of stars half a minute, or 30
+seconds, apart, would form a very wide double star, and with stars
+placed at even this distance, the moon’s disc would cover about 3,000,
+or over six times the actual number visible in the largest telescopes.
+In Dr. Roberts’ photograph of the region surrounding Gamma Cassiopeiæ,
+which shows over 17,000 stars, on four square degrees, or over 4,000
+stars to the square degree, the stars do not seem very crowded, and
+there is a good deal of black sky visible between them.
+
+But, in addition to the conclusive evidence as to the limited number of
+the visible stars derived from actual observation and the results of
+photography, we have indisputable evidence from mathematical
+considerations that the number of the visible stars _must necessarily_
+be limited. For were the stars infinite in number, and scattered through
+infinite space with any approach to uniformity, it may be proved that
+the whole heavens would shine with the brightness of the sun. As the
+surface of a sphere varies as the square of its radius, and light
+inversely as the square of the distance (or radius of the star sphere at
+any point), we have the diminished light of the stars exactly
+counterbalanced by the increased number at any given distance. For a
+distance of say ten times the distance of the nearest fixed star, the
+light of each star would be diminished by the square of 10 or 100 times,
+but the total number of stars would be 100 times greater, so that the
+total star light would be the same. This would be true for _all_
+distances. The total light would therefore—by addition—be proportional
+to the distance, and hence, for an infinite distance we should have an
+infinite amount of light For an infinite number of stars, therefore, we
+should have a continuous blaze of light over the whole surface of the
+visible heavens. Far from this being the case, the amount of light
+afforded by the stars on the clearest nights is, on the contrary,
+comparatively small, and the blackness of the background, “the darkness
+behind the stars,” is very obvious. According to Miss Clerke (“System of
+the Stars,” p. 7), the total light of all the stars, to magnitude 9½, is
+about one-eightieth of full moonlight. M. G. l’Hermite found for the
+total amount of starlight one-tenth of moonlight; but this estimate is
+evidently too high. Assuming the sun’s brightness as 28 magnitudes
+brighter than a star of the first magnitude,[145] and Zöllner’s estimate
+that sunlight is 618,000 times that of moonlight, I find that the total
+light of the stars to magnitude 9½, as stated by Miss Clerke, would be
+equivalent to the combined light of about 320,000 stars of the sixth
+magnitude, or 3,200 stars of the first magnitude. Even taking M.
+l’Hermite’s high estimate of one-tenth of moonlight, the total starlight
+would be represented by 25,600 stars of the first magnitude.
+
+To explain the limited number of the visible stars, several hypothesis
+have been advanced. If space be really infinite, as we seem compelled to
+suppose, it would be reasonable to expect that the number of the stars
+would be practically infinite also. But, as I have shown above, the
+number of the _visible_ stars is certainly finite, and the number
+visible and invisible must be finite also, for otherwise the amount of
+starlight would be much greater than it is. To account for the limited
+number of visible stars, it has been suggested that beyond a certain
+distance in space, there may be an “extinction of light,” caused by
+absorption in the luminiferous ether. In a recent paper on this subject,
+Schiaparelli, the famous Italian astronomer, suggests that if any
+extinction of light really takes place, it may probably be due, not to
+absorption in the ether, but to fine particles of matter scattered
+through interstellar space. In support of this hypothesis, he refers to
+the supposed constitution of comets’ tails, of falling stars, and
+meteorites, and he shows that the quantity of matter necessary to
+produce the required extinction would be very small—so small, indeed,
+that a quantity of this matter scattered through a volume equal to that
+of the earth, if collected into one mass, would only form a ball of less
+than one inch in diameter. We can readily admit the existence of such a
+minute quantity of matter in a fine state of subdivision scattered
+through space, but it seems to me much more probable that the limited
+number of the visible stars is due, not to any extinction of their light
+by absorption in the ether, or by fine particles scattered through
+space, but to a real thinning out of the stars as we approach the limits
+of our sidereal universe. Celoria’s observation, mentioned above, seems
+to prove that near the pole of the Milky Way very few stars fainter than
+the eleventh magnitude are visible, even in a large telescope, and Dr.
+Roberts’ photographs, taken in the vicinity of the celestial pole,
+confirm this conclusion. Now, this paucity of stars of the fainter
+magnitudes cannot be due to any absorption of light in the ether, for
+numerous stars of the sixteenth magnitude, or perhaps fainter, are
+visible in other parts of the heavens, and if in one place, why not in
+another? Sir John Herschel’s observations of the Milky Way in the
+Southern Hemisphere appear to render the hypothesis of any extinction of
+light very improbable. He says that the hypothesis, “if applicable to
+any, is equally so to every part of the Galaxy. We are not at liberty to
+argue that at one part of its circumference our view is limited by this
+sort of cosmical veil, which extinguishes the smaller magnitudes, cuts
+off the nebulous light of distant masses, and closes our view in
+impenetrable darkness; while at another we are compelled, by the
+clearest evidence telescopes can afford, to believe that star-strewn
+vistas _lie open_, exhausting their powers, and stretching out beyond
+their utmost reach, as is proved by that very phænomenon which the
+existence of such a veil would render impossible, _viz._, infinite
+increase of number and diminution of magnitude, terminating in complete
+irresolvable nebulosity.”
+
+How then are we to explain the limited number of the visible stars? If
+space be infinite, as we seem compelled to suppose, the number of the
+stars would probably be infinite also, or at least vastly greater than
+the number actually visible. It has been suggested that, owing to the
+progressive motion of light, the light of very distant stars may
+probably not yet have reached the earth, although travelling through
+space for thousands of years. But considering the vast periods of time
+during which the stellar universe has probably been in existence, this
+hypothesis seems very unsatisfactory. The most probable hypothesis seems
+to be that all the stars, clusters and nebulæ, visible in our largest
+telescopes, form together one vast system, which constitutes our visible
+universe, and that this system is isolated by a starless void from other
+similar systems which probably exist in infinite space. The distance
+between these separate systems—or “island universes,” as they have been
+called—may be very great, compared with the diameter of each system, in
+the same way that the diameter of our visible universe is very great
+compared with the diameter of the solar system. As the sun is a star,
+and the stars are suns, and as our sun is separated from his neighbour
+suns in space by a sunless void, so may our universe be separated from
+other universes by a vast and starless abyss. On this hypothesis, the
+supposed extinction of light—which may have little or no perceptible
+effect within the limits of our visible universe—may possibly come into
+play across the vast and immeasurable distances which probably separate
+the different universes from each other, and may perhaps extinguish
+their light altogether.
+
+Another hypothesis which also seems possible is that the luminiferous
+ether which extends throughout our visible universe may perhaps be
+confined to this universe itself, and that beyond its confines, the
+ether may thin out, as our atmosphere does at a certain distance from
+the earth, and finally cease to exist altogether, ending in an
+_absolute_ vacuum, which would, of course, arrest the passage of all
+light from outer space, and thus produce “the darkness behind the
+stars.”
+
+Let us now consider the apparent distribution of the stars and nebulæ on
+the celestial vault, and their probable relation to each other in space.
+As already stated, Argelander considered the number of stars of the
+first magnitude to be about twenty, but modern photometric measures have
+reduced this number to thirteen or fourteen. According to the Harvard
+measures, the fourteen brightest stars in the heavens, in order of
+magnitude, are: Sirius, Canopus, Arcturus, Capella, Vega, Alpha
+Centauri, Rigel, Procyon, Achernar, Beta Centauri, Betelgeuse, Altair,
+Aldebaran and Alpha Crucis. Seven of these are in the Northern
+Hemisphere, namely: Arcturus, Capella, Vega, Procyon, Betelgeuse,
+Altair, and Aldebaran; and seven in the Southern Hemisphere: Sirius,
+Canopus, Alpha Centauri, Rigel, Achernar, Beta Centauri, and Alpha
+Crucis, so that the brightest stars are pretty evenly distributed
+between the two hemispheres. Of these bright stars, no less than twelve
+lie in or near the Milky Way, Arcturus and Achernar being the only two
+at any considerable distance from the Galaxy. This is very remarkable
+and suggestive, as the area covered by the Milky Way is probably not
+more than one-fourth of the whole star sphere.
+
+Of the stars fainter than the first magnitude, but brighter than
+magnitude 2·0, there are about 10 in the Northern Hemisphere, of which 4
+lie in or near the Milky Way, and about 19 in the Southern Hemisphere,
+of which no less than 14 are situated in or near the Galaxy.
+
+Of those brighter than magnitude 3·0, I find 33 stars in or near the
+Milky Way out of a total of about 95 in both hemispheres. To extend this
+investigation to all stars visible to the naked eye, I made, some years
+since, an examination of all the stars in Heis’ atlas that lie in the
+Milky Way, and found that number to be 1,186 out of a total of 5,356, or
+a percentage of about 22. At my request, Col. Markwick, F.R.A.S., made a
+similar count for the stars in Dr. Gould’s charts of the Southern
+Hemisphere (_Uranometria Argentina_), and found that, down to the fourth
+magnitude, there are 121 stars on the Milky Way out of 228, or a
+percentage of 53, and for all stars to the seventh magnitude inclusive,
+there are 3,072 on the Milky Way out of a total of 6,694, or a
+percentage of nearly 46. Col. Markwick finds that the Milky Way in the
+Southern Hemisphere, as shown on Gould’s charts, covers about one-third
+of the whole hemisphere. As will be seen by the above figures, the
+percentage of stars, even to the fourth magnitude, lying on the Milky
+Way is considerably greater than this proportion.
+
+The above results show that the brighter stars which are apparently
+projected on the Milky Way probably belong to that zone, and are not
+merely fortuitously scattered over the surface of the heavens.
+
+To extend the investigation still further, and include stars to the
+eighth magnitude, I made an examination of the stars shown on Harding’s
+charts to that magnitude, in a zone of 30° in width—15° degrees on each
+side of the Equator—and found a marked increase in the number of stars
+where the zone crossed the Milky Way. The numbers per hour of Right
+Ascension varied from a minimum of 275 (hours I. and II.) to maxima of
+601 in the Milky Way in Monoceros, and 611 in the Galaxy in Serpens and
+Aquila. A valuable investigation by the late Mr. Proctor went further
+still. He plotted all the stars shown in the charts of Argelander’s
+_Durchmusterung_, which contains stars to 9½ or 10th magnitude. In this
+remarkable chart the course of the Milky Way is clearly defined by a
+marked increase of stellar density. Proctor says: “In the very regions
+where the Herschelian gauges showed the minutest telescopic stars to be
+most crowded, my chart of 324,198 stars shows the stars of the higher
+orders (down to the eleventh magnitude) to be so crowded that, by their
+mere aggregation within the mass, they show the Milky Way with all its
+streams and clusterings. This evidence, I venture to affirm, is
+altogether decisive as to the main question, whether large and small
+stars are really intermixed in many regions of space, or whether the
+small stars are excessively remote. It is utterly impossible that
+excessively remote stars could seem to be clustered exactly where
+relatively near stars are richly spread. This might happen, no doubt, in
+a single instance; but that it could be repeated over and over again, so
+as to account for all the complicated features seen in my chart of
+324,198 stars, I maintain to be utterly incredible.”[146]
+
+From a careful examination of the Milky Way in Aquila and Cygnus, Mr.
+Easton finds that “(1) In the zones considered, the distribution of
+stars down to 9·5 magnitude corresponds to the greater or less intensity
+of galactic light. (2) There is a real correspondence of the general
+outlines of the galactic forms with the distribution of 11 magnitude
+stars, and with those of stars between 10 and 15 magnitude. (3) Thus, in
+general, for the zones considered, the faint stars which form the Milky
+Way are thickly or sparsely scattered in respectively the same regions
+as the stars in Argelander’s last class; it follows, therefore, with a
+great degree of probability, that there is a real connexion between the
+distribution of 9 and 10 magnitude stars and that of the very faint
+stars of the Milky Way. Consequently, the very faint stars are at a
+distance which does not greatly exceed that of 9–10 magnitude stars. If
+stars of 13–15 magnitude were at their theoretical distance, there would
+be no reason why they should have the same apparent distribution in
+galactic latitude and longitude as 9–10 magnitude stars separated from
+them by enormous intervals.”[147]
+
+There are some regions in both hemispheres especially rich in naked eye
+stars. Of these the following may be mentioned in the Northern
+Hemisphere:—the region including the Pleiades, and Hyades in Taurus, the
+Northern portion of Orion, and the adjoining part of Gemini, the
+constellation Lyra, the northern portion of Cygnus, Cassiopeia’s Chair,
+and Coma Berenices. In the Southern Hemisphere there are several rich
+spots. A rich region extends from Canis Major to the Southern Cross, and
+nearly coincides with the course of the Milky Way. The richest spot of
+all, and perhaps the richest in the whole heavens in naked eye
+stars—with exception of the Pleiades—is that including the Southern
+Cross. This spot has an average of three stars to five square degrees,
+and if the whole heavens were as richly studded with stars there would
+be about 24,000 visible to the naked eye! The poverty of the adjoining
+“coal sack” is very remarkable. Another rich spot surrounds the variable
+star Eta Argûs, and the great nebula in Argo. There is another rich spot
+in the constellation Hydrus, not far from the greater Magellanic Cloud,
+and another will be found in Centaurus and Lupus, with its centre about
+Alpha of the latter constellation. According to Gould’s maps of the
+Southern Hemisphere, the richest region in stars down to the seventh
+magnitude is the southern portion of that part of the constellation
+Argo, known as Puppis.
+
+In contrast to these rich regions, and in many cases closely adjoining
+them, are some barren regions, very poor in naked eye stars. For
+example, closely following the rich spot in Cassiopeia and between Iota
+Cassiopeiæ and Eta Persei is a remarkably poor spot, where a space of
+some sixty square degrees does not contain a single star brighter than
+the sixth magnitude! There is another poor region south of Alpha Hydræ,
+and another in the southern portion of the constellation Cetus.
+
+A region of considerable extent, remarkably deficient in bright stars,
+will be noticed in the Northern Hemisphere. This comparatively barren
+region, which contains no star brighter than the fourth magnitude, is
+bounded by Cepheus, Cassiopeia, Perseus, Auriga, Gemini, Ursa Major,
+Draco, and Ursa Minor, and forms a conspicuous feature in the
+north-eastern portion of the sky in the early winter evenings. It will
+be noticed that the surrounding constellations all contain bright stars.
+
+Whether the apparent crowding of stars in certain regions of the heavens
+is caused by a real proximity in space, or whether it is merely due to
+their being placed accidentally in the line of sight, is a question
+difficult to determine. In the case of star clusters, and especially the
+globular clusters, there is a high mathematical probability, amounting
+almost to absolute certainty, that they are comparatively close
+together, but in groups scattered over a considerable area, like those
+referred to above, the probability in favour of proximity is not so
+great. As we know the distance of so few stars from the earth, it is
+impossible to say whether the crowding is real or only apparent, but the
+probability seems to be that it is to some extent real.
+
+A tendency to an arrangement of stars in streams was pointed out by
+Proctor in his “Universe and the Coming Transits.” This tendency to
+stream formation may be noticed on a large scale among the naked eye
+stars, for example, in Pisces, Scorpio, the River Eridanus, Aquarius,
+and the festoon of stars in Perseus. In some of these cases, of course,
+the stars are so far apart that the formation may be more apparent than
+real, but the tendency can also be clearly recognised among the fainter
+stars, and even among those only visible in telescopes and stellar
+photographs. This tendency to run in streams is well marked on the
+photographs taken at the Paris Observatory, and on those taken by
+Professor Barnard, Dr. Max Wolf, and others. It is a suggestive fact
+that these star streams are also very noticeable in star clusters, where
+there can be little or no doubt of a physical connexion between the
+component stars. With reference to a photograph of the southern portion
+of Aquila taken by Dr. Max Wolf in July, 1892, the late Mr. Ranyard,
+remarked: “Some of the streams of fainter stars in this region are very
+striking, and must convince the most sceptical of their reality. It is
+possible to draw an arc of a circle through any three stars, and a conic
+section through any five; but where we find ten or twenty stars falling
+into line, not once, but in many cases, and that there is a curious
+similarity between the strange curves and branching streams which these
+phalanges of stars mark out on the heavens, there is no room left for
+doubt that the mind is not being led away by a tendency of the
+imagination similar to that which finds faces in the fire, or sees a man
+carrying sticks on the face of the moon. If it is proved that a group of
+stars is arranged in line or marshalled in any order, it would follow
+that the individuals of the group must be actually as well as apparently
+close to one another, and that they form some kind of system, having all
+of them had a common origin, or been subject to some common
+influence.”[148]
+
+The great majority of the star clusters are found along the course of
+the Milky Way, while the irresolvable nebulæ seem to congregate towards
+the poles of the galactic zone.
+
+Dr. Gould is of opinion that “a belt or stream of bright stars appears
+to girdle the heavens very nearly in a great circle, which intersects
+the Milky Way at about the points of its highest declination, and forms
+with it an angle not far from 20°; the southern node being near the
+margin of the Cross, and the northern in Cassiopeia.” According to
+Gould, this belt covers Orion, Canis Major, Columba, Puppis, Carina, the
+Southern Cross, Centaurus, Lupus, and the head of Scorpion in the
+Southern Hemisphere, its northern course being indicated by the
+brightest stars in Taurus, Perseus, Cassiopeia, Cepheus, Cygnus, and
+Lyra. Dr. Gould considers that our sun may possibly be a member of this
+belt of stars, which perhaps numbers less than 500, and which constitute
+“a small cluster, distinct from the vast organisation of that which
+forms the Milky Way, and of a flattened and somewhat bifid form. The
+southern portion of this supposed stream of bright stars had been
+previously recognised by Sir John Herschel, who says in his ‘Cape
+Observations,’ (p. 385), ‘It is about this region, or, perhaps, somewhat
+earlier, in the interval between η Argus and α Crucis, that the galactic
+circle, or medial line of the Milky Way may be considered as crossed by
+that zone of large stars, which is marked out by the brilliant
+constellation of _Orion_, the bright stars of Canis Major, and almost
+all the more conspicuous stars of _Argo_, the Cross, the Centaur, Lupus,
+and _Scorpion_. A great circle passing through ε Orionis and α Crucis
+will mark out the axis of the zone in question, whose inclination to the
+galactic circle is, therefore, about 20°, and whose appearance would
+lead us to suspect that our nearest neighbours in the sidereal system
+(if really such) form part of a subordinate sheet or stratum deviating
+to that extent from parallelism to the general mass which, seen
+projected on the heavens, forms the Milky Way.’”
+
+These conclusions might seem probable enough when we compare the
+supposed zone of bright stars with the very diagrammatic drawings of the
+Milky Way as shown in many star maps; but when we consider the stars
+referred to with reference to the more artistic and accurate
+delineations of the Milky Way as drawn by Boeddicker, and even by Gould
+himself, we see that most of them are involved in the milky light of the
+Galaxy, and their connexion with the Milky Way itself seems quite as
+probable as that they form a belt distinct from the galactic zone. The
+apparent connexion of the stars in question with the Milky Way does not,
+however, disprove the existence of Dr. Gould’s belt or zone of bright
+stars. If the plane of the supposed belt nearly coincided with that of
+the Milky Way, the apparent connexion might not be real.
+
+Mr. J. R. Sutton advances the theory[149] that the Milky Way consists of
+“a great ring of large stars”—Dr. Gould’s solar cluster above referred
+to—“intersecting an equal ring of small ones (the Milky Way) at the
+extremities of a common diameter.” He considers that “the great star
+belt is a genuine girdle of stars in space, in which also the
+foundations of the sidereal system are laid, the Milky Way being an
+appendant to it of lesser rank.”
+
+That the Milky Way really forms a ring of stars in space there is strong
+evidence to show. Sir William Herschel’s original theory that the
+galactic gleam is due to our sun being situated near the centre of an
+indefinite stratum of stars—the “disc theory,” as it is termed—was
+abandoned by its illustrious author in his later writings, and is now
+considered to be wholly untenable by nearly all astronomers who have
+studied the subject. Sir John Herschel remarks that the general aspect
+of the galaxy near the Southern Cross indicates “that the Milky Way, in
+this neighbourhood, at any rate, is really what it appears to be, a belt
+or zone of stars separated from us by a starless interval.” It certainly
+seems utterly improbable that the nearly circular blank space near the
+Southern Cross, known as “the coal sack,” should represent a tunnel
+through a disc, of which the thickness is comparatively small, while its
+diameter, on the “disc theory,” stretches out almost to infinity. A
+straight, tunnel-shaped opening of great length, pointing directly
+towards the earth, would form an extraordinary phenomenon even in a
+solitary instance; yet there are several somewhat similar openings to be
+found in the Milky Way, as viewed both with the naked eye and with a
+telescope. That _all_ these openings should represent tunnels radiating
+from a common centre is quite beyond the bounds of probability, and,
+indeed, such an hypothesis does not deserve serious consideration. With
+reference to a photograph of the Milky Way in the constellation Cepheus,
+Professor Barnard says, “the sky (or Milky Way) is broken up into
+numerous black cracks or crevices. Looking at these peculiar features, I
+cannot well see how one can avoid the conclusion that they are
+necessarily real vacancies in the Milky Way, through which we look out
+into the blackness of space.”[150] Using a telescope with a low power,
+Mr. S. M. Baird Gemmill says, “December 1, 1886. In sweeping over the
+constellation of Monoceros, I was much struck with the reticulated
+character of the arrangement of the brighter stars upon the glimmering
+background, and the way in which this background seemed to follow the
+reticulation. By ‘brighter stars’ are meant stars of from 8 to 10
+magnitude, for it was among these that I noticed this peculiarity of
+arrangement. It put me in mind of M. M. Henry’s photographs of Cygnus.
+The region seemed, in fact, a vast network of stars, the reticulations
+of which were separated by desert, or comparatively desert spaces.”[151]
+I have noticed the same thing myself while examining the Milky Way with
+a binocular field-glass. On October 26, 1889, I noted as follows: “North
+of Alpha Cygni, and near Xi and Nu Cygni, the nebulous light of the
+Milky Way seems to cling round and follow streams of small stars in a
+very remarkable way; numerous small ‘coal sacks’ and rifts are visible,
+in which comparatively few stars are to be seen with the binocular.”
+This observation has been fully confirmed by photographs of this region,
+taken by Dr. Max Wolf in 1891.
+
+[Illustration:
+
+  FIG. 19.—_Photograph of Milky Way, Sagittarius._
+
+  (From “Visible Universe.”)
+]
+
+That the Milky Way is not indefinitely extended in the line of sight
+seems clearly shown by Sir John Herschel’s observations in the Southern
+Hemisphere. In his “Outlines of Astronomy” (p. 578), he says: “When
+examined with powerful telescopes, the constitution of this wonderful
+zone is found to be no less various than its aspect to the eye is
+irregular. In some regions, the stars of which it is wholly composed are
+scattered with remarkable uniformity over immense tracts, while in
+others the irregularity of their distribution is quite as striking,
+exhibiting a rapid succession of closely clustering rich patches,
+separated by comparatively poor intervals, and indeed, in some
+instances, by spaces absolutely dark _and completely void of any
+star_,[152] even of the smallest telescopic magnitude.... In some, for
+instance, extremely minute stars, though never altogether wanting, occur
+in numbers so moderate, as to lead us irresistibly to the conclusion
+that, in those regions, we see _fairly through_ the starry stratum,
+since it is impossible otherwise (supposing their light not
+intercepted), that the members of the smaller magnitude should not go on
+increasing _ad infinitum_. In such cases, moreover, the ground of the
+heavens, as seen between the stars, is for the most part perfectly dark,
+which again would not be the case if innumerable multitudes of stars,
+too minute to be individually discernible, existed beyond. In other
+regions we are presented with the phænomenon of an almost uniform degree
+of brightness of the individual stars, accompanied with a very even
+distribution of them over the ground of the heavens, both the larger and
+smaller magnitudes being strikingly deficient. In such cases it is
+equally impossible not to perceive that we are looking _through_ a sheet
+of stars nearly of a size and of no great thickness compared with the
+distance which separates them from us. Were it otherwise, we should be
+driven to suppose the more distant stars uniformly the larger, so as to
+compensate by their greater intrinsic brightness for their greater
+distance, a supposition contrary to all probability. In others again,
+and that not unfrequently, we are presented with a double phænomenon of
+the same kind, _viz._, a tissue, as it were, of large stars spread over
+another of very small ones, the intermediate magnitude being wanting.
+The conclusion here seems equally evident that in such cases we look
+through two sidereal sheets separated by a starless interval.”
+
+An examination of the evidence at present available, with reference to
+the distribution of the visible stars in space, has recently been
+undertaken by Professor Kapteyn of Groningen, and an account of the
+conclusions he has arrived at may prove of interest to the reader.
+
+[Illustration:
+
+  FIG. 20.—_The Milky Way._
+
+  (From _Knowledge_, Nov., 1894.)
+]
+
+We must first explain that in order to obtain a clear view of the
+construction of the visible universe, it would be necessary to know the
+relative distances of a large number of stars; but as the distances of
+only a few stars from the earth have yet been determined by actual
+measurement, and the results hitherto obtained are open to much
+uncertainty, we must have recourse to some other method of estimating
+the distances. While travelling in a railway carriage, if we fix our
+attention on trees, buildings, and other objects we pass on our journey,
+it will be noticed that all objects apparently move past us in the
+opposite direction to that in which we are travelling, and that the
+nearer the object is the faster it seems to move with reference to
+distant objects near the horizon. So it is with the stars. As we showed
+in Chapter III., the sun is moving through space, carrying along with
+the earth all the planets, satellites, and comets, forming the solar
+system. The effect of this motion is to cause an apparent small motion
+of the stars in the opposite direction, and the nearer the star is to
+the earth, the greater will this apparent motion seem to be as in the
+case of the railway train. In addition to this apparent motion, the
+stars are themselves—like the sun—moving through space, and this _real_
+motion is also visible. If this real motion takes place in the
+_opposite_ direction to that in which the sun and earth are moving, it
+will add to the apparent motion, and will increase the star’s “proper
+motion,” as it is termed. If, on the other hand, the real motion is in
+the _same_ direction as the earth’s motion, the proper motion will be
+diminished. In either case, the nearer the star is to the earth, the
+greater will be its apparent annual displacement on the background of
+the heavens. The amount of the “proper motion” is, therefore, considered
+by astronomers to form a reliable criterion of the star’s distance from
+the earth, and the actual measures of distance which have been made show
+that this assumption is approximately true. Of fourteen stars which have
+proper motion of over three seconds of arc per annum, eleven have
+yielded a measurable parallax, or displacement, due to the earth’s
+annual motion round the sun; that is to say, eleven out of fourteen
+fast-moving stars are within a measurable distance of the earth, and
+are, therefore, near us, when compared with the great majority of stars
+which are not within measurable distance, or, at least, are beyond the
+reach of our present methods of measurement.
+
+In the case of small groups of stars, we may assume that the real
+motions of the individual stars take place indifferently in all
+directions, and that consequently, taking an average of all the motions
+of the stars composing the group, the effects due to the real motions
+will destroy each other, and there will remain, as the most reliable
+criterion, the effect due to the sun’s motion in space. If, however, we
+compare the proper motions of groups situated in _different parts_ of
+the sky, there is a consideration which, to a great extent, vitiates
+this conclusion. For, near the point of the heavens, towards which the
+sun and earth are moving, known as the “apex of the solar way,” and
+probably situated not far from the bright star Vega, as indicated by
+recent researches, and near the point away _from_ which the sun is
+moving known as the _ant-apex_, about 15° south of Sirius, there will be
+no apparent displacement due to the solar motion through space, as this
+motion takes place in the line of sight with reference to these points
+of the sky. The observed proper motion at these points will, therefore,
+be solely due to the real motions of the stars themselves in those
+regions. In other parts of the heavens, however, the total proper motion
+will be a combination of the apparent and real motions of the stars, and
+for stars in different parts of the sky, it will not follow that stars
+having equal proper motions are necessarily at the same distance from
+the earth. To make this point clearer, let us suppose that there are two
+stars at absolutely the same distance from the earth, one situated at or
+near the solar “apex,” and the other at a point 90° from the apex, and
+let us suppose that both stars are moving through space with exactly the
+same velocity and in the same direction, say at right angles to the
+direction of the solar motion. Then in the case of the star near the
+apex, the observed “proper motion” will be solely due to the star’s real
+motion, and in the star 90° distant from the apex, the proper motion
+will be solely due to the solar motion, as the star’s _real motion_,
+being in the line of sight, will not be visible. Now, unless the stellar
+motion and the solar motion happen to be equal, the observed “proper
+motions” will not be equal, although both stars are at the same distance
+from the earth. If both the stars are really at rest, the star at the
+apex will have no proper motion, while the star 90° distant will have an
+apparent proper motion due to the sun’s motion. To overcome this source
+of error in estimating the distance of a star from its proper motion,
+Professor Kapteyn made use of another measure, which is independent of
+the solar motion. This is the component of the proper motion measured at
+right angles to a great circle of the sphere passing through a star and
+the solar apex. The amount of motion in this direction will evidently
+not be affected by the sun’s motion, and from a discussion of the stars,
+contained in the Draper “Catalogue of Stellar Spectra,” which were
+observed by Bradley (and of which the proper motions are now known with
+accuracy), Professor Kapteyn finds that this motion is “nearly inversely
+proportional to the distance,” that is, the greater the motion, the less
+the distance of the stars, and the smaller the motion, the greater the
+distance. Excluding stars with proper motions greater than half a second
+of arc per annum, Professor Kapteyn found that for stars at various
+distances from the Milky Way this component of the “proper motion” forms
+a good measure of distance.
+
+As the result of his investigations on the subject, Professor Kapteyn
+arrives at the following conclusions. Neglecting stars with small or
+imperceptible proper motions, we have a group of stars which no longer
+show any condensation in a plane. Stars with very small or no proper
+motions show a condensation towards the plane of the Milky Way. This
+applies to stars of the second or solar type, as well as to those of the
+first or Sirian type of spectrum, and evidently indicates that the stars
+composing the Milky Way lie at a great distance from the earth. The
+extreme faintness of the majority of the stars composing the Galaxy
+seems in favour of this conclusion. The condensation of stars of the
+first type is more marked than those of the second, and this agrees with
+the fact which has been noticed by Professor Pickering, that the
+majority of the brighter stars of the Milky Way have spectra of the
+Sirian type.
+
+Professor Kapteyn finds that this condensation of stars with small
+proper motions is very perceptible even for stars visible to the naked
+eye, and is as well marked in those stars which have spectra of the
+second type as for all the stars of the ninth magnitude; but for stars
+of the first type the condensation is still more marked. He considers
+that this condensation is either partly real, or that there is a real
+thinning out of stars near the pole of the Milky Way. As already
+mentioned (in the beginning of this chapter), Celoria’s observations
+with a small telescope, compared with Sir William Herschel’s
+observations with a large telescope, indicate clearly that there _is a
+real thinning out_ of stars near the poles of the Galaxy.
+
+Professor Kapteyn concludes that the arrangement of the stars suggested
+by Struve—a modification of the “disc theory”—has no real
+existence.[153] He attributes the fallacy in Struve’s hypothesis to the
+fact that the mean distance of stars of a given magnitude in the Milky
+Way, and outside it, is not the same.
+
+Professor Kapteyn finds that the vicinity of the sun is almost
+exclusively occupied by stars of the second or solar type, a conclusion
+which evidently tends to strengthen Dr. Gould’s theory of a “solar
+cluster.” He finds that the number of Sirian type stars increases
+gradually with the distance, and that beyond a distance corresponding to
+a proper motion of about ¹⁄₁₄th of a second of arc per annum, the Sirian
+stars largely predominate. In the group of stars known as the Hyades,
+however, the components of which have a common proper motion both in
+amount and direction, stars of the first and second types appear to be
+mixed, and Professor Kapteyn assumes that the two types represent
+different phases of evolution, and that as the brightest stars of the
+group are chiefly of the solar type, these stars must be the largest of
+the group. From this fact he concludes the solar type stars are in a
+less advanced stage of evolution than those of the Sirian type. This
+does not agree with the generally accepted view. Professor Vogel
+considers the Sirian stars to represent an earlier stage of stellar
+evolution. Mr. Proctor held the same opinion, and in Professor Lockyer’s
+hypothesis of increasing and decreasing temperatures in stars of various
+types, he places the Sirian stars at the summit of the evolution curve,
+and the sun and solar stars just below them on the descending branch of
+the curve.[154] These hypotheses are in conformity also with the current
+opinion that the sun is a cooling body. The discrepancy may perhaps be
+explained by supposing that the _brighter_ stars of the Hyades form a
+connected group, and that some, at least, of the fainter stars do not
+belong to the group, but lie at a great distance behind it. In the case
+of the Pleiades, which form a more evident cluster, I find from the
+Draper “Catalogue of Stellar Spectra” that the great majority of the
+brighter stars have spectra of the Sirian type. Most of the stars in the
+Pleiades have a very similar proper motion, both in amount and in
+direction, and there can be no doubt that most of the brighter stars, at
+least, form a connected system. As already stated, it seems highly
+probable that the fainter stars in the Pleiades lie far beyond the
+brighter components, and have merely an optical connexion with them, and
+the same may be the case in the Hyades. The superior brilliancy of the
+stars composing the Hyades would suggest that they are nearer to the
+earth than the Pleiades group, and they may possibly form members of
+Gould’s “solar cluster.”
+
+Assuming that the distances are inversely proportional to the proper
+motions, Professor Kapteyn computes the relative volumes of the
+spherical shells which contain the stars with different proper motions
+(from one-tenth of a second to one second of arc and more). Comparing
+these volumes with the corresponding number of stars, we arrive at an
+estimate of the density of star distribution at various distances. The
+result of this calculation shows that the distribution of stars of the
+Sirian type approaches uniformity when a large number of the faint stars
+(ninth magnitude) are considered. With reference to the stars of the
+second type, however, the larger the proper motion the greater the
+number of the stars; or, in other words, the second type, or solar
+stars, are crowded together in the sun’s vicinity. Evidence in favour of
+this conclusion is afforded by the fact that, of eight stars having the
+largest measured parallax (and whose spectrum has been determined), I
+find that seven have spectra of the solar type. The exception is Sirius,
+which is evidently an exceptional star with reference to its brightness
+and comparative proximity to the earth, no other star of the first
+magnitude having nearly so large a parallax. Indeed, the average
+distance of all the first magnitude stars is about forty times the
+distance of Sirius.
+
+Professor Kapteyn finds that the centre of greatest condensation of the
+solar type stars lies near a point situated about ten degrees to the
+west of the great nebula in Andromeda, and that this centre nearly
+coincides with the point which, according to Struve and Herschel,
+represents the apparent centre of the Milky Way considered as a ring.
+This would indicate that the sun and solar system lie a little to the
+north of the Milky Way, and towards a point situated in the northern
+portion of the constellation of the Centaur. The fact is worth noting,
+that the nearest fixed star to the earth, Alpha Centauri, lies not very
+far from this point. Possibly there may be other stars in this direction
+having a measured parallax, as the southern portion of the heavens has
+not yet been thoroughly explored.
+
+Professor Kapteyn finds that for stars of equal brightness, those of the
+Sirian type are, on an average, about two and three-quarter times
+farther from the earth than those of the solar type. Now, as light
+varies inversely as the square of the distance, this would imply that
+the Sirian stars are intrinsically brighter than those of the solar
+type. This conclusion is confirmed by the great brilliancy of Sirius and
+other stars of the same type in proportion to their mass. I have shown
+in Chapter IV. that Sirius is about ten times brighter than the sun
+would be if placed at the same distance, although its mass is only twice
+the sun’s mass, as computed from the orbit of its satellite.
+
+The general conclusions to be derived from the above results seems to be
+that the sun is a member of a cluster of stars, possibly distributed in
+the form of a ring, and that outside this ring, at a much greater
+distance from us than the stars of the solar cluster, lies a
+considerably richer ring-shaped cluster, the light of which, reduced to
+nebulosity by immensity of distance, produces the Milky Way gleam of our
+midnight skies.
+
+
+
+
+                                 INDEX
+
+
+                                   A
+
+ Aberration of light, discovered, 18;
+   a proof of the earth’s revolution, 57;
+   of meteor-radiants, 396
+
+ Aboul Wefa, the moon’s variation, 5
+
+ Acceleration, 152
+
+ Achromatic lens, 177
+
+ Adams, 449;
+   discovery of Neptune, 32, 349;
+   orbit of November meteors, 393
+
+ Aerolites. _See_ Meteorites
+
+ Airy, reduction of Greenwich observations, 19;
+   search for Neptune, 32
+
+ Albategnius, movement of the sun’s apogee, 5
+
+ Albedo of Mercury, 274;
+   of Venus, 278;
+   of the earth, 289;
+   of the moon, 290;
+   of Mars, 298, 334;
+   of asteroids, 312;
+   of Jupiter, 320;
+   of Jupiter’s satellites, 330, 332;
+   of Saturn, 334;
+   of rings, 338;
+   of Titan, 342;
+   of Uranus, 345;
+   of Neptune, 349
+
+ Alcor, 402
+
+ Alcyone, 499–502
+
+ Aldebaran, 403, 404, 407, 415, 421, 423, 427
+
+ Algol, 407, 415, 453, 457, 469–474
+
+ Almagest, 4, 6
+
+ Al-Mamûm’s school of astronomy at Baghdad, 5
+
+ Alphard, 409, 415
+
+ Alphonsine tables, 6
+
+ Al-Sûfi, description of the stars, 5;
+   Alphard, red, 415;
+   Algol, red, 472
+
+ Altair, 404, 427
+
+ Altazimuth, 184, 202
+
+ Altitude, 65
+
+ Amplitude, 66
+
+ Anderson, Dr., discovery of new star, 489
+
+ Andromeda nebula, 409, 529–532
+
+ Andromedæ, Gamma, 412, 417
+
+ — Nova, 489, 491
+
+ Andromede meteor-showers, 393, 394
+
+ Angelot, lunar volcanic action, 293
+
+ Annular eclipse, 113
+
+ — nebulæ, 526, 527
+
+ Antares, 404, 415
+
+ Anthelmus, new star, 484
+
+ Antlia, 468
+
+ Aphelion, 75
+
+ Apogee, 89
+
+ Apse Line, 75
+
+ Aquilæ, Eta, 467
+
+ Arago, nature of meteorites, 392;
+   parallax of 61 Cygni, 422
+
+ Arc of meridian, 130
+
+ Arcturus, 403, 405, 406, 415, 423, 427
+
+ Argelander, solar translation, 28;
+   survey of the heavens, 38;
+   comet of 1811, 357;
+   estimate of stars of ninth magnitude, 541
+
+ Argo Nebula, 522, 523, 549
+
+ — Eta, 462–464
+
+ Argon, not a solar element, 250;
+   peculiar qualities, 255;
+   found in meteorites, 389
+
+ Aries, first point of, 67
+
+ Aristarchus, heliocentric system, 4
+
+ Aristotle, description of a comet, 358
+
+ Asteroids, position in solar system, 229, 230, 310;
+   discoveries, 311, 314;
+   diameters, 312, 315;
+   computation of orbits, 314;
+   numbers and joint mass, 315;
+   distribution, 316;
+   groups, 317;
+   origin, 318
+
+ Asterope, 498, 499
+
+ Astronomy, Greek, 3, 4;
+   Arab, 4–6;
+   Tartar, 5;
+   of the Invisible, 31;
+   gravitational, 11, 33;
+   spectroscopic, 33–36;
+   photographic, 36–38
+
+ Astrophysics, foundation of, 36
+
+ Atmosphere, of the sun, 240, 271;
+   of Mercury, 277;
+   of Venus, 278, 279;
+   of the earth, 286, 313;
+   of the moon, 294, 313;
+   of Mars, 299, 307;
+   of Vesta, 312–313;
+   presence dependant upon mass, 313;
+   of Jupiter, 326;
+   of Uranus, 345
+
+ Atmospheric refraction, 52
+
+ Augmentation of moon’s diameter, 144
+
+ Aurigæ Beta, 404, 454, 456, 457
+
+ — New Star, 489
+
+ Auroræ, magnetic relations, 17, 288
+
+ Auwers’ reduction of Bradley’s observations, 19;
+   proper motion of Sirius, 437
+
+ Azimuth, 65
+
+
+                                   B
+
+ Babinet, rarity of cometary matter, 366
+
+ Baden-Powell, Sir George, eclipse-expedition, 259;
+   coronal photographs, 271
+
+ Bailey, Prof., 441, 464, 511, 513, 539
+
+ Ball, Sir Robert, 422, 433
+
+ Barnard, Prof., photograph of corona of January 1, 1889, 268–9;
+   effect of totality, 270;
+   zodiacal counterglow, 272;
+   photograph of eclipsed moon, 296;
+   drawing of Mars, 302;
+   seas of Mars, 306;
+   measurements of asteroids, 312;
+   markings on Jupiter’s satellites, 330;
+   discovery of fifth satellite, 331;
+   measures of Saturn, 335;
+   of ring-system, 330;
+   disappearance of rings, 337;
+   eclipse of Japetus, 338;
+   compression of Uranus, 343, 344;
+   Encke’s comet, 366;
+   comet-photographs, 378–381;
+   Swift’s comet, 383;
+   Nova in Auriga, 494;
+   Alcyone, 500;
+   curved nebulosity stretching over constellation of Orion, 520;
+   annular nebulæ, 526;
+   stars in streams, 551;
+   vacancies in the Milky Way, 554
+
+ Base line, 131
+
+ Baxendell, 484
+
+ Bayer, 404, 529
+
+ Behrmann, 400, 433, 541
+
+ Bellatrix, 408
+
+ Bélopolsky, spectrographic determination of Jupiter’s rotation, 325;
+   absolute velocity of 61 Cygni, 427;
+   spectroscopic examination of Castor, 451;
+   observation of Delta Cephei, 456;
+   Beta Lyræ, 466–467
+
+ Berberich, variability of Encke’s comet, 360
+
+ Berson, aeronautic ascent, 286
+
+ Bessel, _Fundamenta Astronomiæ_, 19;
+   astronomy of the invisible, 31, 32;
+   measurement of the Pleiades, 37;
+   Halley’s comet, 355;
+   comet of 1807, 362;
+   Epsilon Lyræ, 411
+
+ Betelgeuse, 404, 408, 415, 427
+
+ Bianchi, 459
+
+ Bianchini, rotation of Venus, 280
+
+ Biela, discovery of a comet, 365
+
+ Bigelow, theory of Zodiacal Light, 272
+
+ Binary stars, 431
+
+ Biot, meteoric fall, 387
+
+ Bird, quadrants, 19, 20
+
+ “Bird, Red,” 415
+
+ Birmingham, 417, 485, 486
+
+ “Blaze Star,” 485, 487
+
+ Bliss, astronomer-royal, 19
+
+ Bode’s law, 145, 232, 311, 317, 349
+
+ Boeddicker, Dr., heat-phases of eclipsed moon, 295
+
+ Bolometer, 226, 239
+
+ Bompas, 430
+
+ Bond, W. C., discoveries of Hyperion and of Saturn’s dusky ring, 25,
+    336, 341;
+   celestial photography, 36, 37;
+   the great nebula, 530
+
+ Bradley, discoveries of aberration and nutation, 18, 20;
+   reduction of his observations, 19;
+   Saturn’s rings, 337;
+   the distance of stars, 419–420;
+   Gamma Virginis, 445–446
+
+ Brahé, Tycho, the moon’s variation, 5;
+   career, 8;
+   scheme of the celestial movements, 9
+
+ Bredichin, theory of comets’ tails, 369, 370.
+   (_See also_ Tycho.)
+
+ Brenner, ashen light of Venus, 279;
+   rotation of Venus, 280
+
+ Brightest stars, 403, 404, 546
+
+ Brinkley, 422
+
+ British catalogue, 15, 16
+
+ Brooks’ cometary discoveries, 365, 371, 380
+
+ Bunsen, foundation of spectrum analysis, 33
+
+ Burnham, 433, 437, 441, 448, 509
+
+
+                                   C
+
+ Calcium, represented in Fraunhofer spectrum, 230;
+   in chromospheric and prominence-spectra, 258, 261, 262
+
+ Calendar, 86
+
+ Callandreau, capture of comets, 372
+
+ Campbell, Prof., spectrum of Mars, 306;
+   mountains on, 307
+
+ Canals of Mars, 301–305
+
+ Cancri, S., 474
+
+ — Zeta, 439, 440
+
+ Canis Majoris, R, 473
+
+ Canopus, 403
+
+ Capella, 403, 406, 415, 427
+
+ Capricornus, 411
+
+ Capture-theory of comets, 372
+
+ Carbon in sun, 242, 250;
+   in comets, 368;
+   in meteorites, 389
+
+ Cardinal points, 51
+
+ Carrington, sun-spot zones, 247;
+   sun’s rotation, 248, 249
+
+ Casey, 433
+
+ Cassegrain telescope, 180
+
+ Cassini, rotation of Venus, 280;
+   red spot on Jupiter, 323;
+   division of Saturn’s rings, 336;
+   discoveries of Saturnian satellites, 341
+
+ Cassiopeia, Chair of, 405, 481, 549
+
+ Cassiopeiæ, Eta, 413, 450
+
+ Castor, 404, 406, 413, 450, 451
+
+ Catalogues of stars, 70
+
+ Celoria, 433, 442, 540
+
+ Centauri, Alpha, 410, 413, 422, 440, 441
+
+ — Omega, 512, 513, 516, 539
+
+ — R, 478
+
+ Cephii Delta, 417, 456, 466
+
+ — U, 474
+
+ Ceraski, luminous night-clouds, 286;
+   discovery of U Cephei, 474
+
+ Ceres, discovery, 311;
+   diameter, 312
+
+ Cerulli, rotation of Venus, 280
+
+ Ceti, Mira, 458
+
+ Challis, search for Neptune, 32
+
+ Chandler, 462, 472, 476
+
+ Charlois, asteroidal discoveries, 314
+
+ Chemistry, universal, 35, 36;
+   solar, 250, 255;
+   of prominences, 256;
+   of chromosphere, 258;
+   of comets, 368, 370, 384;
+   of meteorites, 389
+
+ Chromosphere, 253, 258
+
+ Chronograph, 175
+
+ Chronometer, 175
+
+ Circle, meridian, 198;
+   transit, 198;
+   position, 208
+
+ Circumpolar stars, 46
+
+ Clairaut, verification of Newton’s law, 11;
+   calculation of Halley’s comet, 16
+
+ Clark, Alvan, great refractors, 26
+
+ — — G., detection of the companion of Sirius, 26, 437
+
+ Clarke, dimensions of earth, 134
+
+ Clausen, groups of comets, 361
+
+ Clerke, Agnes, appearance of R Sculptoris, 416;
+   examination of Pickering’s catalogue of stars, 541;
+   estimate of total light of stars to magnitude 9½, 543
+
+ Clock, astronomical, 174;
+   driving, 186;
+   sidereal, 68
+
+ Clock stars, 82
+
+ Clusters, globular, 507–517;
+   irregular, 497–507
+
+ “Coal sacks” in Milky Way, 554
+
+ Coelostat, 194
+
+ Collimation of transit instrument, 200
+
+ Collimator of spectroscope, 215
+
+ Colours of double stars, 417
+
+ Comæ Berenices, 434, 502, 549
+
+ Comet, Aristotle’s, 352, 353;
+   of 1743, 354;
+   Newton’s, 355;
+   of 1843, 358, 359;
+   Tebbutt’s, 362, 368;
+   Donati’s, 362, 369;
+   Lexell’s, 365, 370, 371;
+   Brooks’, of 1889, 365;
+   of 1893, 380;
+   Winnecke’s, 368, 371, 372;
+   Brorsen’s, 370;
+   Tuttle’s, 372, 393;
+   Wolf’s, 377;
+   Rordame’s, 383;
+   Gale’s, 383;
+   Leonid, 393, 395
+
+ — Halley’s, return in 1759, 16, 17;
+   status in solar system, 230, 232;
+   return in 1835, 335, 336;
+   type of tail, 369;
+   a client of Neptune, 371, 372
+
+ — Encke’s, disturbed by Mercury, 273;
+   rarefaction, 366;
+   acceleration, 307;
+   exempt from Jupiter’s influence, 371
+
+ — of 1811, structure, 356, 357;
+   type of tail, 369;
+   bulk, 383
+
+ — of 1843, surprising appearance, 358;
+   conditions of movement, 359
+
+ — of 1882, photographs, 38, 361;
+   transit, 359, 361;
+   period, 300;
+   spectrum, 369
+
+ Comet, Biela’s, discovery, 365;
+   duplication, 366;
+   related meteor-swarm, 393, 394
+
+ — Wells, spectrum, 368
+
+ — photographically detected, 377
+
+ Comets, orbits of, 108;
+   periodic, 109;
+   domiciled in solar system, 230, 371, 372;
+   granular nuclei, 353, 379, 384;
+   tenuity, 354, 384;
+   classification by Olbers, 358, 383;
+   groups, 359, 361, 362;
+   disruption, 360, 366, 379;
+   photographs, 361, 377, 380;
+   chemistry, 368, 370, 385;
+   luminous by electricity, 309, 384;
+   lost, 370;
+   short-period, 370, 371;
+   capture by planets, 371, 372, 384;
+   share sun’s translation, 372;
+   meteoric relationships, 384, 393, 394
+
+ — tails, multiple, 354, 355, 361, 377;
+   electrical theory, 357, 369, 383;
+   passage of the earth through, 302, 365;
+   three types, 369;
+   structure shown in photographs, 377, 383
+
+ Common, Dr., 25, 510, 520, 532, 534
+
+ Conjunctions, 99, 103
+
+ Constant of aberration, 59
+
+ Constellations, 45
+
+ Contacts in eclipse, 114
+
+ Copeland, Dr., cometary spectra, 368;
+   Nova in Auriga, 490;
+   helium, 519
+
+ Copernicus, residence in Italy, 7;
+   theory of planetary revolutions, 8, 9, 418
+
+ Cornelius, Gamma, 479
+
+ Corona Borealis, Eta, 437
+
+ — — Gamma, 442
+
+ — solar, 253;
+   compound nature of light, 202;
+   daylight photography, 267;
+   periodicity of type, 208, 270, 272;
+   photographs, 268–271;
+   rarefaction, 271, 361;
+   connexion with Zodiacal Light, 272, 273
+
+ Coronium, 238, 262
+
+ Co-tidal lines, 165
+
+ Coudé telescope, 27, 296
+
+ Crateris, R, 478
+
+ Craters, lunar, 292, 307
+
+ Crema meteorite, 386
+
+ Cross, Southern, 410, 416, 549
+
+ Crosswires, 195, 199, 206
+
+ Crucis, Kappa, 506
+
+ Cygni Beta, 417
+
+ — Chi, 460
+
+ — (_34_), 482
+
+ — (_61_), 422, 427
+
+ — Rho, new star near, 486
+
+ — Y, 474
+
+ Cygnus, 407
+
+
+                                   D
+
+ D’Alembert, verification of Newton’s Law, 11
+
+ D’Arrest, asteroidal orbits, 316;
+   comet, 371, 372
+
+ Darwin, G. H., tidal friction, 236;
+   origin of the moon, 236, 237;
+   density of Saturn, 333
+
+ Day and night, 52
+
+ — apparent solar, 79;
+   mean solar, 79
+
+ Declination, 66
+
+ De la Rue, celestial photography, 36, 295
+
+ Delphini, Beta, 435
+
+ — Gamma, 412
+
+ Deneb, 407
+
+ Denning, rotation of Saturn, 334;
+   discovery of a comet, 370;
+   August meteors, 391;
+   meteor-radiants, 395, 396
+
+ Density of earth, 160
+
+ Deslandres, prominence-photography, 261;
+   photographs of the sun as a bright-line star, 262;
+   daylight coronal photography, 267;
+   eclipse of 1893, 270;
+   rotation of Jupiter, 325
+
+ Dewar, atmospheric resistance to meteorites, 388
+
+ Dhurmsala meteorite, 389
+
+ Diameters, determination of, 141
+
+ Diamonds in meteorites, 390
+
+ Diffraction grating, 216
+
+ Direct movement, 89
+
+ — vision spectroscope, 216
+
+ Distance of the stars, 417
+
+ Doberck, Dr., 441, 442, 447, 450, 451
+
+ Dollond, invention of achromatic lenses, 21
+
+ Donati, discovery of a comet, 362;
+   cometary spectrum, 368
+
+ Double-slit method of photography, 261
+
+ Draconis, Gamma, 419, 420
+
+ Draper, Henry, photograph of the moon, 36
+
+ Dubjago, 435
+
+ Dunér, spectroscopic measurement of the sun’s rotation, 249;
+   R Hydræ, 462;
+   Y Cygni, 474;
+   Z Herculis, 475
+
+
+                                   E
+
+ Earth, shape of, 41, 134;
+   size of, 42, 134;
+   rotation of, 47, 48, 283, 284;
+   revolution of, 57;
+   orbit of, 59, 72;
+   varying speed of, 75;
+   real path of, 77;
+   shadow of, 110;
+   mass of, 159;
+   internal heat, 284, 285;
+   age, 285;
+   atmosphere, 286, 289;
+   magnetic relations, 287, 288
+
+ Easton, 532, 548
+
+ Eccentricity of ellipse, 74
+
+ Eclipse, solar, of 1842, 253;
+   of 1860, 254;
+   of 1868, 254;
+   of 1870, 258;
+   of 1896, 259, 271;
+   of 1882, 260, 268;
+   of 1878, 268;
+   of 1889, 268, 270;
+   of 1893, 270
+
+ Eclipses, lunar, 111;
+   partial, 111, 114;
+   annular, 113;
+   magnitude of, 113;
+   total of sun, 113;
+   duration of solar, 115;
+   number of in a year, 118;
+   recurrence of, 119;
+   of satellites, 121;
+   varieties of lunar, 295;
+   of Jupiter’s satellites, 329;
+   of Saturn’s, 342
+
+ Ecliptic, 56
+
+ — obliquity of, 61
+
+ Electrical theory of photospheric radiance, 242;
+   of corona, 271, 272;
+   of comets’ tails, 357, 358, 383;
+   of cometary luminosity, 369, 384
+
+ Electra, 498, 499
+
+ Elements of an orbit, 106
+
+ Elevating floor, 193
+
+ Elger, lunar _maria_, 290
+
+ Elkin, Dr., transit of great comet, 359;
+   meteorograph, 396;
+   measurements, 421–423, 425, 433, 438
+
+ Ellipse, properties of, 73;
+   eccentricity of, 74;
+   foci of, 74;
+   to draw an, 74
+
+ Elliptical nebulæ, 529–533
+
+ Elongations, 99
+
+ Encke, discovery of a comet, 366;
+   resisting medium, 367
+
+ Enoch, Book of, 404
+
+ Equation of time, 79
+
+ Equator, terrestrial, 50;
+   celestial, 66
+
+ Equatorial coudé, 193
+
+ Equatorial telescope, 185
+
+ Equinoxes, 55;
+   precession of, 69, 167, 170
+
+ Equulei, Delta, 433
+
+ Eridani, (_40_), 444
+
+ Espin, 460, 482, 494
+
+ Establishment of a port, 165
+
+ Ether of space, 546
+
+ Euler, lunar theory, 11
+
+ Evening star, 100
+
+ Evolution, of solar system, 235, 310;
+   of terrestrial, 236, 237, 283
+
+ Eye-pieces, 182
+
+
+                                   F
+
+ Fabricius, 458, 483
+
+ Fabry, cometary orbits, 372
+
+ Faculæ, associated with sun-spots, 244;
+   rotation, 249;
+   photographed, 262
+
+ Faye, planetary origin, 235, 350;
+   water on Mars, 299
+
+ Fényi, solar eruptions, 259, 260
+
+ Finder of telescope, 187
+
+ First Point of Aries, 67
+
+ Fixed stars, 45, 423
+
+ Flammarion, rotation of Venus, 280;
+   canals of Mars, 304;
+   condition of Mars, 309
+
+ Flamsteed, first astronomer-royal, 15;
+   stellar parallax, 18;
+   Flamsteed’s star, 461
+
+ Fleming, Mrs., 460, 465, 489, 494–496
+
+ Fletcher, 447
+
+ Fomalhaut, 404
+
+ Fontana, pseudo-satellite of Venus, 282
+
+ Forbes, ultra-Neptunian planets, 231
+
+ Foucault’s pendulum, 48–50
+
+ Fraunhofer, improvement of telescopes, 21;
+   solar spectrum mapped by, 34
+
+ Fraunhofer lines, 34, 249, 259, 271;
+   interpreted, 35, 250;
+   reflected in spectrum of Uranus, 346;
+   in spectra of comets, 368
+
+ Fritsche, 433
+
+ Frost, spectrograph of Uranus, 346
+
+ Froley, 434
+
+
+                                   G
+
+ Galaxy. _See_ Milky Way
+
+ Galileo, telescopic observations, 9;
+   double-star method of parallaxes, 28, 418
+
+ Gaseous nebula, 517
+
+ Gemini, star cluster in, 504
+
+ Geminorum, Zeta, 468
+
+ Gemma, Cornelius, 479
+
+ Gemmill, 554
+
+ Geocentric positions, 70
+
+ Geodesy, 129
+
+ Gill, Dr., photographs of comet of 1882, 38, 361;
+   parallax of Sirius, 421;
+   parallax and velocity, Lacaille, 424;
+   Omega Centauri, 513
+
+ Glasenapp, 433–434
+
+ Gledhill, red spot on Jupiter, 323
+
+ Globular clusters, 507
+
+ Gnomon, 125
+
+ Goodricke, 465, 466, 470
+
+ Gould, Dr., photographic measurement of the Pleiades, 37;
+   planetary photography, 327;
+   Pi Gruis and R Sculptoris, 416;
+   Kappa Crucis, 506;
+   stars in Southern Hemisphere, 541;
+   belt of stars intersecting the Milky Way, 551
+
+ Graduated circles, 171
+
+ Grating spectroscope, 216
+
+ Gravity, surface, on Mercury, 274;
+   on Venus, 278;
+   on the moon, 293;
+   on Mars, 298;
+   on Saturn, 335;
+   on Uranus, 345;
+   on Neptune, 349
+
+ Gravitation, laws of, 153;
+   universal, 156
+
+ Greenwich observations, 15, 19, 20
+
+ Groombridge, 424
+
+ Grosch, corona of 1867, 268
+
+ Grubb, Sir Howard, great refractors, 26
+
+ — Thomas, Melbourne reflecting telescope, 24
+
+ Guinand, optical glass, 21
+
+ Gully, Ludovic, 488
+
+ Gylden, 423
+
+ Gyroscope, 50
+
+
+                                   H
+
+ Hadley, improvement of reflecting telescopes, 21
+
+ Hale, spectrographs of prominences, 261;
+   calcium light pictures of sun and surroundings, 262;
+   double-slit method of coronal photography, 267
+
+ Hall, Prof. Asaph, discovery of the moons of Mars, 26, 309;
+   rotation of Saturn, 334
+
+ — Chester More, invention of achromatic lenses, 20
+
+ — Maxwell, 472
+
+ Halley, law of gravitation, 10;
+   acceleration of the moon, 12;
+   astronomer-royal, 16;
+   comet calculated by, 16;
+   transits of Venus, 17;
+   discovery of proper motion in stars, 423;
+   discovery of the star cluster in Hercules, 507
+
+ Harding, 548
+
+ Hartwig, 488
+
+ Harvest moon, 95
+
+ Heavens, diurnal motion of, 45
+
+ Heis, 400, 401, 541
+
+ Heliocentric positions, 70
+
+ Heliometer, 209
+
+ Helium, a chromospheric element, 255, 258;
+   extracted from clevite, 255
+
+ Helmholtz, maintenance of sun’s heat, 234;
+   past duration of sunlight, 285
+
+ Hencke, asteroidal discoveries, 314
+
+ Henderson, 422, 447
+
+ Henry’s belts of Uranus, 343
+
+ Hepidannus, 479
+
+ Herculis, Alpha, 413, 416
+
+ — Zeta, 435
+
+ — Z, 475
+
+ Herschel, Sir John, mathematical analysis at Cambridge, 15;
+   observations of nebulæ, 23, 31;
+   Magellanic clouds, 30, 31;
+   survey of the heavens, 31;
+   photography of sun-spots, 36;
+   telescope, 180;
+   great spot-group in 1837, 244;
+   cyclonic theory of sun-spots, 252;
+   Halley’s comet, 355;
+   comet of 1843, 358;
+   Biela’s comet, 365;
+   red stars, 416;
+   orbit of Gamma Virginis, 446;
+   Kappa Crucis, 506;
+   2 Messier, 511–512;
+   22 Messier, 514;
+   nebula round Eta Argus, 522–523;
+   30 Doradus, 524;
+   the trifid nebula, Sagittarius, 525;
+   planetary nebula, 528–529;
+   the Nubecula Major, 534–536;
+   Milky Way, crossed by zone of large stars, 552;
+   observations in the Southern Hemisphere, 554
+
+ — Sir William, the sun’s translation, 19, 28;
+   reflecting telescopes, 21–23;
+   discovery of Uranus, 21, 22;
+   of binary stars, 28;
+   comprehensive designs, 27, 29;
+   nebular theory, 30, 35;
+   rotation of Jupiter’s satellites, 331;
+   variability of Japetus, 341;
+   discovery of Uranian moons, 347;
+   binary stars, 419, 431;
+   motion real and apparent, 428;
+   Zeta Herculis, 435;
+   Xi Ursæ Majoris, 440;
+   70 Ophiuchi, 441;
+   5 Messier, 510
+
+ Hevelius, 459, 462, 484
+
+ Hind, 433, 474, 482, 484
+
+ Hipparchus, construction of a star catalogue, 3;
+   mathematical standpoint, 4
+
+ Holden, Prof., solar rotation, 249;
+   names of asteroids, 315;
+   helical nebulæ, 528
+
+ Holmes, discovery of a comet, 379
+
+ Holwarda, Phocylides, 458
+
+ Hooke, law of gravitation, 10;
+   observations of Greek letter Draconis, 18;
+   Gamma Arietis, 412;
+   parallax of Gamma Draconis, 419–420
+
+ Horizon, visible, 41;
+   sensible, 44;
+   celestial, 44;
+   rational, 44
+
+ Horrebow, satellite of Venus, 282
+
+ Hour circle, 186
+
+ Howlett, depression of sun-spot umbræ, 251
+
+ Huggins, Dr., stellar and nebular spectra, 35;
+   photographed, 37;
+   observations of prominences, 255;
+   daylight coronal photography, 267;
+   prismatic occultation of a star, 294;
+   spectrum of Mars, 306;
+   of Jupiter, 326;
+   of Uranus, 345;
+   of Winnecke’s comet, 368;
+   spectrograph of Tebbutt’s comet, 368;
+   measurement of motion in the line of sight, 426;
+   spectroscopic examination of new star, 493;
+   spectroscopic examination of the “fish-mouth” nebula, 518;
+   discovery of gaseous spectrum, 528
+
+ Humboldt, meteoric shower of 1799, 392;
+   temporary star of 1572, 479–481
+
+ Hussey, cometary forms, 380;
+   photograph of Rordame’s comet, 383
+
+ Huygens, 417, 517
+
+ Hyades, 407, 549
+
+ Hydræ, R, 462
+
+ Hydrogen, ultra-violet spectrum in stars, 37;
+   a gaseous metal, 250;
+   a constituent of prominences and chromosphere, 255, 258;
+   velocity of molecules, 313;
+   free in atmospheres of Uranus and Neptune, 346, 349;
+   assumed constituent of comets’ tails, 369, 370
+
+ Hypothesis of external galaxies, 546
+
+
+                                   I
+
+ Infinity of Space, 546
+
+
+                                   J
+
+ Jacob, 433, 447
+
+ Jacoby, measures of photographs, 37
+
+ Janssen, photograph of the sun, 243;
+   spectroscopic method of prominence-observation, 254;
+   double-slit method, 261
+
+ Japetus, remarkable eclipse, 338;
+   variability, 341;
+   plane of orbit, 342
+
+ Jesse, luminous night-clouds, 286
+
+ Job, Book of, 404
+
+ Johnson, 450
+
+ Juno, discovery, 311;
+   diameter and albedo, 312, 316;
+   a twin of Clotho, 317
+
+ Jupiter, long inequality, 12, 17;
+   disturbance of Halley’s comet, 16;
+   influence upon asteroidal distribution, 316–318;
+   mass and figure, 318;
+   rotation, 318, 325, 326;
+   density, 319, 326;
+   reflective power, 320;
+   belts and streamers, 321, 322, 326;
+   spots, 323, 325;
+   photographs, 327;
+   disturbance of comets, 371
+
+ Jupiter’s satellites, Galilean quartette, 9, 327, 328;
+   transits, 329;
+   constitution, 330;
+   fifth satellite, 331, 332
+
+
+                                   K
+
+ Kapteyn, 422, 556, 561–563
+
+ Keeler, drawings of Jupiter, 321;
+   description of markings, 322;
+   spectroscopic test of the meteoric constitution of Saturn’s rings,
+      339;
+   measuring velocities of nebula in line of sight, 428;
+   spectra of the Orion nebula, 519–520
+
+ Kelvin, Lord, subterranean temperature, 285
+
+ Kepler’s Laws, 10, 155, 339, 417
+
+ Kirch, 460, 470, 510
+
+ Kirchhoff, spectrum analysis, 33;
+   Fraunhofer’s lines, 34
+
+ Kirkwood, distribution of asteroids, 316, 317;
+   divisions in Saturn’s rings, 338
+
+ Kleiber, number of shooting stars, 390
+
+ Koch, 461
+
+ Kreutz, relations of great southern comets, 360
+
+ Krüger, 442
+
+
+                                   L
+
+ Lacaille, southern nebulæ, 30
+
+ Lagrange, verified principle of gravitation, 11;
+   stability of solar system, 13
+
+ Lajoye, 488
+
+ Lamp, fate of Brorsen’s comet, 370
+
+ Lane’s law, 242
+
+ Langley, solar radiation, 238, 239;
+   spectroscopic effects of sun’s rotation, 249;
+   temperature of the moon, 294;
+   fireball, 386
+
+ Laplace, verified Newton’s law, 11;
+   lunar acceleration, 12;
+   _Mécanique Céleste_, 13, 14;
+   nebular hypothesis, 235
+
+ Lassell, large reflectors, 24;
+   discoveries of Hyperion, Ariel, and Umbriel, 24, 341, 347;
+   Saturn’s dark ring, 336
+
+ Latitude, terrestrial, 50, 125;
+   celestial, 68;
+   of sun, 77;
+   geocentric, 135;
+   geographical, 135;
+   astronomical, 136;
+   variation of, 136
+
+ Leland, Miss, 511
+
+ Leonid meteors, 391–395
+
+ Leonis, Gamma, 413
+
+ — R, 461
+
+ Lepaute, Madame, computation of Halley’s comet, 16
+
+ Leverrier, discovery of Neptune, 32;
+   intra-Mercurian planet, 232;
+   mass of asteroids, 315;
+   orbit of November meteors, 395
+
+ Lewis, 426
+
+ Libræ, Delta, 473
+
+ Librations, of Mercury, 277;
+   of Venus, 281;
+   of the moon, 93, 289
+
+ Lick observatory, 25, 26
+
+ Light-equation, 329
+
+ “Light journey,” 420
+
+ Limited number of visible stars, 538, 545
+
+ Limiting apertures, 212
+
+ Lippershey, inventor of the telescope, 9
+
+ Lockyer, spectroscopic observations at the sun’s limb, 254;
+   classification of prominences, 250;
+   solar tornadoes, 259
+
+ Loewy, Coudé telescope, 27;
+   lunar photography, 296
+
+ Longitude, terrestrial, 50, 125;
+   celestial, 68
+
+ Lowell, rotation of Mercury, 277;
+   observations of Venus, 279, 281;
+   lakes of Mars, 301, 302;
+   relation to canals, 302–304
+
+ Luminous night-clouds, 286
+
+ Lunar distances, 129
+
+ — ecliptic limit, 112
+
+ Lyncis (_12_), 450
+
+ Lyra, annular nebula in, 526
+
+ Lyræ, Beta, 465
+
+ Lyraid meteors, 393, 395
+
+
+                                   M
+
+ Maclear, 464
+
+ Mädler, search for Martian moons, 309;
+   compression of Uranus, 343
+
+ Madrid meteorite, 385
+
+ Magellanic clouds, 30, 534–537
+
+ Magnetism, terrestrial, 287, 288
+
+ Magnitude, of eclipses, 113;
+   of stars, 212
+
+ Magnitudes, star, 403, 404
+
+ Mann, 433
+
+ Maps, 133
+
+ Maraldi, 462, 470, 511
+
+ Marchand, observations of the Zodiacal Light, 273
+
+ Markwick, Col., 547
+
+ Mars, phases of, 104;
+   parallax of, 147;
+   a superior planet, 297;
+   seasons, 298, 301, 302;
+   snow-caps, 299, 303, 306;
+   land and water, 299–301, 305, 306;
+   continents, 300, 301;
+   canals, 301, 304;
+   duplication, 301, 305;
+   spectrum, 306;
+   atmosphere, 307, 313;
+   mountains, 307;
+   climate, 308;
+   moons, 309, 310
+
+ Marth, Neptune’s satellite, 350
+
+ Mascari, rotation of Venus, 280
+
+ Maskelyne, astronomer-royal, 19;
+   founded _Nautical Almanac_, 20;
+   star-motions, 28
+
+ Mass, defined, 151;
+   sun, 156;
+   planets, 157;
+   moon, 158;
+   of asteroids, 158;
+   earth, 159;
+   satellites, 159
+
+ Maunder, 460, 488, 493
+
+ Maxwell, Clerk, constitution of Saturn’s rings, 337, 340
+
+ Mayer, Tobias, lunar tables, 11;
+   star-motions, 28
+
+ Mazapil meteorite, 396
+
+ Measurement, of earth, 42, 129;
+   of sun’s distance, 146;
+   of binary stars, 208;
+   of planets, 208
+
+ _Mécanique Céleste_, character, 13, 14
+
+ Megrez, 402
+
+ Mercury, Copernican theory of movements, 8;
+   transit of, 101;
+   phases of, 101;
+   orbit, 273, 274;
+   atmosphere, 274, 275;
+   rotation, 275–277;
+   as an abode of life, 277;
+   capture of Encke’s comet, 372
+
+ Meridian, 50;
+   line, 51;
+   arc of, 130;
+   circle, 198;
+   photometer, 214
+
+ Merope, 498, 499
+
+ Messier (_3_), 509
+
+ — (_5_), 510
+
+ — (_11_), 506
+
+ — (_22_), 514
+
+ — (_37_), 505
+
+ — (_51_), 533
+
+ — (_57_), 526
+
+ — (_80_), new star in, 485
+
+ — (_92_), 509
+
+ — (_99_), 534
+
+ — discoveries of nebulæ, 30
+
+ Metonic cycle, 92
+
+ Meteoric systems, 231, 390, 391;
+   radiants, 392, 395, 396
+
+ Meteorites, falls, 385–387;
+   legal status, 387;
+   velocities, 387, 388, 390;
+   thumb-marks, 388;
+   chemical composition, 389;
+   enclosed diamonds, 390
+
+ Meteors, Perseid, 391, 393;
+   Leonid, 391–393;
+   Andromede, 393–394, 396;
+   relations to comets, 393, 395
+
+ Micrometer, wire, or pillar, 205;
+   evolution of, 207
+
+ Michell, prevision of binary stars, 28
+
+ Midnight sun, 63
+
+ Milky Way, 402, 430, 555, 557
+
+ — — star streams, 9;
+   disc theory, 29
+
+ Minimum deviation, 215
+
+ Mira Ceti, 458, 459
+
+ Mitchell, 421, 431
+
+ Mizar, 402, 411, 455, 457
+
+ Molyneux, 419, 420
+
+ Montanari, 471
+
+ Month, 91
+
+ Moon, acceleration, 12;
+   _contumax sidus_, 16;
+   observations, 19;
+   apparent motion of, 87;
+   orbit of, 88, 94;
+   phases of, 89;
+   sidereal period of, 89;
+   synodic period of, 91;
+   rotation of, 92;
+   librations of, 93;
+   harvest, 95;
+   high and low, 97;
+   shadow of, 115;
+   distance of, 143;
+   size of, 144;
+   mass of, 158;
+   possible disintegration, 233;
+   origin, 236, 237;
+   rotation, 289;
+   cones and craters, 290, 292, 293;
+   rays and rills, 293;
+   absence of air and water, 294, 313;
+   temperature, 294, 295;
+   eclipses, 295;
+   photography, 295–297
+
+ Morning star, 100
+
+ Müller, surface of Mercury, 275;
+   photometry of asteroids, 312;
+   albedo of Jupiter, 320;
+   of Saturn, 334;
+   of Neptune, 349
+
+ Muscæ, R, 468
+
+
+                                   N
+
+ Nadir, 45
+
+ Nasir Eddin, planetary tables, 5
+
+ Nasmyth, conjunction of Mercury and Venus, 278
+
+ Nearest fixed stars, 417
+
+ Nebula, Orion, 23, 25, 30
+
+ Nebulæ, structure, 23;
+   spiral, 24;
+   photographs, 23, 25;
+   first discoveries, 29, 30;
+   status, 30, 31;
+   gaseous nature, 30, 35;
+   annular, 526, 527;
+   elliptical, 529, 533;
+   gaseous, 517–524;
+   planetary, 527–529;
+   spiral, 533, 534
+
+ Nebular hypothesis, 30, 35, 235, 530
+
+ Nebulous stars, 529
+
+ Neptune, discovery, 32, 229;
+   distance from the sun, 232;
+   dimensions, 349;
+   compression, 351;
+   retrograde rotation, 351;
+   planets as viewed from, 351, 352;
+   family of comets, 371, 372
+
+ Neptune’s satellite, discovery, 24;
+   plane of revolution, 350;
+   precessional disturbance, 351
+
+ Newall, 25-inch refractor, 26
+
+ Newcomb, Prof., past duration of sunlight, 285;
+   light changes of Ariel, 347;
+   satellite of Neptune, 351;
+   the runaway star, 424;
+   proper motion of Alcyone, 501
+
+ New stars, 477–497
+
+ Newton, H. A., capture of comets, 372;
+   meteoric cult, 387;
+   daily number of shooting stars, 390
+
+ — Sir Isaac, law of gravitation, 10, 11;
+   invention of reflecting telescope, 21;
+   comet of 1680, 355;
+   decay of comets, 366
+
+ Newtonian telescope, 179
+
+ Nichol, Dr., 508
+
+ Niesten, rotation of Venus, 280;
+   mass of asteroids, 315
+
+ Nodes, 94
+
+ North polar distance, 66
+
+ Nova Andromedæ, 488
+
+ — Aurigæ, 489
+
+ — Cassiopeiæ, 479
+
+ — Cygni, 486
+
+ — Ophiuchi, 484
+
+ — Serpentarii, 483
+
+ — Vulpeculæ, 484
+
+ Nubecula Major, 534
+
+ — Minor, 535
+
+ Number of visible stars, 538–544
+
+ Nutation, 169
+
+
+                                   O
+
+ Oases of Mars, 302–305
+
+ Object-glass, achromatic, 177;
+   photographic, 195;
+   photo-telescope, 196
+
+ Objective prism, 223
+
+ Obliquity of ecliptic, 61
+
+ Observatories, 191
+
+ — Lick, 189, 190, 202
+
+ — Nice, 192
+
+ — Yerkes, 189
+
+ Occultations, 121
+
+ — of stars, by the moon, 294;
+   by comets, 366
+
+ Olbers, discovery of Pallas and Vesta, 311;
+   origin of asteroids, 311, 316;
+   electrical theory of comets, 357;
+   classification, 358, 383;
+   comet discovered by, 371
+
+ Ophiuchi, Nova, 483–485
+
+ — (_70_), 441
+
+ — U, 473, 476
+
+ Opposition, 103, 105
+
+ Orbit, of earth, 72, 76;
+   of moon, 88;
+   elements of a planetary orbit, 106;
+   of binary stars, 432
+
+ Orion, 406, 408, 417
+
+ — great nebula in, 517–521
+
+
+ Orionis, Alpha (Betelgeuse), 404, 408, 415, 427
+
+ — Iota, 414
+
+ — Sigma, 414
+
+ — Theta, 414
+
+ “Owl,” nebula, 528
+
+
+                                   P
+
+ P (_34_) Cygni, 482
+
+ Palisa, discoveries of asteroids, 314
+
+ Palitzsch, 470
+
+ Pallas, discovery, 311;
+   diameter, 312
+
+ Parallax of stars, 419, 420
+
+ — diurnal, 140;
+   equatorial horizontal, 140;
+   horizontal, 140;
+   of sun, 146;
+   of Mars, 147
+
+ Parmentier, distribution of asteroids, 316
+
+ Pegasus, Square of, 409
+
+ Pegasi, Kappa, 433
+
+ — (_85_), 434
+
+ — U, 469
+
+ Pendulum observations, 135;
+   compensated, 174
+
+ Penumbra, of earth’s shadow, 111
+
+ Percentage of stars in Milky Way, 547, 548
+
+ Perigee, 89
+
+ Perihelion, 75
+
+ Perrotin, rotation of Venus, 280;
+   of Uranus, 343;
+   markings on Uranus, 344
+
+ Persei, Beta (Algol). _See_ Algol
+
+ Perseid meteors, 391;
+   associated with Tuttle’s comet, 393
+
+ Perseus, 407
+
+ — star clusters in, 503
+
+ Perturbations, 158
+
+ Peters, 428
+
+ Phases of moon, 89;
+   of Venus, 101;
+   of Mars, 104
+
+ Phocylides Holwarda, 458
+
+ Photographic telescopes, 194
+
+ Photography of nebulæ, 23, 25, 38;
+   of sun-spots, 36, 243, 244;
+   of the moon, 36, 295–297;
+   of stellar spectra, 37;
+   of comets, 38, 354, 377–383;
+   celestial, 194;
+   of spectra, 219, 223;
+   of the eclipsed sun, 254;
+   of the reversing layer, 259;
+   of prominence-spectra, 260;
+   of prominences and faculæ, 261, 262;
+   of the corona, 267, 269–271;
+   planetary, 327; meteoric, 396
+
+ Photoheliograph, 197
+
+ Photometers, wedge, 213;
+   meridian, 214
+
+ Photosphere, visible structure, 242
+
+ Piazzi, five-foot circle, 20;
+   discovery of Ceres, 311
+
+ Pickering, Prof. E. C., photometric measures of asteroids, 312;
+   photograph of Jupiter, 327;
+   the spectrum of Alpha Centauri, 441;
+   the spectrum of Pleione, 498
+
+ Pickering, W. H., lunar photographs, 296;
+   mounting of telescopes, 297;
+   lakes and canals of Mars, 301, 304;
+   water area on Mars, 305;
+   star collisions, 495;
+   nebula surrounding Zeta Orionis, 520
+
+ Pigott, 467
+
+ “Pilgrim star,” 479–482
+
+ Planetary nebulæ, 527–529
+
+ Planets, apparent movements of, 98;
+   interior and exterior, 98;
+   conjunctions of, 99, 103;
+   phases of, 101, 104;
+   oppositions of, 103;
+   synodic periods of, 107;
+   times of revolution, 107;
+   relative distances of, 144;
+   distances of, 150;
+   terrestrial, 229;
+   giant, 229, 319, 343;
+   trans-Neptunian, 231;
+   intra-Mercurian, 232;
+   decay, 233;
+   comets captured by, 371, 372
+
+ — minor. _See_ Asteroids
+
+ Pleiades, 404, 407, 497–502, 539, 549
+
+ Pleione, 498
+
+ “Plough,” 400–402, 405
+
+ Plummer, short-period comets, 371;
+   Encke’s, 372
+
+ Podmaniczky, Baroness, 488
+
+ Pogson, 485
+
+ Polar axis, 185
+
+ Polaris. _See_ Pole Star
+
+ Pole, celestial, 46; terrestrial, 50;
+   movements of, 138
+
+ Pole Star, 46, 405, 412, 421, 427
+
+ Pollux, 404, 406, 415, 427
+
+ Pond, defects of Greenwich quadrant, 19;
+   astronomer-royal, 20
+
+ Position, angle, 208;
+   circle, 208
+
+ Poynting’s experiment, 160
+
+ Præsepe, 502
+
+ Precession, of equinoxes, 69;
+   effects of, 170;
+   luni-solar, 169
+
+ Prime vertical, 210
+
+ Principia, publication, 10, 13;
+   character, 14
+
+ Prism, action of, 215;
+   objective, 223
+
+ Prismatic camera, 223
+
+ — spectroscope, 215
+
+ Pritchard, Prof., 422, 424
+
+ Proctor, Saturn’s rings, 341;
+   distance of Uranus, 344;
+   Proctor’s chart, 548;
+   stars in streams, 550
+
+ Procyon, supposed satellite, 31, 32;
+   order of magnitude, 404;
+   parallax of Procyon, 421;
+   Procyon approaching the Earth, 427
+
+ Prominences, solar appendages, 253, 254;
+   spectrum, 254, 256, 258, 260;
+   daylight observations, 254, 255;
+   quiescent and eruptive, 256;
+   periodicity, 257;
+   rapid development, 259;
+   spectral photography, 260, 261
+
+ “Proper motions” of stars, 423–431
+
+ Ptolemaic system, 3, 4, 6
+
+
+                                   Q
+
+ Quadrature, 105
+
+
+                                   R
+
+ Rambaud, absorption in solar atmosphere, 240;
+   fireball, 380
+
+ Ramsay, terrestrial discovery of helium, 255
+
+ Ramsden, astronomical circles, 20
+
+ Raynard, the sun a nebulous body, 253;
+   future of Saturn’s ring-system, 340;
+   outflows from comets, 380;
+   star streams, 551
+
+ Ravené, gravitational disturbance by asteroids, 315
+
+ R Centauri, 478
+
+ Reading microscope, 172
+
+ Recurrence of eclipses, 119
+
+ “Red Bird,” 415
+
+ Red spot on Jupiter, 323, 324
+
+ Red stars, 416
+
+ Reduction of observations, 18, 19
+
+ Refracting telescope, 176
+
+ Refraction, 52
+
+ — in Venus, 278
+
+ Reflecting telescope, 178
+
+ Regression of moon’s nodes, 94
+
+ Regulus, 406, 410, 427
+
+ Retrogradation, 89, 103
+
+ Reversing layer, 249, 258, 271;
+   photographed, 259
+
+ Rich and poor regions, 549
+
+ Richaud, 440
+
+ Rigel, 404, 414, 427
+
+ Right ascension, 66
+
+ Roberts, Dr., 23, 107, 502, 503, 508, 511, 520, 525–528, 530, 533, 534,
+    539, 540
+
+ Roberts, A. W., 440, 441, 469
+
+ Roche, minimum distance of satellites, 340
+
+ Römer, velocity of light, 329
+
+ Rosse, Earl of, giant reflector, 24
+
+ Roszl, mass of 311 asteroids, 315
+
+ Rotation of earth, 47, 48;
+   of moon, 92
+
+ Rowland grating, 217
+
+ — solar elements, 250
+
+ Russell, photograph of Swift’s comet, 377;
+   Kappa Crucis, 506;
+   the “key-hole” nebula, 522;
+   the Magellanic clouds, 536
+
+ Rutherfurd, photographs of the moon, 295
+
+
+                                   S
+
+ Sacrobosco, treatise on the sphere, 6
+
+ Sagittarii, Zeta, 434
+
+ Saros, 120
+
+ Satellites, movements of Satellites, 108;
+   masses of Satellites, 159
+
+ — discoveries, 9, 23, 25, 26, 309, 347;
+   apportionment, 230;
+   formation checked by tidal friction, 277, 282;
+   planes of revolution, 328, 347, 348, 350;
+   transits, 329, 330, 342;
+   eclipses, 329, 342;
+   variability, 330, 341, 347;
+   rotation, 331, 341, 342, 347
+
+ Saturn, density, 333;
+   spectrum, 334;
+   rotation, 334, 339;
+   dimensions, 535
+
+ Saturn’s ring-system, dusky member, 25, 336, 338;
+   dimensions, 336;
+   constitution, 337, 339, 340;
+   albedo, 338
+
+ Sawyer, U Ophiuchi discovered, 473;
+   variability of R Canis Majoris detected, 473
+
+ Schaeberle, photographs of corona of 1893, 270;
+   land and water on Mars, 306
+
+ Scheiner, spectra of sun-spots, 251
+
+ Schiaparelli, rotation of Mercury, 275;
+   map of Mercury, 277;
+   rotation of Venus, 280, 281;
+   canals of Mars, 301;
+   duplication, 305;
+   climate of Mars, 308;
+   compression of Uranus, 343;
+   comets and meteors, 393;
+   theory of extinction of light, 544
+
+ Schiehallion experiment, 161
+
+ Schmidt, map of the moon, 290
+
+ Schönfeld, 461, 464, 466, 467, 473, 483
+
+ Schorr, 442
+
+ Schur, 433, 442
+
+ Schuster, photograph of eclipsed sun, 268
+
+ Schwabe, discovery of sun-spot periodicity, 245
+
+ Seasons, 61
+
+ Secchi, observations of prominences, 256;
+   spectrum of Uranus, 345
+
+ See, Dr., 413, 433–435, 440, 442, 447, 448
+
+ Seeliger, photometric measures of Saturn’s rings, 339
+
+ Serpentarii, Nova, 483
+
+ Sextant, 211
+
+ Shackleton, photograph of the reversing layer, 259
+
+ Ship, position of, 128
+
+ “Sickle” in Leo, 406
+
+ Siderostat, 194
+
+ Sidgreaves, elevations of chromosphere, 258
+
+ Sirius, proper motion, 17, 31;
+   companion, 32;
+   spectrum, 37;
+   size, 403;
+   position, 409;
+   colour, 414;
+   distance, 418, 421;
+   discovery of proper motion, 423;
+   a binary star, 437;
+   comparative magnitude, 438
+
+ Smyth, 447, 448, 502, 505, 508
+
+ Solar, constant, 239
+
+ — diagonal, 183
+
+ — eclipses, 113
+
+ — ecliptic limit, 118
+
+ — System, dominated by gravity, 29;
+   constitution, 229, 232;
+   dimensions, 231;
+   stability, 232;
+   origin, 235, 236
+
+ Southern Cross, 410, 416, 549
+
+ Shouting, 68, 198
+
+ Spectroheliograph, 225
+
+ Spectroscope, prismatic, 215;
+   direct vision, 216;
+   grating, 216;
+   Lick star-, 219;
+   Rowland, 217;
+   tele-, 219
+
+ Spectroscopic measurements of rotation;
+   the sun, 248;
+   Venus, 281;
+   Saturn, 339
+
+ Spectrum, solar, 34, 250;
+   of stars and nebulæ, 35, 37;
+   measurement of, 218;
+   sun-spot, 250, 251;
+   prominence, 254, 256, 261;
+   chromospheric, 258;
+   of Mercury, 275;
+   of Venus, 279;
+   auroral, 288;
+   of Jupiter, 326;
+   of Saturn’s rings, 338;
+   of Uranus, 345, 346;
+   of Neptune, 351;
+   of comets, 368
+
+ Spherical excess, 133
+
+ Spica, 404, 410
+
+ Spiral nebulæ, 533, 534
+
+ Spoerer, solar rotation, 249
+
+ Star of Bethlehem, 101
+
+ Star-charting, photographic, 38
+
+ — cluster, 17
+
+ — spectroscope, 219
+
+ — time, 68
+
+ Stars, temporary, 3, 8, 477;
+   proper motions of, 17, 19, 28, 425, 427;
+   fixed, 45;
+   circumpolar, 46;
+   diurnal motion of, 46;
+   aberration of, 58;
+   catalogues of, 71;
+   clock, 82;
+   morning and evening, 100;
+   magnitudes, 403, 404;
+   Pole, 405, 412;
+   double, 410;
+   coloured, 416;
+   red, 416;
+   nearest, 417;
+   binary, 431;
+   variable, 458
+
+ Stationary points, 103
+
+ Stone, mass of Titan, 342
+
+ Stoney, G. Johnstone, atmospheres of planets, 313
+
+ Stratonoff, sun’s rotation from faculæ, 249
+
+ Suess, theory of lunar formations, 292
+
+ Sun, translation, 28, 229;
+   apparent movements of, 55, 77;
+   midnight, 63;
+   apparent diameter of, 72;
+   mean, 79;
+   eclipses of, 113;
+   distance of, 146;
+   mass of, 156;
+   maintenance of heat, 234;
+   radiative power, 237–239, 241, 242;
+   temperature, 239, 240;
+   magnitude, 240, 241;
+   luminous surface, 242;
+   spots, 243–249, 251, 252;
+   periodicity, 246;
+   rotation, 247–249;
+   chemistry, 250;
+   theories, 252
+
+ Sun-dial, 78
+
+ Sun’s motion in space, 428
+
+ Sun-spots, observed by Galileo, 9;
+   construction, 243, 251;
+   zones, 245, 247;
+   periodicity, 245, 247;
+   irregular movements, 247–249;
+   spectra, 250–252
+
+ Sutton, 553
+
+ Swift, Lewis, comet discovered by, 377, 378, 383
+
+ Sykora, elevation of spotted areas on the sun, 252
+
+ Synodic period, of moon, 91;
+   of planets, 107
+
+
+                                   T
+
+ Tacchini, spectrum of Venus, 279;
+   rotation, 280
+
+ Talcott’s latitude method, 124
+
+ Tauri, Alpha. _See_ Aldebaran
+
+ — Lambda, 473
+
+ Tebbutt’s comet, 362, 368
+
+ Telescope, invention of, 9;
+   achromatic, 20, 21;
+   reflecting, 21, 24, 25, 178;
+   refracting, 20, 25–27, 176;
+   future improvement, 26, 27, 297;
+   Newtonian, 179;
+   Cassegrain, 180, 181;
+   Herschellian, 180;
+   Skew Cassegrain, 181;
+   magnifying power of, 184;
+   illuminating power of, 184;
+   altazimuth, 184;
+   equatorial, 185;
+   Rosse, 187;
+   Common, 5-foot, 188;
+   Lick, 190;
+   fixed, 194;
+   photographic, 194
+
+ Telespectroscope, 219
+
+ Tempel, 501
+
+ “Temporary stars,” 477–497
+
+ Theodolite, 205
+
+ Thiele, 435, 447, 451
+
+ Thome, comet of 1887, 360
+
+ Tidal evolution, 167
+
+ Tidal friction, 166;
+   in earth-moon system, 236, 283, 284;
+   on Mercury, 277;
+   effect on satellite-formation, 278, 282;
+   on Venus, 282;
+   on Phobos, 310;
+   on Saturnian satellites, 342
+
+ Tides, 162;
+   spring and neap, 164;
+   priming and lagging, 164
+
+ Time, apparent, 78;
+   equation of, 79;
+   mean solar, 79;
+   determination of, 82;
+   at different places, 83;
+   Greenwich mean, 83;
+   local, 83;
+   telegraphy, 84;
+   zone, 84;
+   balls, 85
+
+ Tisserand, revolutions of Jupiter’s fifth satellite, 331;
+   disturbance of Neptune’s satellite, 351;
+   capture of comets, 372
+
+ Todd, Miss M. L., drawing of corona, 268
+
+ — Prof., trans-Neptunian planet, 231
+
+ Toucani (_41_), 513
+
+ Transit circle, 198–202
+
+ — instrument, 202
+
+ — of Venus, 101, 148
+
+ Triangulation, 53
+
+ Troughton, instrumental improvements, 19, 20
+
+ Trouvelot, mountains of Venus, 279;
+   rotation, 280
+
+ Twilight, 53
+
+ Tycho Brahé, 5, 8, 9, 405, 418, 479, 481, 529
+
+
+                                   U
+
+ Ulugh Beigh, observations at Samarcand, 5
+
+ Umbra of earth’s shadow, 111
+
+ Uranus, discovery, 22, 229;
+   perturbations, 32, 231;
+   dimensions and markings, 343–345;
+   analogy with Neptune, 343, 351;
+   rotation, 344, 348;
+   spectrum, 345, 346;
+   satellites, 347, 348;
+   comets captured by, 371, 395
+
+ Ursa Major, stars in, 400, 401
+
+ Ursæ Majoris, Xi, 440
+
+
+                                   V
+
+ Variation of latitude, 136
+
+ Variable stars, 458
+
+ Vega, 403, 406, 414, 422, 427
+
+ Venus, phases observed by Galileo, 9;
+   transits, 17;
+   phases of, 101;
+   transit of, 101, 148;
+   atmosphere, 278, 281, 282;
+   ashen light, 279;
+   spectrum, 279;
+   rotation, 280, 281;
+   imaginary satellite, 282
+
+ Vernier, 172
+
+ Very, distribution of lunar heat, 295
+
+ Vesta, discovery, 311;
+   diameter and brightness, 312;
+   mass, 313
+
+ Villarceau, 433
+
+ Virginis, Alpha (Spica), 404, 410
+
+ — Gamma, 413, 444–450
+
+ — Tau, 456
+
+ — W, 469
+
+ Visible stars, number of, 538–546
+
+ Vogel, spectrum of Jupiter, 326;
+   of Uranus, 345;
+   binary or multiple system of Beta Lyræ, 466;
+   diameter of Algol, 472
+
+ Volcanic action, terrestrial, 284;
+   lunar, 290, 292
+
+ Von Gothard, 465
+
+ Vulpeculæ, Nova, 484
+
+ — S, 484
+
+
+                                   W
+
+ Ward, 488
+
+ Way, Milky, 402, 430, 549, 557
+
+ Webb, 528
+
+ Wedge, photometer, 213
+
+ Weight, defined, 151;
+   of the earth, 160
+
+ Wells’ comet, 368
+
+ Williams, A. Stanley, rotation of Venus, 280;
+   of Jupiter, 325;
+   photographs of Jupiter, 327;
+   spots on Saturn, 334
+
+ Wilson, Alexander, depression of sun-spots, 251
+
+ — W. E., temperature of the sun, 240
+
+ Winnecke, 422
+
+ Winnecke’s comet, 368, 371, 372
+
+ Wire micrometer, 205
+
+ Wolf, Max, photographic discovery of asteroids, 314;
+   comet discovered by, 377;
+   chart of the Pleiades, 499
+
+ Wrublewsky, 434
+
+
+                                   Y
+
+ Year, 85;
+   sidereal, 85;
+   tropical, 85;
+   leap, 86
+
+ Yendell, 474
+
+ Yerkes, 40-inch refractor, 26, 27
+
+ Young, solar eruption, 256;
+   spectrum of chromosphere, 258;
+   reversing layer, 258;
+   spectrum of Venus, 279;
+   brightness of Phobos, 309;
+   belts of Uranus, 343;
+   size of Uranus, 345;
+   the sun and planets, seen from Neptune, 349, 352;
+   Andromede meteors, 394
+
+
+                                   Z
+
+ Zenith, 45
+
+ — telescope, 210
+
+ Zodiac, 60
+
+ Zodiacal Light, 272, 273
+
+ Zöllner, albedo of Mars, 298, 334;
+   of Jupiter, 320;
+   of Neptune, 349;
+   estimate of sunlight, 543
+
+ Zone time, 84
+
+
+                                THE END.
+
+-----
+
+Footnote 1:
+
+  There is a very complete paper on “How to find Easter,” by Dr.
+  Downing, in the _Journal_ of the British Astronomical Association,
+  vol. ii., p. 264.
+
+Footnote 2:
+
+  The application of Kepler’s third law gives us P = _a_^{³⁄₂} years,
+  but as this is not strictly true, both P and _a_ must be given where
+  the greatest possible accuracy is desired.
+
+Footnote 3:
+
+  The diagram is based upon one given by Prof. Albrech in the
+  _Astronomische Nachrichten_, No. 3333. The dotted part of the curve
+  could not be directly derived on account of insufficient observations.
+
+Footnote 4:
+
+  The focal length of a lens is the distance from its centre at which an
+  image of a very distant object, such as the sun, is formed.
+
+Footnote 5:
+
+  In a British inch there are 25·4 millimetres.
+
+Footnote 6:
+
+  Proctor: “Old and New Astronomy,” p. 327.
+
+Footnote 7:
+
+  Langley: “The New Astronomy,” p. 108.
+
+Footnote 8:
+
+  The “bolometer,” invented by Langley, measures heat with exquisite
+  refinement by means of its electrical effects.
+
+Footnote 9:
+
+  W. E. Wilson: _Monthly Notices_, vol. lv., p. 457.
+
+Footnote 10:
+
+  _Observatory_, vol. xviii., p. 344.
+
+Footnote 11:
+
+  Frost-Scheiner: “Astronomical Spectroscopy,” p. 177.
+
+Footnote 12:
+
+  _Astronomische Nachrichten_, No. 3330.
+
+Footnote 13:
+
+  _Knowledge_, vol. vi., p. 13.
+
+Footnote 14:
+
+  “The Sun,” p. 206, first edition.
+
+Footnote 15:
+
+  “Memoirs of the Royal Astronomical Society,” vol. xli., p. 435.
+
+Footnote 16:
+
+  _Astronomy and Astro-Physics_, vol. xiii., p. 122.
+
+Footnote 17:
+
+  _Comptes Rendus_, December 26, 1893.
+
+Footnote 18:
+
+  _Knowledge_, vol. iv., p. 105.
+
+Footnote 19:
+
+  “Rapport de la Mission envoyée an Sénégal,” p. 31.
+
+Footnote 20:
+
+  “Harvard Annals,” vol. xix., part ii.; 1893.
+
+Footnote 21:
+
+  “The Solar Corona discussed by Spherical Harmonics;” Washington, 1889.
+
+Footnote 22:
+
+  _Bulletin Astronomique_, April, 1896.
+
+Footnote 23:
+
+  According to G. Müller, _Potsdam Publicationen_, No. 30, p. 369,
+  Zöllner fixed the albedo of Mercury at 0·13.
+
+Footnote 24:
+
+  _Astr. Nach._, No. 3171.
+
+Footnote 25:
+
+  _Astr. Nach._, No. 3406.
+
+Footnote 26:
+
+  _Ibid._, No. 2944.
+
+Footnote 27:
+
+  _Astr. Nach._, No. 3332.
+
+Footnote 28:
+
+  This was in principle suggested by Proctor in “The Old and New
+  Astronomy.”
+
+Footnote 29:
+
+  _Nature_, vol. li., p. 227.
+
+Footnote 30:
+
+  Kelvin, _Nature_, p. 440; Clarence King, _American Journal of
+  Science_, January, 1893.
+
+Footnote 31:
+
+  _Ciel et Terre_, 16th March, 1895.
+
+Footnote 32:
+
+  _Himmel und Erde_, Feb., 1889; _Astr. Nach._, No. 3347; A. Battandier,
+  _L’Astronomie_, 1894.
+
+Footnote 33:
+
+  Balfour Stewart: “Ency. Brit.,” vol. xvi. pp. 164, 165.
+
+Footnote 34:
+
+  A. Paulsen: _Ciel et Terre_, 1 Juillet, 1895, p. 202.
+
+Footnote 35:
+
+  Elger: “The Moon,” p. 73.
+
+Footnote 36:
+
+  “Publications, Astronomical Society of the Pacific,” vol. vii., p.
+  144.
+
+Footnote 37:
+
+  “Harvard Annals,” vol. xxxii., part i., p. 109.
+
+Footnote 38:
+
+  _Astronomy and Astro-Physics_, Nov., 1894, p. 718.
+
+Footnote 39:
+
+  “Popular Astronomy,” 1895, p. 347.
+
+Footnote 40:
+
+  _Astr. Nach._, No. 3271 (Schiaparelli).
+
+Footnote 41:
+
+  “Popular Astronomy,” vol. i., p. 348.
+
+Footnote 42:
+
+  _Scientific American_, Feb. 29, 1896.
+
+Footnote 43:
+
+  Schiaparelli: _Astronomy and Astro-Physics_, Nov., 1894, p. 720.
+
+Footnote 44:
+
+  _Astronomy and Astro-Physics_, August, 1894, p. 554.
+
+Footnote 45:
+
+  “Publ. Astro. Soc. of the Pacific,” vol. iv., p. 196.
+
+Footnote 46:
+
+  _Monthly Notices_, vol. lvi., p. 166.
+
+Footnote 47:
+
+  Campbell: “Publ. A. S. P.,” vol. vi., p. 273.
+
+Footnote 48:
+
+  _Ibid._, vol. ii., p. 248.
+
+Footnote 49:
+
+  _Ibid._, vol. vi., p. 110.
+
+Footnote 50:
+
+  _Astronomy and Astro-Physics_, October, 1894, p. 640.
+
+Footnote 51:
+
+  _Potsdam Publicationen_, No. 30, 1893.
+
+Footnote 52:
+
+  Barnard: _Monthly Notices_, vol. lvi., p. 55.
+
+Footnote 53:
+
+  John Hopkins’ _University Circular_, Jan., 1895.
+
+Footnote 54:
+
+  _Astr. Nach._, No. 3359.
+
+Footnote 55:
+
+  _Monthly Notices_, vol. lvi., p. 250.
+
+Footnote 56:
+
+  Barnard: _Astr. Journal_, No. 325, 1894.
+
+Footnote 57:
+
+  “Publ. A. S. P.,” vol. ii., p. 286.
+
+Footnote 58:
+
+  Maunder: _Knowledge_, vol. xix., p. 5.
+
+Footnote 59:
+
+  _Monthly Notices_, vol. lvi., p. 143.
+
+Footnote 60:
+
+  “Scientific Proceedings, R. Dublin Society,” vol. viii., p. 398.
+
+Footnote 61:
+
+  _Astro.-Phys. Journal_, May, 1896, p. 394; “Rapport de l’Observatoire
+  de Paris,” 1895, p. 22.
+
+Footnote 62:
+
+  Proctor: “Old and New Astronomy,” p. 584.
+
+Footnote 63:
+
+  “The subject of slant-markings,” Mr. Stanley Williams remarks (_loc.
+  cit._), “has only just begun to be investigated.”
+
+Footnote 64:
+
+  “Jupiter and his System,” by Ellen M. Clerke, p. 43.
+
+Footnote 65:
+
+  _Comptes Rendus_, t. cxix., p. 581.
+
+Footnote 66:
+
+  G. H. Darwin: _Harper’s Magazine_, June, 1889.
+
+Footnote 67:
+
+  Barnard, _Monthly Notices_, vol. lvi., p. 163.
+
+Footnote 68:
+
+  Lewis: _Observatory_, vol. xviii., p. 379.
+
+Footnote 69:
+
+  _Monthly Notices_, vol. lii., p. 419.
+
+Footnote 70:
+
+  “Abhandlungen Akad. der Wissensch.” München, Bl. xvi., p. 403.
+
+Footnote 71:
+
+  _Astro-Physical Journal_, May, June, 1895.
+
+Footnote 72:
+
+  “Old and New Astronomy,” p. 640.
+
+Footnote 73:
+
+  “Phil. Trans.,” vol. lxxxii., p. 17.
+
+Footnote 74:
+
+  “Publications Astr. Soc. of the Pacific,” vol. iii., p. 284.
+
+Footnote 75:
+
+  _Astr. Journal_, No. 370.
+
+Footnote 76:
+
+  Perrotin: “Vierteljahrsschrift Astr. Ges.,” Jahrg. xxiv., p. 267.
+
+Footnote 77:
+
+  “Annales de l’Observatoire de Nice,” t. ii., 1887.
+
+Footnote 78:
+
+  Keeler: _Astr. Nach._, No. 2927.
+
+Footnote 79:
+
+  Gregory: _Nature_, vol. xl., p. 236.
+
+Footnote 80:
+
+  “General Astronomy,” p. 372.
+
+Footnote 81:
+
+  _Astronomical Journal_, No. 342.
+
+Footnote 82:
+
+  Tisserand: _Astronomy and Astro-Physics_, vol. xiii., p. 291 (1894).
+
+Footnote 83:
+
+  _Comptes Rendus_, t. cvii., p. 804.
+
+Footnote 84:
+
+  _Astronomical Journal_, No. 186.
+
+Footnote 85:
+
+  “General Astronomy,” p. 372.
+
+Footnote 86:
+
+  “Observations at the Cape of Good Hope,” p. 396.
+
+Footnote 87:
+
+  “Monat. Correspondenz,” Bd. xxv., pp. 3–22, 1812.
+
+Footnote 88:
+
+  Fessenden: _Astro-Physical Journal_, vol. iii., p. 40.
+
+Footnote 89:
+
+  _Astr. Nach._, No. 2837.
+
+Footnote 90:
+
+  Guillemin: “The World of Comets,” p. 282.
+
+Footnote 91:
+
+  _Astr. Nach._, No. 2437.
+
+Footnote 92:
+
+  _Knowledge_, Feb., 1896, p. 41.
+
+Footnote 93:
+
+  For an account of its spectral changes, see Campbell in _Astr. and
+  Astr.-Physics_, vol. xi., p. 698.
+
+Footnote 94:
+
+  Barnard, _Knowledge_, vol. viii., p. 229.
+
+Footnote 95:
+
+  Denning: _Astronomy and Astro-Physics_, vol. xii., p. 371.
+
+Footnote 96:
+
+  _Astroph. Journal_, Jan., 1896, p. 42.
+
+Footnote 97:
+
+  Ranyard: _Knowledge_, vol. ix., p. 159.
+
+Footnote 98:
+
+  “Publications Astr. Pac. Society,” vol. vii., p. 166.
+
+Footnote 99:
+
+  Hussey: _loc. cit._, p. 171.
+
+Footnote 100:
+
+  Holden: “Publ. Astr. Pac. Society,” vol. ii., p. 19. H. A. Newton:
+  _Ibid._, vol. iii., p. 91.
+
+Footnote 101:
+
+  “Report Bri. Ass.,” 1891, p. 805.
+
+Footnote 102:
+
+  S. Meunier: “Encycl. Chimique,” t. ii., p. 461.
+
+Footnote 103:
+
+  Young: “Gen. Astr.,” p. 435.
+
+Footnote 104:
+
+  Cornish: _Knowledge_, vol. vi., p. 163.
+
+Footnote 105:
+
+  _Journal Brit. Astr. Ass._, vol. vi., p. 432.
+
+Footnote 106:
+
+  H. A. Newton: “Proc. Amer. Phil. Society,” vol. xxxii.
+
+Footnote 107:
+
+  Quoted by Sir F. Palgrave: “Phil. Trans.,” vol. cxxx., p. 175.
+
+Footnote 108:
+
+  _Observatory_, April, 1895.
+
+Footnote 109:
+
+  _Observatory_, Jan., 1896.
+
+Footnote 110:
+
+  It has been recently seen again in America.
+
+Footnote 111:
+
+  _Journal of the British Astronomical Association_, March, 1891.
+
+Footnote 112:
+
+  _Nature_, Feb. 13, 1896.
+
+Footnote 113:
+
+  “Planetary and Stellar Studies,” p. 257.
+
+Footnote 114:
+
+  See Chapter V.
+
+Footnote 115:
+
+  _Comptes Rendus_, March 30, 1896.
+
+Footnote 116:
+
+  _Nature_, April 30, 1896.
+
+Footnote 117:
+
+  “Cape Observations,” p. 34.
+
+Footnote 118:
+
+  _Journal of the British Astronomical Association_, vol. iv., No. 11,
+  p. 21.
+
+Footnote 119:
+
+  _Journal of the British Astronomical Association_, vol. vi., No. 6, p.
+  312.
+
+Footnote 120:
+
+  Recent observations show that the total variation is 2·71
+  magnitudes—the largest variation known for an Algol star.
+
+Footnote 121:
+
+  “Cosmos,” Bohn’s edition, vol. iii., p. 205.
+
+Footnote 122:
+
+  It was, however, asserted by Herlicius that he had seen it on Sept.
+  27.
+
+Footnote 123:
+
+  The spectrum, however, seems to have since become continuous.
+
+Footnote 124:
+
+  _Astronomical Journal_, No. 100.
+
+Footnote 125:
+
+  _Journal of the British Astronomical Association_, March, 1892.
+
+Footnote 126:
+
+  _Journal of the British Astronomical Association_, February, 1895,
+  vol. v. No. 4.
+
+Footnote 127:
+
+  _Ibid._, April, 1895, p. 328.
+
+Footnote 128:
+
+  _Journal of the British Astronomical Association_, February, 1892.
+
+Footnote 129:
+
+  _The Observatory_, December, 1895.
+
+Footnote 130:
+
+  “Planetary and Stellar Studies,” p. 188.
+
+Footnote 131:
+
+  _Nature_, September 6, 1894.
+
+Footnote 132:
+
+  _Nature_, June 4, 1896.
+
+Footnote 133:
+
+  “Cosmos,” vol. iii., Bohn’s edition, p. 192.
+
+Footnote 134:
+
+  Humboldt’s “Cosmos,” Bohn’s edition, vol. iv., pp. 327, 328.
+
+Footnote 135:
+
+  _Monthly Notices_, Royal Astronomical Society, June, 1888.
+
+Footnote 136:
+
+  “Old and New Astronomy,” p. 794.
+
+Footnote 137:
+
+  _Nature_, June 4, 1896.
+
+Footnote 138:
+
+  _Nature_, September, 1894.
+
+Footnote 139:
+
+  _Ibid._, October 4, 1894.
+
+Footnote 140:
+
+  “Outlines of Astronomy,” tenth edition, p. 657.
+
+Footnote 141:
+
+  _Nature_, November, 21, 1895.
+
+Footnote 142:
+
+  _Nature_, January 16, 1896.
+
+Footnote 143:
+
+  _Nature_, August 9, 1888.
+
+Footnote 144:
+
+  Humboldt’s “Cosmos,” Bohn’s edition, vol, iii., p. 143.
+
+Footnote 145:
+
+  See _Knowledge_, June, 1895.
+
+Footnote 146:
+
+  “The Universe and the Coming Transits,” p. 200.
+
+Footnote 147:
+
+  _Journal of the British Astronomical Association_, May, 1895, p. 383.
+
+Footnote 148:
+
+  _Knowledge_, May, 1896.
+
+Footnote 149:
+
+  _Knowledge_, July, 1891.
+
+Footnote 150:
+
+  _Knowledge_, January, 1894, p. 17.
+
+Footnote 151:
+
+  _Journal of the British Astronomical Association_, April, 1895, p.
+  304.
+
+Footnote 152:
+
+  The Italics are Herschel’s.
+
+Footnote 153:
+
+  A full discussion of Struve’s views will be found in Chapter XVI. of
+  “The Visible Universe,” by the present writer.
+
+Footnote 154:
+
+  “The Meteoritic Theory,” pp. 380, 381.
+
+------------------------------------------------------------------------
+
+
+
+
+                D. APPLETON AND COMPANY’S PUBLICATIONS.
+
+
+  _THE SUN._ By C. A. YOUNG, Ph. D., LL. D., Professor of Astronomy in
+    Princeton University. New and revised edition, with numerous
+    Illustrations. 12mo. Cloth, $2.00.
+
+  “In this book we see a master’s hand. Professor Young has no
+  superiors, if he has rivals, among astronomers in this country....
+  ‘The Sun’ is a book of facts and achievements, and not a discussion of
+  theories, and it will be read and appreciated by all scientific
+  students, and not by them alone. Being written in untechnical
+  language, it is equally adapted to a large class of educated readers
+  not engaged in scientific pursuits.”—_Journal of Education, Boston._
+
+  “Professor Young’s work is essentially a record of facts and
+  achievements, rather than of theories and attempts at the
+  interpretation of mysteries; yet the great questions still remaining
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+
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+
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+  Interior._
+
+
+  _THE STORY OF THE SUN._ By Sir ROBERT S. BALL, F. R. S., author of “An
+    Atlas of Astronomy,” “The Cause of an Ice Age,” etc. 8vo. Cloth,
+    $5.00.
+
+  “Sir Robert Ball has the happy gift of making abstruse problems
+  intelligible to the ‘wayfaring man’ by the aid of simple language and
+  a few diagrams. Science moves so fast that there was room for a volume
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+  admirably fills.”—_London Chronicle._
+
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+
+
+  _AN ATLAS OF ASTRONOMY._ By Sir ROBERT S. BALL, F. R. S., Professor of
+    Astronomy and Geometry at the University of Cambridge; Author of
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+    Explanatory Text, and Complete Index. Small 4to. Cloth, $4.00.
+
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+  recommendation of his newly published ‘Atlas of Astronomy.’ The plates
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+  more striking aspects of the more important heavenly bodies are very
+  beautifully executed. The introduction is written with Sir Robert
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+  altogether the Atlas is admirably adapted to meet the needs and smooth
+  the difficulties of young and inexperienced students of astronomy, as
+  well as materially to assist the researches of those are more
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+
+
+  _OUTLINES OF ASTRONOMY._ By Sir J. J. W. HERSCHEL. With Plates and
+    Woodcuts. Eleventh edition. 8vo. Cloth, $4.00.
+
+  _ORIGIN OF THE STARS, AND THE CAUSES OF THEIR MOTIONS AND THEIR
+    LIGHT._ By JACOB ENNIS. 12mo. Cloth, $2.00.
+
+  _SPECTRUM ANALYSIS, IN ITS APPLICATION TO TERRESTRIAL SUBSTANCES, and
+    the Physical Constitution of the Heavenly Bodies._ Familiarly
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+    Translated from the second enlarged and revised German edition by
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+
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+
+
+  _STUDIES IN SPECTRUM ANALYSIS._ By J. NORMAN LOCKYER, F.R.S.,
+    Correspondent of the Institute of France, etc. With 60
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+
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+  the matter before him, nor putting down the book until the last page
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+
+
+  _ASTRONOMY AND GEOLOGY COMPARED._ By Lord ORMATHWAITE. 18mo. Tinted
+    paper. Cloth, $1.00.
+
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+
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+  doubly attractive.”—_Boston Home Journal._
+
+
+                      RICHARD A. PROCTOR’S WORKS.
+
+
+  _OTHER WORLDS THAN OURS. The Plurality of Worlds, Studied under the
+    Light of Recent Scientific Researches._ By RICHARD ANTHONY PROCTOR.
+    With Illustrations, some colored. 12mo. Cloth, $1.75.
+
+  CONTENTS.—Introduction. What the Earth teaches us. What we learn from
+  the Sun. The Inferior Planets. Mars, the Miniature of our Earth.
+  Jupiter, the Giant of the Solar System. Saturn, the Ringed World.
+  Uranus and Neptune, the Arctic Planets. The Moon and other Satellites.
+  Meteors and Comets: Their Office in the Solar System. Other Suns than
+  Ours. Of Minor Stars, and of the Distribution of Stars in Space. The
+  Nebulæ: Are they External Galaxies? Supervision and Control.
+
+
+  _OUR PLACE AMONG INFINITIES._ A Series of Essays contrasting our
+    Little Abode in Space and Time with the Infinities around us. To
+    which are added Essays on the Jewish Sabbath and Astrology. 12mo.
+    Cloth, $1.75.
+
+  CONTENTS.—Past and Future of the Earth. Seeming Wastes in Nature. New
+  Theory of Life in other Worlds. A Missing Comet. The Lost Comet and
+  its Meteor Train. Jupiter. Saturn and its System. A Giant Sun. The
+  Star Depths. Star Gauging. Saturn and the Sabbath of the Jews.
+  Thoughts on Astrology.
+
+
+  _THE EXPANSE OF HEAVEN._ A Series of Essays on the Wonders of the
+    Firmament. 12mo. Cloth, $2.00.
+
+  CONTENTS.—A Dream that was not all a Dream. The Sun. The Queen of
+  Night. The Evening Star. The Ruddy Planet. Life in the Ruddy Planet.
+  The Prince of Planets. Jupiter’s Family of Moons. The Ring-Girdled
+  Planet. Newton and the Law of the Universe. The Discovery of Two Giant
+  Planets. The Lost Comet. Visitants from the Star Depths. Whence come
+  the Comets? The Comet Families of the Giant Planets. The Earth’s
+  Journey through Showers. How the Planets Grew. Our Daily Light. The
+  Flight of Light. A Cluster of Suns. Worlds ruled by Colored Suns. The
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+  Star Depths. The Star Depths Astir with Life. The Drifting Stars. The
+  Milky Way.
+
+
+  _THE MOON: Her Motions, Aspect, Scenery, and Physical Conditions._
+    With Three Lunar Photographs, Map, and many Plates, Charts, etc.
+    12mo. Cloth, $2.00.
+
+  CONTENTS.—The Moon’s Distance, Size, and Mass. The Moon’s Motions. The
+  Moon’s Changes of Aspect, Rotation, Libration, etc. Study of the
+  Moon’s Surface. Lunar Celestial Phenomena. Condition of the Moon’s
+  Surface. Index to the Map of the Moon.
+
+
+  _LIGHT SCIENCE FOR LEISURE HOURS._ A Series of Familiar Essays on
+    Scientific Subjects, Natural Phenomena, etc. 12mo. Cloth, $1.75.
+
+  _POPULAR ASTRONOMY._ A General Description of the Heavens. By CAMILLE
+    FLAMMARION. Translated from the French by J. Ellard Gore. With 3
+    Plates and 288 Illustrations. 8vo. Cloth, $4.50.
+
+  “The fullest and most elaborate compendium of popular knowledge of
+  astronomy.... The book might reasonably be pronounced the most
+  desirable of its kind.”—_New York Sun._
+
+  “M. Flammarion has produced a work that charms while it interests. He
+  has classified astronomy so perfectly that any person of ordinary
+  intelligence may learn from his book practically all the men in the
+  observatories know.”—_New York Times._
+
+  “Flammarion talks, and his conversation is free from those technical
+  expressions which make the obscure style more obscure. He treats the
+  most abstruse problems in such a fashion that you see through them
+  more clearly than you ever thought it possible to do without years of
+  study.”—_New York Herald._
+
+  “While the translator has done excellent work, he has also added
+  largely to the value of the book by his carefully prepared notes, in
+  which he brings every astronomical theme down to date.”—_Chicago
+  Inter-Ocean._
+
+  “The book is one of extreme interest, and to our mind far surpasses in
+  fascination any novel that was ever written.”—_London Literary World._
+
+
+  _ASTRONOMY WITH AN OPERA-GLASS._ A Popular Introduction to the Study
+    of the Starry Heavens with the Simplest of Optical Instruments. By
+    GARRETT P. SERVISS. 8vo. Cloth, $1.50.
+
+  “The glimpses he allows to be seen of far-stretching vistas opening
+  out on every side of his modest course of observation help to fix the
+  attention of the negligent, and lighten the toil of the painstaking
+  student.... Mr. Serviss writes with freshness and vivacity.”—_London
+  Saturday Review._
+
+  “By its aid thousands of people who have resigned themselves to the
+  ignorance in which they were left at school, by our wretched system of
+  teaching by the book only, will thank Mr. Serviss for the suggestions
+  he has so well carried out”—_New York Times._
+
+  “We are glad to welcome this popular introduction to the study of the
+  heavens.... There could hardly be a more pleasant road to astronomical
+  knowledge than it affords.... A child may understand the text, which
+  reads more like a collection of anecdotes than anything else, but this
+  does not mar its scientific value.”—_Nature._
+
+  “Mr. Garrett P. Serviss’s book, ‘Astronomy with an Opera-Glass,’
+  offers us an admirable handbook and guide in the cultivation of this
+  noble æsthetic discipline (the study of the stars).”—_New York Home
+  Journal._
+
+  “The book should belong to every family library.”—_Boston Home
+  Journal._
+
+
+              WORKS BY ARABELLA B. BUCKLEY (MRS. FISHER).
+
+
+  “Mrs. Fisher has a positive genius for presenting the
+  science of living things in shapes that boys and girls can
+  understand.”—_Philadelphia Ledger._
+
+  “Mrs. Fisher’s apprenticeship as secretary to Sir Charles Lyell has
+  resulted in a life-long devotion to the natural sciences, the fruits
+  of which have been a series of charming books valuable to the young,
+  and indeed to grown-ups.”—_The Critic._
+
+
+  _THE FAIRY-LAND OF SCIENCE._ With 74 Illustrations. Cloth, $1.50.
+
+  CONTENTS.—The Fairy-Land of Science: How to Enter it; How to Use it;
+  How to Enjoy it.—Sunbeams, and the Work they do.—The Aërial Ocean in
+  which we Live.—A Drop of Water on its Travels.—The Two Great
+  Sculptors, Water and Ice.—The Voices of Nature, and How we Hear
+  them.—The Life of a Primrose.—The History of a Piece of Coal.—Bees in
+  the Hive.—Bees and Flowers.
+
+
+  _THROUGH MAGIC GLASSES, and other Lectures._ A Sequel to “The
+    Fairy-Land of Science.” Cloth, $1.50.
+
+  CONTENTS.—The Magician’s Chamber by Moonlight.—Magic Glasses and How
+  to Use Them.—Fairy Rings and How They are Made.—The Life History of
+  Lichens and Mosses.—The History of a Lava-Stream.—An Hour with the
+  Sun.—An Evening with the Stars.—Little Beings from a Miniature
+  Ocean.—The Dartmoor Ponies.—The Magician’s Dream of Ancient Days.
+
+
+  _LIFE AND HER CHILDREN: Glimpses of Animal Life, from the Amœba to the
+    Insects._ With over 100 Illustrations. Cloth, $1.50.
+
+  CONTENTS.—Life’s Simplest Children; How they Live, and Move, and
+  Build.—How Sponges Live.—The Lasso-Throwers of the Ponds and
+  Oceans.—How Star-Fish Walk and Sea-Urchins Grow.—The Mantle-covered
+  Animals, and How they Live with Heads and without them.—The Outcasts
+  of Animal Life, and the Elastic-ringed Animals by Sea and by Land.—The
+  Mailed Warriors of the Sea, with Ringed Bodies and Jointed Feet.—The
+  Snare-Weavers and their Hunting Relations.—Insect Suckers and Biters
+  which Change their Coats but not their Bodies.—Insect Sippers and
+  Gnawers which Remodel their Bodies within their Coats.—Intelligent
+  Insects with Helpless Children, as illustrated by the Ants.
+
+
+  _WINNERS IN LIFE’S RACE; or, The Great Backboned Family._ With
+    numerous Illustrations. Cloth, $1.50.
+
+  CONTENTS.—The Threshold of Backboned Life.—How the Quaint Old Fishes
+  of Ancient Times have Lived on into Our Day.—The Bony Fish, and How
+  they have Spread over Sea, and Lake, and River.—How the Backboned
+  Animals pass from Water-breathing to Air-breathing, and find their Way
+  out upon the Land.—The Cold-blooded Air-breathers of the Globe in
+  Times both Past and Present.—The Feathered Conquerors of the
+  Air.—Their Wanderings over Sea and Marsh, Desert and Plain.—From
+  Running to Flying, from Mound-laying to Nest-building, from Cry to
+  Song.—The Mammalia or Milk-Givers, the simplest Suckling Mother, the
+  active Pouchbearers, and the Imperfect-toothed Animals.—From the Lower
+  and Small Milk-Givers which find Safety in Concealment, to the
+  Intelligent Apes and Monkeys.—The Large Milk-givers which have
+  conquered the World by Strength and Intelligence.—How the Backboned
+  Animals have returned to the Water, and Large Milk-givers Imitate the
+  Fish.—A Bird’s-eye View of the Rise and Progress of Backboned Life.
+
+
+  _A SHORT HISTORY OF NATURAL SCIENCE; and of the Progress of Discovery
+    from the Time of the Greeks to the Present Time._ New edition,
+    revised and rearranged. With 77 Illustrations. Cloth, $2.00.
+
+  “The work, though mainly intended for children and young persons, may
+  be most advantageously read by many persons of riper age, and may
+  serve to implant in their minds a fuller and clearer conception of
+  ‘the promises, the achievements, and claims of science.’”—_Journal of
+  Science._
+
+
+  _PRINCIPLES OF GEOLOGY; or, The Modern Changes of the Earth and its
+    Inhabitants, considered as illustrative of Geology._ By Sir CHARLES
+    LYELL, Bart. Illustrated with Maps, Plates, and Woodcuts. Two vols.,
+    royal 8vo. Cloth, $8.00.
+
+  The “Principles of Geology” may be looked upon with pride not only as
+  a representative of English science, but as without a rival of its
+  kind anywhere. Growing in fullness and accuracy with the growth of
+  experience and observation in every region of the world, the work has
+  incorporated with itself each established discovery, and has been
+  modified by every hypothesis of value which has been brought to bear
+  upon, or been evolved from, the most recent body of facts.
+
+
+  _THE AUTOBIOGRAPHY OF THE EARTH._ A Popular Account of Geological
+    History. By Rev. H. N. HUTCHINSON, B. A., F. G. S. With 27
+    Illustrations. Crown 8vo. Cloth, $1.50.
+
+  “A scientific work, divested of technicalities, and put into a bright,
+  narrative form which can not but attract even the most general
+  reader.”—_Boston Transcript._
+
+  “It is hardly conceivable that the rich results of the science of
+  geology can be so treated as to prove uninteresting to thinking
+  people, be they young or old. When, therefore, we say that Mr.
+  Hutchinson’s book is extremely interesting, no more might be implied
+  than that the author has skillfully used the vast materials at his
+  hand. But Mr. Hutchinson has successfully carried out a difficult
+  design on an admirable plan, and has adhered to that plan throughout.
+  His sketch of historical geology has a genuine continuity.”—_Saturday
+  Review._
+
+
+  _A REPRINT OF ANNUAL REPORTS AND OTHER PAPERS OF THE GEOLOGY OF THE
+    VIRGINIAS._ By the late WILLIAM BARTON ROGERS, LL. D., etc.,
+    Director of the Geological Survey of Virginia from 1835 to 1841.
+    With numerous Maps, Colored Charts, etc. 12mo. Cloth, $5.00.
+
+
+  _NATURAL RESOURCES OF THE UNITED STATES._ By JACOB HARRIS PATTON, M.
+    A., Ph. D. 8vo, 523 pages. Cloth, $3.00.
+
+  “This portly octavo of over 500 pages is an encyclopedic directory to
+  its subject, and a work of comprehensive scope, exhaustive research,
+  scientific spirit, and good literary form.... Capitalists, investors,
+  railroad projectors, land speculators, and all who need to know the
+  distribution of land values, will find a vast amount of information in
+  this well-arranged work, the contents of which, it is safe to say,
+  could not be found assembled in similar compass elsewhere.”—_Boston
+  Literary World._
+
+  “As interesting to read as it is valuable to consult. By the
+  employment of fine white paper and large, clear type, the publishers
+  have made it an elegant specimen of the printer’s art.”—_New York
+  Sun._
+
+
+                  BOOKS BY PROF. G. FREDERICK WRIGHT.
+
+
+  _GREENLAND ICEFIELDS, AND LIFE IN THE NORTH ATLANTIC_. With a New
+    Discussion of the Causes of the Ice Age. By G. FREDERICK WRIGHT, D.
+    D., LL. D., F. G. S. A., author of “The Ice Age in North America,”
+    “Man and the Glacial Period,” etc., and WARREN UPHAM, A. M., F. G.
+    S. A., late of the Geological Surveys of New Hampshire, Minnesota,
+    and the United States. With numerous Maps and Illustrations. 12mo.
+    Cloth, $2.00.
+
+  The immediate impulse to the preparation of this volume arose in
+  connection with a trip to Greenland by Professor Wright in the summer
+  of 1894 on the steamer Miranda. The work aims to give within moderate
+  limits a comprehensive view of the scenery, the glacial phenomena, the
+  natural history, the people, and the explorations of Greenland. The
+  photographs are all original, and the maps have been prepared to show
+  the latest state of knowledge concerning the region. The volume treats
+  of the ice of the Labrador current, the coast of Labrador, Spitzbergen
+  ice in Davis Strait, the Greenland Eskimos, Europeans in Greenland,
+  explorations of the inland ice, the plants and animals of Greenland,
+  changes of level since the advent of the Glacial period, and includes
+  a summary of the bearing of the facts upon glacial theories. The work
+  is of both popular and scientific interest.
+
+
+  _THE ICE AGE IN NORTH AMERICA, and its Bearings upon the Antiquity of
+    Man._ With an Appendix on “The Probable Cause of Glaciation,” by
+    WARREN UPHAM, F. G. S. A., Assistant on the Geological Surveys of
+    New Hampshire, Minnesota, and the United States. New and enlarged
+    edition. With 150 Maps and Illustrations. 8vo, 625 pages, and Index.
+    Cloth, $5.00.
+
+  “The author has seen with his own eyes the most important phenomena of
+  the Ice age on this continent from Maine to Alaska. In the work
+  itself, elementary description is combined with a broad, scientific,
+  and philosophic method, without abandoning for a moment the purely
+  scientific character. Professor Wright has contrived to give the whole
+  a philosophical direction which lends interest and inspiration to it,
+  and which in the chapters on Man and the Glacial Period rises to
+  something like dramatic intensity.”—_The Independent._
+
+
+  _MAN AND THE GLACIAL PERIOD._ International Scientific Series. With
+    numerous Illustrations. 12mo. Cloth, $1.75.
+
+  “The earlier chapters describing glacial action, and the traces of it
+  in North America—especially the defining of its limits, such as the
+  terminal moraine of the great movement itself—are of great interest
+  and value. The maps and diagrams are of much assistance in enabling
+  the reader to grasp the vast extent of the movement.”—_London
+  Spectator._
+
+
+  _PIONEERS OF SCIENCE IN AMERICA._ Sketches of their Lives and
+    Scientific Work. Edited and revised by WILLIAM JAY YOUMANS, M. D.
+    With Portraits. 8vo. Cloth, $4.00.
+
+  Impelled solely by an enthusiastic love of Nature, and neither asking
+  nor receiving outside aid, these early workers opened the way and
+  initiated the movement through which American science has reached its
+  present commanding position. This book gives some account of these
+  men, their early struggles, their scientific labors, and, whenever
+  possible, something of their personal characteristics. This
+  information, often very difficult to obtain, has been collected from a
+  great variety of sources, with the utmost care to secure accuracy. It
+  is presented in a series of sketches, some fifty in all, each with a
+  single exception accompanied with a well-authenticated portrait.
+
+  “Fills a place that needed filling, and is likely to be widely
+  read.”—_New York Sun._
+
+  “It is certainly a useful and convenient volume, and readable too, if
+  we judge correctly of the degree of accuracy of the whole by critical
+  examination of those cases in which our own knowledge enables us to
+  form an opinion.... In general, it seems to us that the handy volume
+  is specially to be commended for setting in just historical
+  perspective many of the earlier scientists who are neither very
+  generally nor very well known.”—_New York Evening Post._
+
+  “A wonderfully interesting volume. Many a young man will find it
+  fascinating. The compilation of the book is a work well done, well
+  worth the doing.”—_Philadelphia Press._
+
+  “One of the most valuable books which we have received.”—_Boston
+  Advertiser._
+
+  “A book of no little educational value.... An extremely valuable work
+  of reference.”—_Boston Beacon._
+
+  “A valuable handbook for those whose work runs on these same lines,
+  and is likely to prove of lasting interest to those for whom ‘_les
+  documents humain_’ are second only to history in importance—nay, are a
+  vital part of history.”—_Boston Transcript._
+
+  “A biographical history of science in America, noteworthy for its
+  completeness and scope.... All of the sketches are excellently
+  prepared and unusually interesting.”—_Chicago Record._
+
+  “One of the most valuable contributions to American literature
+  recently made.... The pleasing style in which these sketches are
+  written, the plans taken to secure accuracy, and the information
+  conveyed, combine to give them great value and interest. No better or
+  more inspiring reading could be placed in the hands of an intelligent
+  and aspiring young man.”—_New York Christian Work._
+
+  “A book whose interest and value are not for to-day or to-morrow, but
+  for indefinite time.”—_Rochester Herald._
+
+  “It is difficult to imagine a reader of ordinary intelligence who
+  would not be entertained by the book.... Conciseness, exactness,
+  urbanity of tone, and interestingness are the four qualities which
+  chiefly impress the reader of these sketches.”—_Buffalo Express._
+
+  “Full of interesting and valuable matter.”—_The Churchman._
+
+
+                      THE ANTHROPOLOGICAL SERIES.
+
+
+                               NOW READY.
+
+
+  _THE BEGINNINGS OF ART._ By ERNST GROSSE, Professor of Philosophy in
+    the University of Freiburg. A new volume in the Anthropological
+    Series, edited by Professor Frederick Starr. Illustrated. 12mo.
+    Cloth, $1.75.
+
+  “This book can not fail to interest students of every branch of art,
+  while the general reader who will dare to take hold of it will have
+  his mind broadened and enriched beyond what he would conceive a work
+  of many times its dimensions might effect.”—_Brooklyn Eagle._
+
+  “The volume is clearly written, and should prove a popular exposition
+  of a deeply interesting theme.”—_Philadelphia Public Ledger._
+
+
+  _WOMAN’S SHARE IN PRIMITIVE CULTURE._ By OTIS TUFTON MASON, A. M.,
+    Curator of the Department of Ethnology in the United States National
+    Museum. With numerous Illustrations. 12mo. Cloth, $1.75.
+
+  “A most interesting _résumé_ of the revelations which science has made
+  concerning the habits of human beings in primitive times, and
+  especially as to the place, the duties, and the customs of
+  women.”—_Philadelphia Inquirer._
+
+
+  _THE PYGMIES._ By A. DE QUATREFAGES, late Professor of Anthropology at
+    the Museum of Natural History, Paris. With numerous Illustrations.
+    12mo. Cloth, $1.75.
+
+  “Probably no one was better equipped to illustrate the general subject
+  than Quatrefages. While constantly occupied upon the anatomical and
+  osseous phases of his subject, he was none the less well acquainted
+  with what literature and history had to say concerning the pygmies....
+  This book ought to be in every divinity school in which man as well as
+  God is studied, and from which missionaries go out to convert the
+  human being of reality and not the man of rhetoric and
+  text-books.”—_Boston Literary World._
+
+
+  _THE BEGINNINGS OF WRITING._ By W. J. HOFFMAN, M. D. With numerous
+    Illustrations. 12mo. Cloth, $1.75.
+
+  This interesting book gives a most attractive account of the rude
+  methods employed by primitive man for recording his deeds. The
+  earliest writing consists of pictographs which were traced on stone,
+  wood, bone, skins, and various paperlike substances. Dr. Hoffman shows
+  how the several classes of symbols used in these records are to be
+  interpreted, and traces the growth of conventional signs up to
+  syllabaries and alphabets—the two classes of signs employed by modern
+  peoples.
+
+
+                            IN PREPARATION.
+
+              _THE SOUTH SEA ISLANDERS._ By Dr. SCHMELTZ.
+              _THE ZUÑI._ By FRANK HAMILTON CUSHING.
+              _THE AZTECS._ By Mrs. ZELIA NUTTALL.
+
+         RECENT VOLUMES OF THE INTERNATIONAL SCIENTIFIC SERIES.
+
+
+  _THE AURORA BOREALIS._ By ALFRED ANGOT, Honorary Meteorologist to the
+    Central Meteorological Office of France. With 18 Illustrations.
+    $1.75.
+
+  While there have been many monographs in different languages upon
+  various phases of this subject, there has been a want of a convenient
+  and comprehensive survey of the whole field. Professor Angot has cited
+  a few illustrations of each class of phenomena, and, without
+  encumbering his book with a mass of minor details, he presents a
+  picture of the actual state of present knowledge, with a summary both
+  of definite results and of the points demanding additional
+  investigation.
+
+
+  _THE EVOLUTION OF THE ART OF MUSIC._ By C. HUBERT H. PARRY, D. C. L.,
+    M. A., etc. $1.75.
+
+  Dr. Parry’s high rank among modern writers upon music assures to this
+  book a cordial welcome. It was first published as “The Art of Music,”
+  in octavo form. The title of this revised edition has been slightly
+  amplified, with a view of suggesting the intention of the work more
+  effectually.
+
+
+  _WHAT IS ELECTRICITY?_ By JOHN TROWBRIDGE, S. D., Rumford Professor
+    and Lecturer on the Applications of Science to the Useful Arts,
+    Harvard University. Illustrated. $1.50.
+
+  Professor Trowbridge’s long experience both as an original
+  investigator and as a teacher imparts a peculiar value to this
+  important work. Finding that no treatise could be recommended which
+  answers the question, What is Electricity? satisfactorily, he has
+  explained in a popular way the electro-magnetic theory of light and
+  heat, and the subject of periodic currents and electric waves, seeking
+  an answer for his titular question in the study of the transformation
+  of energy and a consideration of the hypotheses of movements in the
+  ether.
+
+
+  _ICE-WORK, PRESENT AND PAST._ By T. G. BONNEY, D. Sc., F. R. S., F. S.
+    A., etc., Professor of Geology at University College, London. $1.50.
+
+  In his work Professor Bonney has endeavored to give greater prominence
+  to those facts of glacial geology on which all inferences must be
+  founded. After setting forth the facts shown in various regions, he
+  has given the various interpretations which have been proposed, adding
+  his comments and criticisms. He also explains a method by which he
+  believes we can approximate to the temperature at various places
+  during the Glacial epoch, and the different explanations of this
+  general refrigeration are stated and briefly discussed.
+
+
+  _MOVEMENT._ By E. J. MAREY, Member of the Institute and of the Academy
+    of Medicine; Professor at the College of France; Author of “Animal
+    Mechanism.” Translated by Eric Pritchard, M. A. With 200
+    Illustrations. $1.75.
+
+  The present work describes the methods employed in the extended
+  development of photography of moving objects attained in the last few
+  years, and shows the importance of such researches in mechanics and
+  other departments of physics, the fine arts, physiology, and zoölogy,
+  and in regulating the walking or marching of men and the gait of
+  horses.
+
+
+                   D. APPLETON AND COMPANY, NEW YORK.
+
+------------------------------------------------------------------------
+
+
+
+
+                          TRANSCRIBER’S NOTES
+
+
+ Page           Changed from                      Changed to
+
+  475 the diameter of our sun, I find,  the diameter of our sun, I find,
+      from the above date, that the     from the above data, that the
+
+ ● Typos fixed; non-standard spelling and dialect retained.
+ ● Used numbers for footnotes, placing them all at the end of the last
+     chapter.
+ ● Enclosed italics font in _underscores_.
+ ● Enclosed bold or blackletter font in =equals=.
+ ● The caret (^) serves as a superscript indicator, applicable to
+     individual characters (like 2^d) and even entire phrases (like
+     1^{st}).
+ ● Subscripts are shown using an underscore (_) with curly braces { },
+     as in H_{2}O.
+ ● HTML alt text was added for images that didn’t have captions.
+
+*** END OF THE PROJECT GUTENBERG EBOOK 77516 ***
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+  <body>
+<div style='text-align:center'>*** START OF THE PROJECT GUTENBERG EBOOK 77516 ***</div>
+
+<div class='tnotes covernote'>
+
+<p class='c000'><strong>Transcriber’s Note:</strong></p>
+
+<p class='c000'>New original cover art included with this eBook is granted to the public domain.</p>
+
+</div>
+
+<div class='lg-container-r c001'>
+  <div class='linegroup'>
+    <div class='group'>
+      <div class='line'><span class='xlarge'><i>THE</i></span></div>
+      <div class='line'><span class='xlarge'><i>CONCISE</i></span></div>
+      <div class='line'><span class='xlarge'><i>KNOWLEDGE</i></span></div>
+      <div class='line'><span class='xlarge'><i>LIBRARY</i></span></div>
+    </div>
+  </div>
+</div>
+
+<div class='nf-center-c0'>
+<div class='nf-center c002'>
+    <div><span class='xxlarge'>ASTRONOMY</span></div>
+  </div>
+</div>
+
+<div  class='figcenter id001'>
+<img src='images/i_frontis.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p>Photograph of the Moon taken at Paris by MM. Loewy and Puiseux with the great Coudé Equatorial, February 14, 1894.</p>
+</div>
+</div>
+
+<p class='c003'><span class='xlarge'>THE CONCISE KNOWLEDGE LIBRARY</span></p>
+<div class='titlepage'>
+
+<div>
+  <h1 class='c004'>ASTRONOMY</h1>
+</div>
+
+<div class='nf-center-c0'>
+<div class='nf-center c005'>
+    <div>❧</div>
+    <div class='c005'>BY</div>
+    <div class='c005'><span class='large'>AGNES M. CLERKE</span></div>
+    <div class='c005'><span class='small'>AUTHOR OF A POPULAR HISTORY OF ASTRONOMY DURING THE NINETEENTH CENTURY</span></div>
+    <div class='c005'><span class='large'>A. FOWLER, A.R.C.S., F.R.A.S.</span></div>
+    <div class='c005'><span class='small'>DEMONSTRATOR TO THE ROYAL COLLEGE OF SCIENCE</span></div>
+    <div class='c005'><span class='large'>J. ELLARD GORE, F.R.A.S., M.R.I.A</span>.</div>
+    <div class='c005'><span class='small'>AUTHOR OF THE SCENERY OF THE HEAVENS, THE WORLDS OF SPACE, ETC.</span></div>
+  </div>
+</div>
+
+<div  class='figcenter id002'>
+<img src='images/i_title.jpg' alt='Black-and-white publisher’s emblem with a tree above a shield featuring stylized letters, flanked by scrolls reading ‘Inter Folia Fructus.’' class='ig001'>
+</div>
+
+<div class='nf-center-c0'>
+  <div class='nf-center'>
+    <div>NEW YORK</div>
+    <div>D. APPLETON AND COMPANY</div>
+    <div>1898</div>
+  </div>
+</div>
+
+</div>
+
+<div class='nf-center-c0'>
+<div class='nf-center c002'>
+    <div><span class='small'><i>Authorized Edition.</i></span></div>
+  </div>
+</div>
+
+<div class='chapter'>
+  <span class='pageno' id='Page_vii'>vii</span>
+  <h2 class='c006'>PREFACE</h2>
+</div>
+
+<p class='c003'>This work aims to present in concise form a popular
+synopsis of Astronomical Knowledge to date.</p>
+
+<p class='c007'>In Section I., Miss Agnes M. Clerke, author of “A
+Popular History of Astronomy during the Nineteenth Century,”
+gives a brief historical sketch of the science from
+Hipparchus to the present time. In Section II., an attempt
+is made by Mr. A. Fowler, A.R.C.S., F.R.A.S., Demonstrator
+of Astronomical Physics to the Royal College of
+Science, to briefly outline the general principles of spherical
+and gravitational Astronomy, and to describe the instrumental
+means now at the command of observers in the
+various branches of Astronomical research. The author begs
+to record his indebtedness to Loomis’ “Treatise on Astronomy,”
+and Young’s “General Astronomy,” which have
+been frequently consulted, especially for memorial data; to
+Mr. W. Shackleton, for assistance in proof-reading; and to
+Mr. C. P. Butter, for valuable help in preparing the diagrams.
+Section III., contributed by Miss Agnes M. Clerke,
+deals with the Solar System; and Section IV., written by
+Mr. J. Ellard Gore, F.R.A.S., M.R.I.A., treats of the
+Sidereal Heavens.</p>
+
+<p class='c007'>The work is illustrated by a large number of diagrams and
+other illustrations, prepared expressly for its pages, as well as
+by a number of photographic and other reproductions of
+photographs and drawings made by distinguished astronomers
+<span class='pageno' id='Page_viii'>viii</span>in Europe and America. In this connexion numerous
+acknowledgments are due.</p>
+
+<p class='c007'>The Editor begs to express his sense of indebtedness to
+the following astronomers and publishers, for kind permission
+to reproduce original photographs and drawings from their
+works:</p>
+
+<p class='c007'>To M. Loewy, Director de l’Observatoire, Paris, for permission
+to reproduce the photograph of the Moon, which
+forms the frontispiece of this volume; to Professor Edward
+S. Holden, Director of the Lick Observatory, for permission
+to reproduce drawings and photographs of the Observatory
+at Nice, p. 2; the Thirty-six Inch Reflector of Lick Observatory,
+p. 40; the Meridian Circle of the Paris Observatory,
+p. 203; the Spectroscope adapted to the eye end of the Lick
+Telescope, p. 221; and Jupiter showing the Red Spot, p. 322; to
+Dr. Isaac Roberts, for permission to reproduce his photograph
+of the photographic telescope used by him, p. 196; to Messrs.
+Trichnor &#38; Co., of Berlin, for permission to reproduce two
+illustrations of Donati’s Comet, pp. 228 and 363; and one of
+Sun-spots and Magnetic Variations, p. 246—all from Langley’s
+“New Astronomy”; to Messrs. Witherby &#38; Co., for permission
+to reproduce the photograph of a Sun-spot by Janssen, p. 243;
+the photograph of Jupiter, p. 328; the photographs of
+Swift’s Comet, pp. 374 and 375, Brooks’ Comet, p. 381, and
+the Milky Way, p. 557, from <cite>Knowledge</cite>; to Messrs.
+Taylor &#38; Francis, for the diagram of curves showing the
+development of Sun-spots, p. 257; to Professor George E.
+Hale, of Kenwood Observatory, Chicago, for his illustrations
+of Eruptive Prominences photographed at Kenwood, March
+24th, 1896, pp. 264 and 265, reproduced from the <cite>Astrophysical
+Journal</cite>; to the Council of the Royal Society, for the
+illustration of the Eclipsed Sun, p. 267, reproduced from
+“Philosophical Transactions”; to Professor Barnard, for the
+photographs of the Corona, reproduced on p. 269; and the
+<span class='pageno' id='Page_ix'>ix</span>drawings of the Transit of Jupiter’s Satellite, on p. 330, reproduced
+from the <cite>Monthly Notices</cite> of the Royal Astronomical
+Society; the Eye of Mars, p. 302; and of Saturn and his
+Rings, p. 335; to the editor of the <cite>Astronomische Nachrichten</cite>,
+for the map of Mercury, by Schiaparelli, reproduced
+on p. 276; to the editor of <cite>Nature</cite>, for the drawing of
+Venus by Mascari, on p. 280; to Messrs. George Philip &#38;
+Son, for the map of the Moon, given on p. 291, from Fowler’s
+“Telescopic Astronomy”; to Messrs. Longman, Green &#38; Co.,
+for the Chart of Mars, p. 300, and the diagram of the Apparent
+Orbit of the Companion of Sirius, p. 439, from Proctor’s “Old
+and New Astronomy”; to Professor W. W. Payne of Goodsell
+Observatory, for the use of the drawing of the Oases of Mars,
+p. 304, from “Popular Astronomy”; and the photograph of
+Holmes’ Comet, p. 378, from the same work; to Messrs.
+A. &#38; C. Black, for the illustrations of the Great Comet of
+September, 1882, p. 361, reproduced from Miss Clerke’s
+“History of Astronomy”; to Messrs. Crosby, Lockwood
+&#38; Co., for permission to reproduce the illustration of the
+Nebula in Andromeda 31 Messier, p. 398, from the frontispiece
+of Mr. J. E. Gore’s “Visible Universe”; and also for
+the same authority, regarding the diagrams, showing the
+Stars visible in the Northern Hemisphere, p. 401; the Stars
+visible in the Southern Hemisphere, p. 403; the drawing
+showing the position of the Solar Apex, according to different
+computers, p. 429; and the photographs of the Spiral
+Nebula 51 Messier, p. 533; and the Milky Way in Sagittarius,
+p. 555, all from the same work; to Messrs. A. D. Innes &#38; Co.,
+for the use of the diagram, Apparent Orbit of Zeta Herculis,
+p. 436; Triple Stars, p. 451; and for permission to reproduce
+the photographs, 37 Messier, p. 505; the star cluster, Omega
+Centauri, p. 512; the Nebulæ of Orion, p. 521; and the
+Magellanic Clouds, p. 537, from “The Worlds of Space”;
+and to Messrs. Sutton &#38; Co., for the use of the diagram
+<span class='pageno' id='Page_x'>x</span>of the Apparent Orbit of 70 Ophiuchi, p. 443; the photographs
+of the Double Cluster of Perseus, p. 503; the Star
+Cluster in Gemini, p. 504; the Star Cluster in Hercules, p. 507;
+and the drawing of the Trifid Nebula, Sagittarius, p. 525,
+from “The Scenery of the Heavens”; and the drawing of the
+Temporary Star of 1572, p. 481, from “Planetary and Stellar
+Studies,” both published by them.</p>
+
+<div class='lg-container-r'>
+  <div class='linegroup'>
+    <div class='group'>
+      <div class='line'>A. H. M.</div>
+    </div>
+  </div>
+</div>
+
+<div class='chapter'>
+  <span class='pageno' id='Page_xi'>xi</span>
+  <h2 class='c006'>CONTENTS</h2>
+</div>
+
+<table class='table0'>
+  <tr><td class='c008' colspan='3'>SECTION I.—HISTORY      By Agnes M. Clerke.</td></tr>
+  <tr>
+    <th class='c009'>Chap.</th>
+    <th class='c010'>&#160;</th>
+    <th class='c011'>Page</th>
+  </tr>
+  <tr>
+    <td class='c009'>I.</td>
+    <td class='c010'>FROM HIPPARCHUS TO LAPLACE</td>
+    <td class='c011'><a href='#Page_3'>3</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>II.</td>
+    <td class='c010'>A CENTURY OF PROGRESS</td>
+    <td class='c011'><a href='#Page_21'>21</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>&#160;</td>
+    <td class='c010'>&#160;</td>
+    <td class='c011'>&#160;</td>
+  </tr>
+  <tr>
+    <td class='c009'>&#160;</td>
+    <td class='c010'>&#160;</td>
+    <td class='c011'>&#160;</td>
+  </tr>
+  <tr><td class='c008' colspan='3'>SECTION II.—GEOMETRICAL ASTRONOMY AND ASTRONOMICAL INSTRUMENTS.</td></tr>
+  <tr><td class='c012' colspan='3'>By A. Fowler, A.R.C.S., F.R.A.S.</td></tr>
+  <tr>
+    <td class='c009'>&#160;</td>
+    <td class='c010'>&#160;</td>
+    <td class='c011'>&#160;</td>
+  </tr>
+  <tr>
+    <td class='c009'>I.</td>
+    <td class='c010'>THE EARTH AND ITS ROTATION</td>
+    <td class='c011'><a href='#Page_41'>41</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>II.</td>
+    <td class='c010'>THE EARTH’S REVOLUTION ROUND THE SUN</td>
+    <td class='c011'><a href='#Page_55'>55</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>III.</td>
+    <td class='c010'>HOW THE POSITIONS OF THE HEAVENLY BODIES ARE DEFINED</td>
+    <td class='c011'><a href='#Page_65'>65</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>IV.</td>
+    <td class='c010'>THE EARTH’S ORBIT</td>
+    <td class='c011'><a href='#Page_72'>72</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>V.</td>
+    <td class='c010'>MEAN SOLAR TIME</td>
+    <td class='c011'><a href='#Page_78'>78</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>VI.</td>
+    <td class='c010'>THE MOVEMENTS OF THE MOON</td>
+    <td class='c011'><a href='#Page_87'>87</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>VII.</td>
+    <td class='c010'>MOVEMENTS OF PLANETS, SATELLITES, AND COMETS</td>
+    <td class='c011'><a href='#Page_98'>98</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>VIII.</td>
+    <td class='c010'>ECLIPSES AND OCCULTATIONS</td>
+    <td class='c011'><a href='#Page_110'>110</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>IX.</td>
+    <td class='c010'>HOW TO FIND OUR SITUATION ON THE EARTH</td>
+    <td class='c011'><a href='#Page_122'>122</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>X.</td>
+    <td class='c010'>THE EXACT SIZE AND SHAPE OF THE EARTH</td>
+    <td class='c011'><a href='#Page_129'>129</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>XI.</td>
+    <td class='c010'>THE DISTANCES AND DIMENSIONS OF THE HEAVENLY BODIES</td>
+    <td class='c011'><a href='#Page_139'>139</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>XII.</td>
+    <td class='c010'>THE MASSES OF CELESTIAL BODIES</td>
+    <td class='c011'><a href='#Page_151'>151</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>XIII.</td>
+    <td class='c010'>GRAVITATIONAL EFFECTS OF SUN AND MOON UPON THE EARTH</td>
+    <td class='c011'><a href='#Page_162'>162</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>XIV.</td>
+    <td class='c010'>INSTRUMENTAL MEASUREMENT OF ANGLES AND TIME</td>
+    <td class='c011'><a href='#Page_171'>171</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>XV.</td>
+    <td class='c010'>TELESCOPES</td>
+    <td class='c011'><a href='#Page_176'>176</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>XVI.</td>
+    <td class='c010'>INSTRUMENTS OF PRECISION</td>
+    <td class='c011'><a href='#Page_198'>198</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>XVII.</td>
+    <td class='c010'>ASTROPHYSICAL INSTRUMENTS</td>
+    <td class='c011'><a href='#Page_211'>211</a></td>
+  </tr>
+  <tr>
+    <td class='c009'><span class='pageno' id='Page_xii'>xii</span>&#160;</td>
+    <td class='c010'>&#160;</td>
+    <td class='c011'>&#160;</td>
+  </tr>
+  <tr>
+    <td class='c009'>&#160;</td>
+    <td class='c010'>&#160;</td>
+    <td class='c011'>&#160;</td>
+  </tr>
+  <tr><td class='c008' colspan='3'>SECTION III.—THE SOLAR SYSTEM.</td></tr>
+  <tr><td class='c012' colspan='3'>By Agnes M. Clerke.</td></tr>
+  <tr>
+    <td class='c009'>&#160;</td>
+    <td class='c010'>&#160;</td>
+    <td class='c011'>&#160;</td>
+  </tr>
+  <tr>
+    <td class='c009'>I.</td>
+    <td class='c010'>THE SOLAR SYSTEM AS A WHOLE</td>
+    <td class='c011'><a href='#Page_229'>229</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>II.</td>
+    <td class='c010'>THE SUN</td>
+    <td class='c011'><a href='#Page_237'>237</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>III.</td>
+    <td class='c010'>THE SUN’S SURROUNDINGS</td>
+    <td class='c011'><a href='#Page_253'>253</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>IV.</td>
+    <td class='c010'>THE INTERIOR PLANETS</td>
+    <td class='c011'><a href='#Page_273'>273</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>V.</td>
+    <td class='c010'>THE EARTH AND MOON</td>
+    <td class='c011'><a href='#Page_283'>283</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>VI.</td>
+    <td class='c010'>THE PLANET MARS</td>
+    <td class='c011'><a href='#Page_297'>297</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>VII.</td>
+    <td class='c010'>THE ASTEROIDS</td>
+    <td class='c011'><a href='#Page_310'>310</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>VIII.</td>
+    <td class='c010'>THE PLANET JUPITER</td>
+    <td class='c011'><a href='#Page_318'>318</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>IX.</td>
+    <td class='c010'>THE SATURNIAN SYSTEM</td>
+    <td class='c011'><a href='#Page_333'>333</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>X.</td>
+    <td class='c010'>URANUS AND NEPTUNE</td>
+    <td class='c011'><a href='#Page_343'>343</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>XI.</td>
+    <td class='c010'>FAMOUS COMETS</td>
+    <td class='c011'><a href='#Page_352'>352</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>XII.</td>
+    <td class='c010'>NATURE AND ORIGIN OF COMETS</td>
+    <td class='c011'><a href='#Page_368'>368</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>XIII.</td>
+    <td class='c010'>METEORITES AND SHOOTING STARS</td>
+    <td class='c011'><a href='#Page_385'>385</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>&#160;</td>
+    <td class='c010'>&#160;</td>
+    <td class='c011'>&#160;</td>
+  </tr>
+  <tr>
+    <td class='c009'>&#160;</td>
+    <td class='c010'>&#160;</td>
+    <td class='c011'>&#160;</td>
+  </tr>
+  <tr><td class='c008' colspan='3'>SECTION IV.—THE SIDEREAL HEAVENS.</td></tr>
+  <tr><td class='c012' colspan='3'>By J. E. Gore, F.R.A.S.</td></tr>
+  <tr>
+    <td class='c009'>&#160;</td>
+    <td class='c010'>&#160;</td>
+    <td class='c011'>&#160;</td>
+  </tr>
+  <tr>
+    <td class='c009'>I.</td>
+    <td class='c010'>THE STARS AND CONSTELLATIONS</td>
+    <td class='c011'><a href='#Page_399'>399</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>II.</td>
+    <td class='c010'>DOUBLE, MULTIPLE, AND COLOURED STARS</td>
+    <td class='c011'><a href='#Page_410'>410</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>III.</td>
+    <td class='c010'>THE DISTANCES AND MOTIONS OF THE STARS</td>
+    <td class='c011'><a href='#Page_417'>417</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>IV.</td>
+    <td class='c010'>BINARY STARS</td>
+    <td class='c011'><a href='#Page_431'>431</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>V.</td>
+    <td class='c010'>VARIABLE AND TEMPORARY STARS</td>
+    <td class='c011'><a href='#Page_458'>458</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>VI.</td>
+    <td class='c010'>CLUSTERS AND NEBULÆ</td>
+    <td class='c011'><a href='#Page_497'>497</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>VII.</td>
+    <td class='c010'>THE CONSTRUCTION OF THE HEAVENS</td>
+    <td class='c011'><a href='#Page_538'>538</a></td>
+  </tr>
+</table>
+
+<div class='chapter'>
+  <span class='pageno' id='Page_xiii'>xiii</span>
+  <h2 class='c006'>LIST OF ILLUSTRATIONS</h2>
+</div>
+
+<table class='table0'>
+  <tr>
+    <td class='c009'>&#160;</td>
+    <td class='c010'><i>Frontispiece to Volume</i>—<span class='sc'>Photograph of the Moon</span>.</td>
+    <td class='c011'>&#160;</td>
+  </tr>
+  <tr>
+    <td class='c009'>&#160;</td>
+    <td class='c010'>&#160;</td>
+    <td class='c011'>&#160;</td>
+  </tr>
+  <tr>
+    <td class='c009'>&#160;</td>
+    <td class='c010'>&#160;</td>
+    <td class='c011'>&#160;</td>
+  </tr>
+  <tr><td class='c008' colspan='3'>SECTION I.—HISTORY.</td></tr>
+  <tr>
+    <th class='c009'></th>
+    <th class='c010'>&#160;</th>
+    <th class='c011'><i>Page</i></th>
+  </tr>
+  <tr>
+    <td class='c009'>&#160;</td>
+    <td class='c010'><i>Frontispiece</i>—<span class='sc'>The Observatory at Nice</span></td>
+    <td class='c011'><a href='#i_002'>2</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>&#160;</td>
+    <td class='c010'>&#160;</td>
+    <td class='c011'>&#160;</td>
+  </tr>
+  <tr>
+    <td class='c009'>&#160;</td>
+    <td class='c010'>&#160;</td>
+    <td class='c011'>&#160;</td>
+  </tr>
+  <tr><td class='c008' colspan='3'>SECTION II.—GEOMETRICAL ASTRONOMY.</td></tr>
+  <tr>
+    <td class='c009'>&#160;</td>
+    <td class='c010'>&#160;</td>
+    <td class='c011'>&#160;</td>
+  </tr>
+  <tr>
+    <td class='c009'>&#160;</td>
+    <td class='c010'><i>Frontispiece</i>—<span class='sc'>The Lick Refractor of Thirty-Six Inches Aperture</span></td>
+    <td class='c011'><a href='#i_040'>40</a></td>
+  </tr>
+  <tr>
+    <th class='c009'><i>Fig.</i></th>
+    <th class='c010'>&#160;</th>
+    <th class='c011'>&#160;</th>
+  </tr>
+  <tr>
+    <td class='c009'>1.—</td>
+    <td class='c010'><span class='sc'>Rough Measurement of Earth’s Diameter</span></td>
+    <td class='c011'><a href='#i_043'>43</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>2.—</td>
+    <td class='c010'><span class='sc'>Horizons at Two Places on the Earth</span></td>
+    <td class='c011'><a href='#i_044'>44</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>3.—</td>
+    <td class='c010'><span class='sc'>Foucault’s Pendulum Experiment</span></td>
+    <td class='c011'><a href='#i_049'>49</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>4.—</td>
+    <td class='c010'><span class='sc'>Sun’s Rays—Day and Night</span></td>
+    <td class='c011'><a href='#i_052'>52</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>5.—</td>
+    <td class='c010'><span class='sc'>Atmospheric Refraction</span></td>
+    <td class='c011'><a href='#i_054'>54</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>6.—</td>
+    <td class='c010'><span class='sc'>Apparent Paths of Sun at Equinoxes and Solstices</span></td>
+    <td class='c011'><a href='#i_056'>56</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>7.—</td>
+    <td class='c010'><span class='sc'>Aberrational Orbit of a Star</span></td>
+    <td class='c011'><a href='#i_058'>58</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>8.—</td>
+    <td class='c010'><span class='sc'>The Sun’s Altitude in Summer and Winter</span></td>
+    <td class='c011'><a href='#i_062'>62</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>9.—</td>
+    <td class='c010'><span class='sc'>The Sun’s Altitude at the Equinoxes</span></td>
+    <td class='c011'><a href='#i_063'>63</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>10.—</td>
+    <td class='c010'><span class='sc'>The Midnight Sun</span></td>
+    <td class='c011'><a href='#i_064'>64</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>11.—</td>
+    <td class='c010'><span class='sc'>Altitude and Azimuth</span></td>
+    <td class='c011'><a href='#i_066'>66</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>12.—</td>
+    <td class='c010'><span class='sc'>Right Ascension, Declination, etc.</span></td>
+    <td class='c011'><a href='#i_069'>69</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>13.—</td>
+    <td class='c010'><span class='sc'>Elliptic Form of Earth’s Orbit</span></td>
+    <td class='c011'><a href='#i_072'>72</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>14.—</td>
+    <td class='c010'><span class='sc'>The Ellipse</span></td>
+    <td class='c011'><a href='#i_073'>73</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>15.—</td>
+    <td class='c010'><span class='sc'>How to draw an Ellipse</span></td>
+    <td class='c011'><a href='#i_074'>74</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>16.—</td>
+    <td class='c010'><span class='sc'>Illustrating Kepler’s Second Law</span></td>
+    <td class='c011'><a href='#i_076'>76</a></td>
+  </tr>
+  <tr>
+    <td class='c009'><span class='pageno' id='Page_xiv'>xiv</span>17.—</td>
+    <td class='c010'><span class='sc'>Effect of Obliquity of Ecliptic upon the Equation of Time</span></td>
+    <td class='c011'><a href='#i_081'>81</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>18.—</td>
+    <td class='c010'><span class='sc'>The Moon’s Phases</span></td>
+    <td class='c011'><a href='#i_090'>90</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>19.—</td>
+    <td class='c010'><span class='sc'>The Lunar Month</span></td>
+    <td class='c011'><a href='#i_091'>91</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>20.—</td>
+    <td class='c010'><span class='sc'>The Moon’s Rotation</span></td>
+    <td class='c011'><a href='#i_093'>93</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>21.—</td>
+    <td class='c010'><span class='sc'>The Moon’s Nodes</span></td>
+    <td class='c011'><a href='#i_094'>94</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>22.—</td>
+    <td class='c010'><span class='sc'>Position of Ecliptic at Sunset at Vernal Equinox</span></td>
+    <td class='c011'><a href='#i_096'>96</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>23.—</td>
+    <td class='c010'><span class='sc'>Movement of an Interior Planet</span></td>
+    <td class='c011'><a href='#i_099'>99</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>24.—</td>
+    <td class='c010'><span class='sc'>Morning and Evening Stars</span></td>
+    <td class='c011'><a href='#i_100'>100</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>25.—</td>
+    <td class='c010'><span class='sc'>Movement of an Exterior Planet</span></td>
+    <td class='c011'><a href='#i_103'>103</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>26.—</td>
+    <td class='c010'><span class='sc'>Apparent Paths of Ceres, Pallas, Juno, and Vesta, in 1896</span></td>
+    <td class='c011'><a href='#i_104'>104</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>27.—</td>
+    <td class='c010'><span class='sc'>Opposition of Mars</span></td>
+    <td class='c011'><a href='#i_105'>105</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>28.—</td>
+    <td class='c010'><span class='sc'>Elements of an Elliptic Orbit</span></td>
+    <td class='c011'><a href='#i_107'>107</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>29.—</td>
+    <td class='c010'><span class='sc'>The Earth’s Shadow</span></td>
+    <td class='c011'><a href='#i_110'>110</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>30.—</td>
+    <td class='c010'><span class='sc'>The Lunar Ecliptic Limit</span></td>
+    <td class='c011'><a href='#i_112'>112</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>31.—</td>
+    <td class='c010'><span class='sc'>Eclipses of the Sun</span></td>
+    <td class='c011'><a href='#i_114'>114</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>32.—</td>
+    <td class='c010'><span class='sc'>Duration of a Solar Eclipse</span></td>
+    <td class='c011'><a href='#i_115'>115</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>33.—</td>
+    <td class='c010'><span class='sc'>Track of Eclipse of April 16, 1893</span></td>
+    <td class='c011'><a href='#i_117'>117</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>34.—</td>
+    <td class='c010'><span class='sc'>Determination of Latitude</span></td>
+    <td class='c011'><a href='#i_124'>124</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>35.—</td>
+    <td class='c010'><span class='sc'>Ancient Mode of measuring Latitude</span></td>
+    <td class='c011'><a href='#i_125'>125</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>36.—</td>
+    <td class='c010'><span class='sc'>Triangulation</span></td>
+    <td class='c011'><a href='#i_132'>132</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>37.—</td>
+    <td class='c010'><span class='sc'>Movements of the Earth’s Pole, 1890–95</span></td>
+    <td class='c011'><a href='#i_138'>138</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>38.—</td>
+    <td class='c010'><span class='sc'>Parallax of a Heavenly Body</span></td>
+    <td class='c011'><a href='#i_140'>140</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>39.—</td>
+    <td class='c010'><span class='sc'>Diameter of a Heavenly Body</span></td>
+    <td class='c011'><a href='#i_142'>142</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>40.—</td>
+    <td class='c010'><span class='sc'>Measurement of the Moon’s Distance</span></td>
+    <td class='c011'><a href='#i_143'>143</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>41.—</td>
+    <td class='c010'><span class='sc'>Relative Distance of Venus</span></td>
+    <td class='c011'><a href='#i_145'>145</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>42.—</td>
+    <td class='c010'><span class='sc'>The Parallax of Mars</span></td>
+    <td class='c011'><a href='#i_147'>147</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>43.—</td>
+    <td class='c010'><span class='sc'>The Transit of Venus</span></td>
+    <td class='c011'><a href='#i_148'>148</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>44.—</td>
+    <td class='c010'><span class='sc'>The Moon’s Curvilinear Path</span></td>
+    <td class='c011'><a href='#i_155'>155</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>45.—</td>
+    <td class='c010'><span class='sc'>The Tides</span></td>
+    <td class='c011'><a href='#i_163'>163</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>46.—</td>
+    <td class='c010'><span class='sc'>Nutation</span></td>
+    <td class='c011'><a href='#i_169'>169</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>47.—</td>
+    <td class='c010'><span class='sc'>Section of Reading Microscope</span></td>
+    <td class='c011'><a href='#i_172'>172</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>48.—</td>
+    <td class='c010'><span class='sc'>The Reading Microscope</span></td>
+    <td class='c011'><a href='#i_173'>173</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>49.—</td>
+    <td class='c010'><span class='sc'>The Achromatic Object-Glass</span></td>
+    <td class='c011'><a href='#i_177'>177</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>50.—</td>
+    <td class='c010'><span class='sc'>The Newtonian Reflector</span></td>
+    <td class='c011'><a href='#i_179'>179</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>51.—</td>
+    <td class='c010'><span class='sc'>The Cassegrain Reflector</span></td>
+    <td class='c011'><a href='#i_181'>181</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>52.—</td>
+    <td class='c010'><span class='sc'>The Photographic Telescope</span></td>
+    <td class='c011'><a href='#i_196'>196</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>53.—</td>
+    <td class='c010'><span class='sc'>The Meridian Circle of the Paris Observatory</span></td>
+    <td class='c011'><a href='#i_203'>203</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>54.—</td>
+    <td class='c010'><span class='sc'>The Micrometer applied to a Binary Star</span></td>
+    <td class='c011'><a href='#i_208'>208</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>55.—</td>
+    <td class='c010'><span class='sc'>The Spectroscope adapted to the Eye End of the Lick Telescope</span></td>
+    <td class='c011'><a href='#i_221'>221</a></td>
+  </tr>
+  <tr>
+    <td class='c009'><span class='pageno' id='Page_xv'>xv</span>&#160;</td>
+    <td class='c010'>&#160;</td>
+    <td class='c011'>&#160;</td>
+  </tr>
+  <tr>
+    <td class='c009'>&#160;</td>
+    <td class='c010'>&#160;</td>
+    <td class='c011'>&#160;</td>
+  </tr>
+  <tr><td class='c008' colspan='3'>SECTION III.—THE SOLAR SYSTEM.</td></tr>
+  <tr>
+    <td class='c009'>&#160;</td>
+    <td class='c010'>&#160;</td>
+    <td class='c011'>&#160;</td>
+  </tr>
+  <tr>
+    <td class='c009'>&#160;</td>
+    <td class='c010'><i>Frontispiece</i>—<span class='sc'>Donati’s Comet</span></td>
+    <td class='c011'><a href='#i_228'>228</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>&#160;</td>
+    <td class='c010'>&#160;</td>
+    <td class='c011'>&#160;</td>
+  </tr>
+  <tr>
+    <td class='c009'>1.—</td>
+    <td class='c010'><span class='sc'>Photograph of a Sun-spot</span></td>
+    <td class='c011'><a href='#i_243'>243</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>2.—</td>
+    <td class='c010'><span class='sc'>Sun-spots and Magnetic Variations</span></td>
+    <td class='c011'><a href='#i_246'>246</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>3.—</td>
+    <td class='c010'><span class='sc'>Curves showing the Development of Sun-spots</span></td>
+    <td class='c011'><a href='#i_257'>257</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>4.—</td>
+    <td class='c010'><span class='sc'>Eruptive Prominence</span></td>
+    <td class='c011'><a href='#i_264'>264</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>5.—</td>
+    <td class='c010'><span class='sc'>The same, 18 minutes later</span></td>
+    <td class='c011'><a href='#i_265'>265</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>6.—</td>
+    <td class='c010'><span class='sc'>The Eclipsed Sun</span></td>
+    <td class='c011'><a href='#i_267'>267</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>7.—</td>
+    <td class='c010'><span class='sc'>The Corona of January 1, 1889</span></td>
+    <td class='c011'><a href='#i_269'>269</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>8.—</td>
+    <td class='c010'><span class='sc'>Map of Mercury</span></td>
+    <td class='c011'><a href='#i_276'>276</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>9.—</td>
+    <td class='c010'><span class='sc'>Venus, from a drawing by Mascari</span></td>
+    <td class='c011'><a href='#i_280'>280</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>10.—</td>
+    <td class='c010'><span class='sc'>Map of the Moon</span></td>
+    <td class='c011'><a href='#i_291'>291</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>11.—</td>
+    <td class='c010'><span class='sc'>Photograph of the Totally Eclipsed Moon</span></td>
+    <td class='c011'><a href='#i_296'>296</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>12.—</td>
+    <td class='c010'><span class='sc'>Chart of Mars</span></td>
+    <td class='c011'><a href='#i_300'>300</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>13.—</td>
+    <td class='c010'><span class='sc'>The “Eye of Mars”</span></td>
+    <td class='c011'><a href='#i_302'>302</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>14.—</td>
+    <td class='c010'><span class='sc'>The Oases of Mars</span></td>
+    <td class='c011'><a href='#i_304'>304</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>15.—</td>
+    <td class='c010'><span class='sc'>Jupiter, showing the Red Spot</span></td>
+    <td class='c011'><a href='#i_322'>322</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>16.—</td>
+    <td class='c010'><span class='sc'>Photograph of Jupiter</span></td>
+    <td class='c011'><a href='#i_328'>328</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>17.—</td>
+    <td class='c010'><span class='sc'>Transit of Jupiter’s first Satellite</span></td>
+    <td class='c011'><a href='#i_330'>330</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>18.—</td>
+    <td class='c010'><span class='sc'>Saturn and his Rings</span></td>
+    <td class='c011'><a href='#i_335'>335</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>19.—</td>
+    <td class='c010'><span class='sc'>Great Comet of September, 1882</span></td>
+    <td class='c011'><a href='#i_361'>361</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>20.—</td>
+    <td class='c010'><span class='sc'>Donati’s Comet</span></td>
+    <td class='c011'><a href='#i_363'>363</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>21.—</td>
+    <td class='c010'><span class='sc'>Photograph of Swift’s Comet</span></td>
+    <td class='c011'><a href='#i_374'>374</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>22.—</td>
+    <td class='c010'><span class='sc'>The same, 24 hours later</span></td>
+    <td class='c011'><a href='#i_375'>375</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>23.—</td>
+    <td class='c010'><span class='sc'>Photograph of Holmes’ Comet</span></td>
+    <td class='c011'><a href='#i_378'>378</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>24.—</td>
+    <td class='c010'><span class='sc'>Photograph of Brooks’ Comet</span></td>
+    <td class='c011'><a href='#i_381'>381</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>&#160;</td>
+    <td class='c010'>&#160;</td>
+    <td class='c011'>&#160;</td>
+  </tr>
+  <tr>
+    <td class='c009'>&#160;</td>
+    <td class='c010'>&#160;</td>
+    <td class='c011'>&#160;</td>
+  </tr>
+  <tr><td class='c008' colspan='3'>SECTION IV.—THE SIDEREAL HEAVENS.</td></tr>
+  <tr>
+    <td class='c009'>&#160;</td>
+    <td class='c010'>&#160;</td>
+    <td class='c011'>&#160;</td>
+  </tr>
+  <tr>
+    <td class='c009'>&#160;</td>
+    <td class='c010'><i>Frontispiece</i>—<span class='sc'>Nebula in Andromeda 31 Messier</span></td>
+    <td class='c011'><a href='#i_398'>398</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>&#160;</td>
+    <td class='c010'>&#160;</td>
+    <td class='c011'>&#160;</td>
+  </tr>
+  <tr>
+    <td class='c009'>1.—</td>
+    <td class='c010'><span class='sc'>Stars visible in the Northern Hemisphere</span></td>
+    <td class='c011'><a href='#i_401'>401</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>2.—</td>
+    <td class='c010'><span class='sc'>Stars visible in the Southern Hemisphere</span></td>
+    <td class='c011'><a href='#i_403'>403</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>3.—</td>
+    <td class='c010'><span class='sc'>Diagram showing “Solar Apex” as roundly computed</span></td>
+    <td class='c011'><a href='#i_429'>429</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>4.—</td>
+    <td class='c010'><span class='sc'>Apparent Orbit of Zeta Herculis</span></td>
+    <td class='c011'><a href='#i_436'>436</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>5.—</td>
+    <td class='c010'><span class='sc'>Apparent Orbit of the Companion of Sirius</span></td>
+    <td class='c011'><a href='#i_439'>439</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>6.—</td>
+    <td class='c010'><span class='sc'>Apparent Orbit of 70 Ophiuchi</span></td>
+    <td class='c011'><a href='#i_443'>443</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>7.—</td>
+    <td class='c010'><span class='sc'>Triple Stars</span></td>
+    <td class='c011'><a href='#i_451'>451</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>8.—</td>
+    <td class='c010'><span class='sc'>The Temporary Star of 1572</span></td>
+    <td class='c011'><a href='#i_481'>481</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>9.—</td>
+    <td class='c010'><span class='sc'>The Double Star Cluster in Perseus</span></td>
+    <td class='c011'><a href='#i_503'>503</a></td>
+  </tr>
+  <tr>
+    <td class='c009'><span class='pageno' id='Page_xvi'>xvi</span>10.—</td>
+    <td class='c010'><span class='sc'>Star Cluster in Gemini</span></td>
+    <td class='c011'><a href='#i_504'>504</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>11.—</td>
+    <td class='c010'><span class='sc'>37 Messier</span></td>
+    <td class='c011'><a href='#i_505'>505</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>12.—</td>
+    <td class='c010'><span class='sc'>Star Cluster in Hercules</span></td>
+    <td class='c011'><a href='#i_507'>507</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>13.—</td>
+    <td class='c010'><span class='sc'>The Star Cluster, Omega Centauri</span></td>
+    <td class='c011'><a href='#i_512'>512</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>14.—</td>
+    <td class='c010'><span class='sc'>The Orion Nebulæ</span></td>
+    <td class='c011'><a href='#i_521'>521</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>15.—</td>
+    <td class='c010'><span class='sc'>The Nebula round Eta Argus</span></td>
+    <td class='c011'><a href='#i_523'>523</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>16.—</td>
+    <td class='c010'><span class='sc'>The Trifid Nebula, Sagittarius</span></td>
+    <td class='c011'><a href='#i_525'>525</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>17.—</td>
+    <td class='c010'><span class='sc'>Spiral Nebula, 51 Messier</span></td>
+    <td class='c011'><a href='#i_533'>533</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>18.—</td>
+    <td class='c010'><span class='sc'>Magellanic Clouds</span></td>
+    <td class='c011'><a href='#i_537'>537</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>19.—</td>
+    <td class='c010'><span class='sc'>Photograph of Milky Way, Sagittarius</span></td>
+    <td class='c011'><a href='#i_555'>555</a></td>
+  </tr>
+  <tr>
+    <td class='c009'>20.—</td>
+    <td class='c010'><span class='sc'>The Milky Way</span></td>
+    <td class='c011'><a href='#i_557'>557</a></td>
+  </tr>
+</table>
+
+<div class='chapter ph1'>
+
+<div class='nf-center-c0'>
+<div class='nf-center c001'>
+    <div>ASTRONOMY</div>
+  </div>
+</div>
+
+</div>
+
+<div id='i_002'  class='figcenter id003'>
+<img src='images/i_002.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p>THE OBSERVATORY AT NICE.<br>  <br>  [<i>See page <a href='#Page_192'>192</a></i></p>
+</div>
+</div>
+
+<div class='chapter'>
+  <span class='pageno' id='Page_3'>3</span>
+  <h2 class='c006'>SECTION I.—HISTORY.</h2>
+</div>
+
+<div class='nf-center-c0'>
+<div class='nf-center c002'>
+    <div><span class='large'><span class='sc'>By Agnes M. Clerke.</span></span></div>
+  </div>
+</div>
+
+<h3 class='c013'>CHAPTER I.<br> <span class='c014'>FROM HIPPARCHUS TO LAPLACE.</span></h3>
+
+<p class='c003'>In the year 134 <span class='fss'>B.C.</span>, a temporary star blazed out in the constellation
+Scorpio. It was observed by a man of extraordinary
+genius, and furnished the incentive to one of his most memorable
+works. This was the construction, on essentially modern
+principles, of a catalogue of 1,080 stars. Hipparchus thus,
+with deliberation and singular prescience, furnished a standard
+by which future changes in the heavens might be detected.
+He was a native of Rhodes, but belonged to the school of
+Alexandria; and at Alexandria, after three centuries, he
+found an able and ambitious successor.</p>
+
+<p class='c007'>Claudius Ptolemæus was one of the many “inheritors of unfulfilled
+renown.” He combined, completed, and preserved
+what his predecessors, eminent or obscure, had done. Gathering
+materials from all quarters, and adding much of his own,
+he reared an astronomical edifice so imposing, coherent, and
+substantial, that the lapse of fourteen centuries left it virtually
+<span class='pageno' id='Page_4'>4</span>unassailed, and, to a superficial judgment, unassailable. Fitly,
+then, this monument of industry and ingenuity kept the title
+bestowed upon it by the Arabs of “Almagest,” signifying
+“the Greatest.” It bears, nevertheless, perennial witness to
+the possibility of satisfying the human mind with the truth of
+appearances, apart from the truth of things. For although
+the Almagest embodies a large amount of real knowledge,
+that knowledge is throughout falsely interpreted. The
+Ptolemaic system was constructed on the principle of “saving
+the phenomena”—that is, of providing expedients geometrically
+valid, even if physically inadmissible, by which to represent
+the apparent movements of the heavenly bodies.
+That they might, to a great extent, be apparent only, was
+obvious to the cultivated Greek mind. The rotation of the
+earth on an axis was a familiar Pythagorean doctrine; it was
+adopted by Plato, and Aristarchus of Samos went to the
+length of ranking our green world as a planet revolving yearly
+round the sun. The idea, however, was too recondite for
+vulgar apprehension; it was tainted with a suspicion of impiety,
+and its development would, besides, have proved
+extremely embarrassing to the nascent science of that age.
+So Hipparchus chose the prudent alternative of treating
+astronomy from the purely mathematical standpoint; he
+submitted to the restrictions imposed by the hypothesis of
+equable circular motion; and, with wonderful skill, fitted the
+Apollonian eccentrics and epicycles to expound celestial
+wanderings. Ptolemy inevitably followed suit. He set some
+five dozen spheres in motion, while leaving the earth at rest;
+and at rest it remained until, in long meditations by the foggy
+shores of the Baltic, a grave-browed ecclesiastic elaborated
+certain cogent arguments in favour of its motion.</p>
+
+<p class='c007'>During the interval between Ptolemy and Copernicus,
+astronomy kept in the Alexandrian groove. Early in the
+eighth century, the seat of learning having been transferred to
+Baghdad, the charge of its crystalline machinery devolved
+upon Arabs and Jews, men of fine technical acquirements,
+but small originative power, men of the kind described in the
+<span class='pageno' id='Page_5'>5</span>“Vicar of Wakefield,” who, “had they been bred cobblers,
+would all their lives have only mended shoes, but never made
+them.” Not but that they executed the necessary repairs
+with uncommon ingenuity, modifying the cumbrous structure
+given into their keeping to suit the fresh inequalities brought
+to light by their patient watchfulness. But their improvements
+consisted in adding to already intolerable complications—in
+piling orb on orb, in devising “trepidations” and oscillations,
+of which nature took small heed; so that the better
+they observed, the worse their system became.</p>
+
+<p class='c007'>The science was diligently cultivated. Al-Mamûm, son
+and successor of “good Harûn-al-Raschid,” founded at
+Baghdad a school of astronomy, of which Albategnius, called
+“the Ptolemy of the Arabs,” was the brightest ornament. He
+discovered, early in the tenth century, the movement of the
+“sun’s apogee”—that slow revolution of the longer axis of the
+earth’s orbit, regarded by astronomical glacialists as a factor
+in the production of recurring Ice Ages. The Persian grandee,
+Al-Sûfi (903–986) belonged to the same group. His “Description
+of the Stars” was a revised edition, not a simple reprint,
+of the Alexandrian list, and has the value derived from
+personal consultation with the skies. Thus, Algol, now purely
+white, is recorded in it as a decidedly red star. About a
+century later, Aboul Wefa detected the moon’s “variation,”
+independently noted, after five centuries, by Tycho Brahé.
+Then the Tartars had their turn. Nasir Eddin (1201–1274)
+was a native of Khorassan; but his love of learning drew him
+to the city of the Khalifs, where he assembled a band of
+experts for the construction of new planetary tables, the old
+ones having lapsed into hopeless disaccord with the heavens.
+Last came Ulugh Beigh, grandson of the furious Tamerlane.
+He founded at Samarcand a kind of astronomical Solomon’s
+House, built a grand observatory, and worked in it assiduously.
+His vigorous and ennobling reign of forty years was
+terminated by one of those domestic catastrophes which
+ordinarily fix the chronology of Eastern dynasties. He was
+murdered by his son in 1447, and the sands of the desert
+<span class='pageno' id='Page_6'>6</span>thereupon closed, so to speak, over his civilising efforts. His
+star catalogue, edited by Francis Baily in 1843, is the outcome
+of fresh observations made in the old way. A Tartar prince,
+he ranks as an Arab astronomer.</p>
+
+<p class='c007'>Mohammedan science had already fulfilled its appointed
+task. A torch, still alight, had been handed on from East to
+West. Its extinction would have been a calamity. A total
+break in the cultivation of astronomy, for instance, would have
+cost ages to repair. The Ptolemaic system, it is true, disguised
+rather than revealed nature; yet it constituted a regulated
+body of knowledge, only looked at from a wrong point of view.
+An unbiassed spectator had merely to shift his position and
+open his eyes, in order to perceive the simplicity of the real
+celestial mechanism. No better illustration could be adduced
+of Bacon’s aphorism that “truth emerges more easily from
+error than from confusion.”</p>
+
+<p class='c007'>It was from the Moors in Spain that Christian Europe took
+its first lessons in antique science. The Alphonsine Tables
+were due to Oriental industry. They were compiled at Toledo
+about 1270 by an assemblage of Arab experts directed by
+Hassan, the Jew delegate of Alfonso X. of Castile. But they
+caught Western attention, and drew Western intelligence towards
+the abstruse art they exemplified. Thus a little treatise
+on the Sphere composed about 1230, by John Holywood, a
+Yorkshireman, known to cosmopolitan fame as Johannes de
+Sacrobosco, obtained astonishing popularity; at least sixty-five
+Latin editions of it appearing between 1472 and 1647,
+besides French, Italian, German, and Spanish versions, and
+endless commentaries. With the revival of classical learning,
+the Almagest, previously known in blundering Latin translations
+from the Arabic, came to be read in the original Greek,
+and thus re-emergent, roused fresh enthusiasm. Inspired by
+the afflatus, George Purbach (1423–1461) and his brilliant
+pupil, Johannes Müller of Königsberg in Franconia (Regiomontanus),
+successively professors of mathematics at Vienna,
+applied themselves to burnishing up the ancient epicyclical
+apparatus; while in Italy, the seductive opinions of the
+<span class='pageno' id='Page_7'>7</span>Pythagorean school gained ground, as evidence came to light,
+that there had been astronomers before Ptolemy no less than
+kings before Agamemnon. The orthodox doctrine naturally
+continued to be taught at the universities; but some of the
+professors held esoteric opinions of a different cast, which they
+freely imparted to privileged disciples. The earth’s rotation
+was spoken of as a matter of common knowledge by
+Lionardo da Vinci; it was inculcated in rhyme, before the
+close of the fifteenth century, by Girolamo Tagliavia, a
+Calabrese poet; it was debated by scholars and pedants; on
+all sides influences wrought to shatter the integrity of
+Ptolemaic convictions.</p>
+
+<p class='c007'>True progress, however, consists less in destruction than in
+re-organisation. And this demands powers of a high order.
+They were brought into play just at the right moment.
+Nicholas Copernicus was born at Thorn on the Vistula,
+February 19, 1473. At the age of twenty-three, having exhausted
+the teaching resources of the university of Cracow, he
+crossed the Alps in quest of instruction in Greek and mathematics.
+Towards the close of 1496, then, he was enrolled as
+a student at Bologna, and shortly afterwards became the pupil,
+assistant, and friend of the Ferrarese astronomer, Domenico
+Maria Novara. Here, beyond reasonable doubt, Copernicus
+adopted Copernican opinions. The question, <i>An terra moveatur?</i>
+was incessantly mooted at Bologna; advanced thinkers
+replied in the affirmative; Novara himself most likely took his
+intellectual beliefs from Plato and Aristarchus, while looking
+to Ptolemy for his daily bread. The transalpine scholar, at
+any rate, brought back with him to Poland in 1505, an unalterable
+persuasion that the heliocentric system belonged to
+the reality of things. He devoted eighteen years of his abode
+within the cathedral precincts of Frauenburg—from 1512 to
+1530—to demonstrating its detailed conformity with the
+phenomena of the heavens; but allowed only a sketch of his
+results to be published. It was only at the earnest request of
+the Bishop of Culm that he finally delivered up to him the
+manuscript of “De Revolutionibus Orbium Coelestium,” the
+<span class='pageno' id='Page_8'>8</span>first printed copy of which was laid on his deathbed, May 24,
+1543.</p>
+
+<p class='c007'>The immediate effect was small. The new system of astronomy
+was admired, but not adopted. It indeed contradicted
+the evidence of the senses, and failed to compel assent from
+the understanding. For its author had not completely broken
+with tradition. He unfortunately retained the false supposition
+of equable circular motion, and thus greatly marred the
+simplicity of his scheme of the heavens. Orbs still kept
+rolling upon orbs, Mercury alone demanding a combination
+of seven to bear him over his course. But if seven, it might
+have been asked, why not seven times seven? The principle of
+representing appearances by transcendental means remained
+the same as before. Ignorance of the laws of motion raised
+other formidable objections. A whirling earth, it was thought,
+should leave behind all detached objects; absolute repose was
+taken to be the condition <i>sine quâ non</i> of stability. Then the
+seeming immobility of the stars implied for them a remoteness
+so extravagant, according to prevalent ideas, that even Kepler
+admitted it to be “a big pill to swallow.” Copernicus was fully
+aware that the earth’s orbital revolution must occasion stellar
+perspective displacements; indeed, he staked the truth of his
+theory upon future measurements of annual parallax. Nevertheless,
+four centuries passed before they were successfully
+executed.</p>
+
+<p class='c007'>Tycho Brahé was the last great mediæval observer. Like
+Hipparchus, he was summoned by a star—the marvellous
+“new star” of 1572; and, having obtained from Frederick II.
+of Denmark the grant of an islet in the Sound, he built upon
+it a mansion “royal, rich and wide,” erected magnificent
+instruments, and used them, not only with consummate skill,
+but also with a certain princely pomp, donning robes of state
+before admitting the bright “populace of heaven” to audience.
+His stormy temper, however, led to disputes with the young
+King Christian IV.; he forsook Uraniborg, and died at
+Prague in 1601. Curiously enough, the very accuracy of his
+observations led him astray from speculative truth. For it
+<span class='pageno' id='Page_9'>9</span>enabled him to perceive the incompatibility of many facts with
+Copernican expedients for harmonising them, and intensified
+the difficulty raised to Copernican views by the absence of
+stellar parallax. So he devised a system of his own, in which
+the planets revolved round the sun, but the sun round the
+earth. It scarcely survived its contriver.</p>
+
+<p class='c007'>The invention of the telescope created descriptive astronomy.
+Without it, the mechanism of the solar system could
+have been laid bare, and the law of force regulating its action
+discovered; and in point of fact, Kepler’s achievements owed
+nothing, and Newton’s very little, to the optician’s art. Inquiries,
+on the other hand, into the nature of the heavenly
+bodies were wholly inspired by it; it disclosed the amazing
+multitude of the stars, and opened endless vistas of research.
+No one could at first have divined the momentous character
+of the accident by which Hans Lippershey, a spectacle-maker
+at Middleburg in Holland, hit upon an arrangement of lenses
+serving virtually to abridge distance. It happened in 1608;
+and Galileo Galilei (1564–1642), hearing of it shortly afterwards
+at Venice, prepared on the hint a “glazed optic tube,”
+and viewed with it, early in 1610, the satellites of Jupiter, the
+mountains of the moon, the star streams of the Milky Way,
+and in 1611, the phases of Venus, the spots on the sun, and
+the strange appendages of Saturn. Thus, amid a tumult of
+applause, the telescopic revelation of the heavens began. It
+was brilliantly illustrative, although not demonstrative, of
+Copernican theory; and Galileo drove his own vivid conviction
+on the subject home to general apprehension by the
+literary skill with which he treated it in his famous “Dialogues”
+(1632). He most substantially promoted the new
+views, however, by his recognition of the laws of motion, and
+of force as the cause of motion. The problem of the heavens,
+stript thereby of metaphysical obscurities, was laid bare to the
+reason as one of pure mechanics; the planets came to be
+treated as ordinary projectiles, and distinct reasoning about
+the nature of their paths was rendered possible. Newton’s
+great task was thus prepared and defined by Galileo.</p>
+
+<p class='c007'><span class='pageno' id='Page_10'>10</span>Kepler’s (1573–1630) three generalisations formed a still
+more indispensable prelude to its accomplishment. Their
+immediate effect was to sweep away the Copernican remnants
+of Ptolemaic lumber, and to disclose the harmonious plan
+upon which our system is ordered. But it was a geometrical
+plan only. Kepler indeed divined the influence of a central
+power, which he surmised to be of a magnetic nature;
+and he aspired towards the establishment of a truly physical
+astronomy. Yet he was far from perceiving the full implications
+of the laws he had himself, after half a lifetime of
+trial and failure, at last triumphantly discovered. These laws
+are:</p>
+
+<p class='c007'>(I.) The planets travel in ellipses of which the sun occupies
+one focus.</p>
+
+<p class='c007'>(II.) They travel at rates varying in such a manner that the
+“radius vector”—or imaginary line joining each to the sun—describes
+equal areas in equal times.</p>
+
+<p class='c007'>(III.) The cubes of their mean distances from the sun are
+proportional to the squares of their periods of revolution.</p>
+
+<p class='c007'>Now these are precisely the conditions under which
+planetary circulation should proceed if governed by a force
+emanating from the sun, and decreasing as the square of the
+distance from him increased. Moreover, Hooke, Halley, and
+Wren separately got so far as to perceive that it could be explained
+on this principle. But Isaac Newton alone could
+demonstrate what they divined, and even his supreme faculties
+were dangerously strained by the laborious process. This was
+not all. He showed that the earth exerts on the moon just
+the same kind of pull that the sun exerts on the planets; a
+pull identical with the familiar “attraction of gravitation,” by
+which the globe we inhabit holds integrally together, retains
+its oceans in their beds, and bears with it through space its
+“cloud of all-sustaining air.” Its domestic affairs are thus
+guided by the same unchanging rule that dominates its foreign
+relations.</p>
+
+<p class='c007'>The publication in 1687 of Newton’s “Principia” marked
+an unprecedented advance in knowledge. The advance consisted
+<span class='pageno' id='Page_11'>11</span>in unification. A science of celestial physics, capable
+of indefinite future expansion, was founded on the sure basis
+of terrestrial experience. Canons of interpretation, derived
+from immediate perception, were proved applicable to the
+phenomena of the heavens. The line drawn in antique
+philosophy between the “corruptible” things under our feet
+and the “incorruptible” over our heads was forever rubbed
+out. Sublunary and empyreal regions were thrown together
+into one vast domain.</p>
+
+<p class='c007'>Although Newton’s law is, in itself, of extreme simplicity,
+its actual workings are highly intricate. Because dependent
+upon a universal and unintermittent influence, they are self-modifying,
+so that each consequence becomes a cause, and to
+each cause is attached an endless train of effects. They can
+be dealt with only with the aid of the infinitesimal calculus,
+and then, not directly, but by successive and tedious approximations,
+or by arts and devices of almost superhuman
+ingenuity. Hence Newton’s laurels would have remained comparatively
+barren had he not found successors in a group of
+men of extraordinary ability. What he had begun, Clairaut,
+D’Alembert, Euler, Lagrange, and Laplace carried on by
+showing the adequacy of a single law to account for every
+traceable deviation from undisturbed elliptical motion. In
+the course of a long and arduous campaign, they carried every
+position that they attacked. Over and over again, the principle
+of gravitation seemed to be compromised; over and
+over again, it was vindicated by these intrepid champions.</p>
+
+<p class='c007'>This process of gradual verification began in 1747, when
+Clairaut and D’Alembert sent to the Paris Academy of
+Sciences, on the same day, the first satisfactory solutions of
+the “Problem of three Bodies.” The motions of the moon,
+nevertheless, did not at once fall in with the general theory;
+they were rendered amenable only after years of anxious
+toil. Barely the initial difficulties had been overcome when
+Euler, in 1753, published his “Theory of the Moon,” from
+which Tobias Mayer of Göttingen constructed lunar tables.
+Now tables are the test of theories. Every row of figures
+<span class='pageno' id='Page_12'>12</span>they contain is a prediction, by the fulfilment, or non-fulfilment
+of which the underlying scheme must stand or fall.
+Through such comparisons, mathematical astronomers find
+out the shortcomings of their methods, or the insufficiency of
+their hypotheses, and are incited to refine the first, and correct
+the second. Demands for the application of the nicer
+criteria thus afforded suggest observational improvements,
+which seldom fail to bring to light minor discrepancies with
+theory, impelling to fresh efforts for their abolition. Such
+alternations of advance along the abstract and the practical
+lines result in a continual diminution in the <i>scale</i> of error,
+although not in its annihilation; absolute exactitude being,
+as it were, an asymptote, continually approached, but touched
+only at infinity—that is, never, under subsisting conditions.
+Even now the length of the moons tether is four or five miles.
+To that extent, she may go astray from her computed path,
+not without occasioning disquietude to the responsible
+authorities.</p>
+
+<p class='c007'>So far as could be ascertained in the eighteenth century,
+her subjection to known law was completed by the dispersal
+of the mystery surrounding a slight, continuous acceleration
+of her orbital velocity detected by Halley in 1693. It had
+been in progress since the earliest recorded eclipse in 721 <span class='fss'>B.C.</span>,
+if not longer; there was no sign of its cessation or reversal,
+and the grave question arose, Was the principle of universal
+attraction, elsewhere unreservedly obeyed, here fatally complicated
+by the action of a resisting medium involving the
+eventual collapse of the earth-moon system? Laplace gave
+the answer, November 19, 1787, by proving the observed
+quickening of pace to be a necessary and simple consequence
+of a secular diminution in the ellipticity of the earths orbit.
+This, however, will not go on for ever in the same direction;
+after many ages the tide of change will turn, and a complete
+restoration to the <i>status quo ante</i> will ensue.</p>
+
+<p class='c007'>Another master-stroke of Laplace’s genius was his explanation,
+also in 1787, of the “long inequality” of Jupiter and
+Saturn. He demonstrated its strictly gravitational origin in
+<span class='pageno' id='Page_13'>13</span>the mutual disturbance of the two giant planets, rendered up
+to a certain point cumulative by the approximate commensurability
+of their periods. While Jupiter performs five circuits
+Saturn accomplishes nearly two, and the perturbation set up
+at their conjunction is hence both intensified and balked of
+compensation for 918 years.</p>
+
+<p class='c007'>The epoch of trial and confirmation immediately following
+the publication of the “Principia” lasted then a full century.
+During its course, difficulties had arisen only to be overcome;
+suggested qualifications of the single and simple
+law of gravity had proved unnecessary; at its close, recalcitrance
+had everywhere been overcome, and there was
+victory all along the line. And not only were the workings
+of the planetary system exhibited as depending upon
+an elementary principle, but they were further shown to
+be perfectly equilibrated. It contained within itself, so far
+as could be ascertained, no seeds of decay; its destruction
+could only come from without. This remarkable conclusion
+was established in a series of splendid treatises by Lagrange
+and Laplace. The special adaptation to permanence of the
+solar mechanism was demonstrated in them. Ruinous disturbances
+were shown to be excluded by the overwhelming
+disparity of mass between the central body and its attendants,
+no less than by the regularity and harmony of their movements
+and distribution. Thus only slight oscillatory changes
+can occur. Millions of years will elapse without producing
+any fundamental alteration. The machine is so beautifully
+adjusted as to right itself automatically through the mutual
+action of its various parts. And it is the force which perturbs
+that eventually restores.</p>
+
+<p class='c007'>The astronomical acquisitions of the century were embodied
+in Laplace’s “Mécanique Céleste,” published 1799–1805.
+This “Almagest of the eighteenth century,” as it has
+been termed, is in a rare degree comprehensive and complete.
+It leaves nothing enigmatical. Every question propounded
+in it receives an answer, if not definitive, at least highly
+authoritative; and the range of these questions is very wide.
+<span class='pageno' id='Page_14'>14</span>All the phenomena which the Greeks and Arabs had rightly
+observed, but wrongly interpreted, are not merely “saved”
+by geometrical artifices, but derived as a connected whole
+from one physical cause, absolutely prescribing that they
+should be thus, and no otherwise. The work is a record of
+unmixed triumphs. It seems as if the author, for want of
+more worlds to conquer, had laid down the sword of the
+calculus to take up the pen of the chronicler. With grave
+exultation, he proceeds from point to point, recounting the
+events of the campaign, commemorating the battles won by
+the brilliant staff of mathematical heroes to which he himself
+belonged, and expatiating in the broad subjugated plain.
+He scarcely looked beyond. There was indeed at that time no
+“beyond” where his methods of investigation were applicable.
+The “Mécanique Céleste” hints at no unsatisfied ambitions;
+it is a book of the <i>teres atque rotundus</i> sort—a world in itself
+well arranged and compact, to which outlying perplexities are
+allowed no access. Nor should this be counted a defect. As
+a monument to one of the greatest periods in the history of
+science, its fitting character was that of an ordered collection
+of acquired certainties.</p>
+
+<p class='c007'>The countrymen of Newton took no part in the striking
+series of operations by which the intricate consequences of
+the law of gravity were deduced and shown to correspond
+with reality. During the whole of the eighteenth century,
+they stood aside from the race towards verification. Their
+effacement was due to no lack of ability, but to a mistaken
+choice of means. Newton’s synthetic method was a veritable
+Bow of Ulysses. It was too tough to be bent by other hands
+than his own. Thus, no sequel could be given to the
+“Principia.” There was no possibility of following up the
+line of demonstration pursued in it. Newton himself would
+have vainly attempted to carry it much further. In order to
+advance, it was necessary, as Dr. Whewell remarked, to begin
+afresh. This, British mathematicians were unwilling to do.
+The easy and flexible analytical method brought to perfection
+on the continent remained strange to them. With inadequate
+<span class='pageno' id='Page_15'>15</span>strength, they persisted in wielding the cumbrous weapon of
+a giant—in using main force, so to speak, where skill and
+agility were required. Our insularity in this respect lasted
+until about 1816, when, by the joint efforts of the younger
+Herschel, Charles Babbage, and George Peacock (afterwards
+Dean of Ely), mathematical studies were revolutionised at the
+University of Cambridge.</p>
+
+<p class='c007'>The neglect in England of theoretical research was, however,
+partly compensated by the steady progress of practical
+astronomy. For a century and a half after its foundation in
+1675, the Royal Observatory at Greenwich continued to be
+the main—almost the only source of information regarding
+the places of the heavenly bodies. Thence were obtained the
+data necessary for the correction of theory, since there alone
+the visible positions of the sun, moon, and planets were
+systematically determined. <i>Actual</i>, compared with <i>predicted</i>,
+movements gave so-called “tabular errors”; and tabular errors
+indicated theoretical shortcomings, the rectification of which
+led gradually, but surely, towards a higher plane of knowledge.</p>
+
+<p class='c007'>John Flamsteed (1646–1719), the first astronomer-royal,
+was, in Professor De Morgan’s phrase, “Tycho Brahé with a
+telescope.” By his diligence and insight he set on foot
+modern astronomy of precision. The “British Catalogue”
+of nearly 3,000 stars, was, in its day, an unique and most
+valuable work. His lunar observations were indispensable to
+Newton’s calculations, which, indeed, through the insufficient
+supply of them, now and again came to a halt; he constructed
+new solar tables, and kept watch over the careers of planets
+and comets. His completion, in 1689, of a seven-foot mural
+quadrant, constituted a marked advance in the art of
+instrument-making. It was firmly fixed in the meridian, so
+that the distances from the zenith of the heavenly bodies at
+the moment of culmination could be read off on the limb, the
+time being simultaneously noted by a clock. Their positions
+in the sky relative to a set of forty otherwise known stars
+were thus completely determined, and they were determined
+essentially after the manner still in use.</p>
+
+<p class='c007'><span class='pageno' id='Page_16'>16</span>On Flamsteed’s death in 1719, Edmund Halley (1656–1742)
+succeeded to his place. An expedition to St. Helena
+in 1677, for the purpose of observing stars invisible in these
+latitudes, got him the name of the “southern Tycho.” They
+were the very first so situated to be located on the sphere
+(except those few that came within Ptolemy’s range), and a
+list of them, to the number of 341, was appended to the
+“British Catalogue.” The purpose to which Halley devoted
+most sustained attention was, unluckily, that in which he
+was least successful. Early in life he formed the design of
+observing the moon through an entire revolution of its nodes,
+so as to bring lunar tables to the perfection required for
+solving the prize-problem of longitudes. But the <i>contumax
+sidus</i>—his opprobrious term for our satellite—proved more
+than a match for him. The eighteen years’ watch was kept,
+notwithstanding that the watcher had reached the age of
+sixty-five before he was able to set about it; but in vain;
+nothing came of it. Halley’s varied performances were,
+nevertheless, so considerable as to warrant Lalande in describing
+him as “the greatest of English astronomers”; and he
+ranked next to Newton among contemporary English men of
+science.</p>
+
+<p class='c007'>His cometary labours alone sufficed to perpetuate his
+name. He initiated the computation, on Newtonian principles,
+of the orbits traversed by such bodies—then a most
+toilsome process; and, among twenty-four, found three so
+much alike as to suggest the identity of the great comets of
+1531, 1607, and 1682. A renewed apparition might then be
+expected in 1758, and he appealed to “candid posterity to
+acknowledge that this was first discovered by an Englishman.”
+The prediction roused widespread interest, and as
+the epoch for its fulfilment drew near, Clairaut undertook the
+formidable task of determining to what extent it might be
+postponed by the retarding influence of Jupiter and Saturn.
+Many times he despaired of its execution, even with the
+efficient aid of Lalande and Madame Lepaute, the wife of a
+Paris clock-maker; and at last, after months of wearisome
+<span class='pageno' id='Page_17'>17</span>calculation, having succeeded in forming the differential
+equations representing the comet’s disturbed motion, he
+threw down the paper on which they were written, with the
+exclamation, “Now, integrate them who can!” Eventually
+this, too, was done; and the comet, caught sight of on Christmas
+Day, 1758, by Palitzsch, a rustic star-gazer in Saxony,
+passed the sun within the month’s “law” permitted to it by
+the French geometer. This signal triumph laid the sure
+foundation of cometary astronomy.</p>
+
+<p class='c007'>In 1679, Halley drew attention to the importance of transits
+of Venus for measuring the sun’s distance; and developed
+later a method extensively used in observing the eighteenth
+century pair of transits in 1761 and 1769. But the accuracy
+actually attained in determining the instants of contact between
+the limbs of the sun and planet fell far short of what he
+had anticipated as attainable. The “black drop” interposed
+its pernicious effects, and occasioned wide discrepancies. The
+margin of uncertainty regarding the value of the great unit
+was, none the less, diminished, although it still remained uncomfortably
+wide; while the public interest excited by such
+rare events, the adventurous character of the expeditions sent
+to the uttermost parts of the earth for their utilisation, and the
+combined efforts of various nations towards the same end,
+served to popularise astronomy, and to give it something of
+that cosmopolitan stamp now borne by it.</p>
+
+<p class='c007'>Besides the discovery of the secular acceleration of the
+moon’s motion, that of the long inequality of Jupiter and
+Saturn was due to Halley; he ascertained, in 1718, the proper
+movements of Sirius, Aldebaran, and Arcturus, thereby virtually
+demonstrating the non-existence of “fixed” stars; he
+associated auroræ with terrestrial magnetism; noted the
+globular star clusters in Hercules and Centaur; and divined
+nebulæ to be composed of “a lucid medium shining with its
+own proper lustre,” and filling “spaces immensely great.”
+Yet, in spite of the comprehensiveness of his genius, his administration
+at Greenwich was a failure. He was a better
+astronomer than astronomer-royal.</p>
+
+<p class='c007'><span class='pageno' id='Page_18'>18</span>James Bradley (1693–1762), who came after him, gave a
+narrower scope to his abilities, yet was of unsurpassed
+sagacity in connecting effects with their causes. Robert
+Hooke (1635–1703) had observed, in 1669, annual displacements
+of γ Draconis, a star nearly crossing the zenith of
+London, which he took for results of parallax; and Flamsteed,
+in 1694, had similarly interpreted a similar affection
+of the pole-star. They had both been misled by an “aberration,”
+due to the progressive transmission of light combined
+with the advance of the earth in its orbit. Bradley determined
+to sift the matter thoroughly, and observed Hooke’s
+star continuously from 1725 until 1728, first at Kew with
+Molyneux, then at Wanstead in Essex. It evidently described
+a small ellipse in the sky with a period of one
+year; yet its place in the ellipse was not what it should
+have been on the parallactic hypothesis; so he remained for
+some time in the dark about it. During a water-party on the
+Thames, however, in September 1728, he noticed that the
+slant of the pennant varied with changes in the boat’s course,
+the wind remaining steady throughout. This gave him the
+clue he wanted; and his discovery of the “aberration of light”
+was communicated to the Royal Society in the month of
+January following. That of the nutation of the earth’s axis
+followed in 1748. Both, setting aside their importance in
+themselves, were indispensable as preliminaries to accuracy in
+fixing the places of the heavenly bodies. For they are vital
+elements in the process of “reduction,” by which the ore of
+truth contained in observations is extricated from the dross of
+casual circumstances. The raw material, collected by timing
+transits and reading circles, must be so refined and purified that
+the facts contained in it become mutually comparable. Before
+Bradley’s time allowance was indeed roughly made for refraction
+in our atmosphere, and for the precession of the equinoxes;
+and, in the case of the moon, for parallax; but the effects of
+aberration and nutation had remained mixed up with a mass
+of disguising errors. Their elimination constituted an inestimable
+improvement.</p>
+
+<p class='c007'><span class='pageno' id='Page_19'>19</span>In the immediate art of observation Bradley was a master.
+He did not live to possess an achromatic telescope; neither
+astronomical circles nor equatorial mountings were at his
+disposal. His leading instrument was an eight-foot quadrant,
+by John Bird, certainly of admirable workmanship; although
+of a type long since, and for good reasons, superseded. He
+amassed with it, nevertheless, a treasure of high-class observations.
+The bulk of them remained in manuscript until
+1798, so that it was reserved for this century to turn them to
+account; but their value has only developed with the efflux
+of years. Those relating to the moon and planets, reduced
+by Sir George Airy, lent efficient aid towards perfecting the
+theories of those bodies. Those of 3,222 stars formed into a
+catalogue by Bessel were published in 1818 with the proud,
+but not unmerited title of “Fundamenta Astronomiæ.” The
+same original data, again in 1886 reduced with the utmost
+nicety of care by Dr. Auwers of Berlin, afforded a splendid
+accession to knowledge of stellar proper motions. Acquaintance
+with Bradley’s stars now extends over 144 years; and
+the amount and direction of their progress across the sphere
+during that long interval have, for the most part, become
+defined with tolerable certainty.</p>
+
+<p class='c007'>Nathaniel Bliss (1700–1764), the fourth astronomer-royal,
+filled the post only two years. Yet the observations made
+under his care form a sequel to Bradley’s well worth having.
+The reign of his successor, Nevil Maskelyne (1732–1811),
+extended over forty-six years. His determinations of the
+sun, moon, and planets, were in great demand abroad for the
+correction of tables, and as criteria of theories; while, of the
+stars, he paid attention only to thirty-six, catalogued as
+reference-points in 1790. Their proper motions served
+Herschel for his second investigation, in 1805, of the sun’s
+translation through space. By the close of the century,
+Maskelyne’s instruments had lapsed into decrepitude; and
+only the stimulus supplied by Pond’s strictures roused him to
+order one of Troughton’s improved circles. But he died
+before it was mounted, and its employment fell to the share
+<span class='pageno' id='Page_20'>20</span>of his critic, John Pond (1767–1836), the sixth astronomer-royal.
+Maskelyne’s most enduring title to fame is his foundation,
+in 1767, of the “Nautical Almanac.”</p>
+
+<p class='c007'>English observers were ably seconded by English artists.
+Graham, Sisson, Cary, Bird, Ramsden, had, from the beginning
+to the end of the eighteenth century, no foreign competitors
+of note. Their quadrants and sectors were distinguished
+both for stability and for refinement of execution.
+The mechanical skill displayed in their construction was no
+less necessary for the promotion of practical astronomy than
+the subtlety of eye and hand needed to employ them to the
+best advantage. Bradley’s work was conditioned by the
+performances of Graham and Bird. Without Graham’s
+sector he could not have discovered the aberration of light;
+without Bird’s quadrant the perennial worth of his Greenwich
+observations would have been impaired, if not destroyed.
+Observatories all over the continent were furnished in the
+latter half of the eighteenth century with instruments of
+English make; the art of accurately dividing circular limbs
+was invented in England, and nowhere else successfully
+practised. The innovation of substituting entire circles for
+quadrants was effectively introduced by Ramsden; and
+Piazzi came from Palermo in 1788 for the purpose of securing
+from him a five-foot altazimuth, at that date the finest sky-measuring
+machine in the world. Edward Troughton (1753–1835)
+ably carried on the tradition of his predecessors, and
+brought the altazimuth, transit circle, and equatorial up to
+the modern standard of efficiency. But they were no longer
+in exclusive demand. The foundation, in 1804, of Reichenbach’s
+Institute at Munich finally abolished the British
+monopoly in supplying astronomers with their exquisite and
+ingenious tools.</p>
+
+<p class='c007'>The improvement of refracting telescopes ran a somewhat
+similar course. The essential step of combining flint and
+crown glass, so as to bring differently-coloured rays to one
+focus, was taken in 1733 by Chester More Hall, a gentleman
+of fortune in Essex; but he published nothing, and the
+<span class='pageno' id='Page_21'>21</span>re-invention of the “achromatic” lens was left to John
+Dollond (1706–1761) a Spitalfields weaver. “I obtained,” he
+wrote in 1758, “a perfect theory for making object-glasses, to
+the apertures of which I could scarcely conceive any limits.”
+The excise duty on glass, however, which was repealed only
+in 1845, drew these limits very narrowly in this country;
+and it was through the extraordinary perseverance of a Swiss
+artisan named Guinand, in overcoming the difficulties connected
+with glass-making, and the genius of Joseph Fraunhofer
+(1787–1826) in moulding the material thus placed at his
+disposal, that refractors began at Munich to rise towards their
+present power and perfection.</p>
+
+<p class='c007'>The history of the reflecting telescope is British throughout.
+It was invented by Newton, made practically effective by
+John Hadley (1682–1744), and brought very near to theoretical
+perfection by James Short of Edinburgh (1710–1768); yet it
+is remarkable that not a single observation of lasting
+interest was made with any of his instruments, a few of
+which have survived, and are regarded with admiration to
+this day. The career of reflectors as engines of discovery
+began, but did not end, with William Herschel.</p>
+
+<h3 class='c004'>CHAPTER II.<br> <span class='c014'>A CENTURY OF PROGRESS.</span></h3>
+
+<p class='c003'>On March 13, 1781, an event occurred without precedent
+in the history of astronomy. A new member of the sun’s
+immediate retinue was disclosed. A hard-worked music-teacher
+at Bath performed this startling—indeed, according
+<span class='pageno' id='Page_22'>22</span>to antique notions—impossible feat; and the name of
+Herschel became known <i>urbi et orbi</i>. It was far from being
+by chance that the “new planet swam into his ken.” The
+Octagon Chapel organist was no ordinary lucky amateur.
+He had, some time previously, made two notable resolutions.
+The first was to push the improvement of telescopes to the
+furthest verge of what was possible; the second, to leave no
+corner of the starry heavens unexplored. And he applied
+himself with marvellous energy, in despite of accumulated
+professional engagements, to carry them into execution. He
+thus rapidly grew to be an adept in the art of constructing
+specula, and a master in the art of using them.</p>
+
+<p class='c007'>Two lines of effort, accordingly, converged, in his case,
+towards celestial discoveries. With all his diligence in “reviewing”
+the heavens, he could not have distinguished at
+sight Uranus from a fixed star, but for the uncommon
+excellence of his seven-foot reflector; nor would the reflector,
+had it been used in the ordinary erratic fashion of casual stargazers,
+been at all likely to have encountered the little bluish
+disc of the remote orb then slowly wending its way through
+the constellation of the Twins. The direct, and a momentous
+result of the discovery was to secure for astronomy the
+undivided powers of the extraordinary man who had made it.
+George III. attached him to his Court, delivered him from
+the drudgery of teaching, and gave him the means of carrying
+out his grand designs.</p>
+
+<p class='c007'>Their fulfilment involved the construction of great light-gathering
+machines. Herschel ardently desired to see as
+far and as much as the conditions of mortality permitted;
+he was the first to connect depth of penetration into space
+with extent of reflective surface; and he accordingly
+strained every nerve to secure the means by which to
+compass the end he had mainly in view. Nor was he
+content with mere size. His mirrors were as remarkable
+for beauty of figure as for breadth of aperture. They
+bore, on proper occasions, enormously high magnifying
+powers, and the precise roundness of the star-images formed
+<span class='pageno' id='Page_23'>23</span>by them excited the incredulous wonder of contemporaries.
+The quality of some of his largest instruments was guaranteed
+by the heavens themselves. Their approval was signified
+to the seven-foot reflector through the detection with it
+of Uranus; the “large twenty-foot,” with a speculum of
+eighteen inches, revealed in January 1787, two Uranian moons,
+Oberon and Titania; and the monster forty-foot, through the
+tube of which George III. promenaded with the Archbishop
+of Canterbury, brought into view, within three weeks of its
+completion, Enceladus and Mimas, the innermost and hardest
+to observe of Saturn’s numerous family of satellites.</p>
+
+<p class='c007'>The forty-foot was “Herschel’s furthest”; he fully recognised
+that with it he had touched the line which divides failure
+from success. If, indeed, he had not overpassed it; for the
+subsequent career of the great telescope hardly bore out the
+promise of its start. It was an unwieldy engine, demanding
+vastly more time and labour to bring into play than the
+twenty-foot; and Herschel took such account of minutes as
+few men do of hours or days. His fiftieth birthday had in
+fact gone by before his optical ambition was satisfied; while
+his appetite for exploration was only whetted by what he had
+already accomplished. He estimated, however, that a “review
+of the heavens” with the forty-foot would have occupied 800
+years; hence it was used only on special occasions. The
+Orion nebula was the last celestial object upon which, January
+19, 1811, “its broad, bright eye” rested; and it was then,
+with due honour, placed on the retired list.</p>
+
+<p class='c007'>Two years before his death, which occurred August 25,
+1822, the elder Herschel initiated his son into the secrets of
+speculum-building. The pupil was worthy of the master.
+John Herschel (1792–1871) aimed only at producing generally
+available instruments, and his success was easy and unqualified.
+His eighteen-inch mirrors seem to have been all but
+faultless. They certainly afforded him better views of the
+nebulæ than had been obtained by his father. Thus he first
+saw the “Dumb-bell” in its true oval shape; and his remarks
+upon annular lines of structure in elliptical nebulæ prove that
+<span class='pageno' id='Page_24'>24</span>features unmistakably imprinted upon Dr. Roberts’ photographs
+had been antecedently visible to him, and probably to
+him alone.</p>
+
+<p class='c007'>The next stride in the enlargement of reflectors was made
+by an Irish nobleman, the third Earl of Rosse (1800–1867).
+His leviathan telescope, six feet in aperture, and fifty-four in
+length, has, in point of actual size, never been surpassed. Distinguished
+rather for light-grasp than for precise definition, it
+found its appropriate field in the nebular realms of the sphere;
+and the discovery of spiral nebulæ, with which it made its
+début, was one of high and wide significance.</p>
+
+<p class='c007'>William Lassell (1800–1881) of Starfield, near Liverpool, set
+the example, in 1840, of mounting reflectors equatorially, so
+as to enable them, by the application of clock-work, to follow
+automatically the diurnal movement of the heavens. His
+specula were of almost unrivalled perfection in form and finish.
+One twenty-four inches in diameter, now at Greenwich, left a
+splendid record. With it Lassell detected, October 10, 1846,
+the satellite of Neptune; September 18, 1848, simultaneously
+with W. C. Bond of Cambridge, U.S., Hyperion, the seventh
+in order of distance and last in order of discovery of Saturn’s
+eight moons; and October 24, 1851, Ariel and Umbriel, the
+inner pair of Uranian satellites, of which Sir William Herschel
+had possibly, although not very probably, caught transient
+glimpses. He erected a similar instrument of fourfold capacity
+at Malta in 1861, registered with its aid 600 new nebulæ, and
+delineated the complex structure of many others, previously
+less well seen.</p>
+
+<p class='c007'>The four-foot reflector built in 1870 by Thomas Grubb
+of Dublin for the Melbourne Observatory disappointed expectation.
+An apparatus so delicate that the abrasion of
+1/20,000th of an inch makes all the difference between good
+and bad definition, is ill-fitted to endure the rough-and-tumble
+experiences of an ocean-voyage; and that it in some way
+“suffered a sea-change” is scarcely doubtful. It was the last
+great telescope of its kind, metallic specula, having, in the
+seventies, been superseded by mirrors made of glass upon
+<span class='pageno' id='Page_25'>25</span>which a thin layer of silver has been chemically deposited.
+These have many advantages over their predecessors. They
+are considerably more reflective; they are more easily constructed;
+their shape is less liable to injury; their brilliancy,
+although more evanescent, can be readily restored. They
+have the drawback, however, of being extremely sensitive to
+changes of temperature. A three-foot mirror of this description
+by Calver, was employed by Dr. Common at
+Ealing with surprising success, early in 1883, for the purpose
+of photographing the Orion nebula. It was mounted at the
+Lick Observatory, California, in 1896. Dr. Common has since
+himself constructed a similar instrument of five feet aperture,
+which is the most potent light-collector ever yet turned to the
+skies. It is curious to learn that the silver spread over its
+surface weighs less than one of the “fourpenny bits” some
+time ago withdrawn from circulation; the reflecting film is in
+fact only 1/280,000 inch thick.</p>
+
+<p class='c007'>Reflectors are perfectly, and <i>naturally</i>, achromatic, rays of
+all colours being thrown back at the same angle, and consequently
+meeting at the same focus. This gives additional
+brilliancy to the images formed by them, compared with
+those given by object-glasses, the colour-correction of which
+has hitherto been so imperfect that much light has to be
+“thrown away” as worse than useless. New kinds and
+combinations of optical glass have, however, of late been
+invented, by which this grave defect may be cured. Reflecting
+telescopes, on the other hand, are less manageable, and
+suffer more from distortion through change of position.
+Their cheapness recommends them to amateurs; but they
+should, on principle, be reserved for special departments of
+work, such as nebular photography and the chemical delineation
+of stellar and nebular spectra.</p>
+
+<p class='c007'>The growth of refractors, like that of reflectors, has obtained
+from time to time the sanction of unexpected disclosures.
+Thus a superb fifteen-inch, turned out at Munich in 1847,
+for Harvard College, Cambridge, U.S., showed Hyperion
+to Bond, September 16, 1848, and on November 15, 1850,
+<span class='pageno' id='Page_26'>26</span>surprised him with a view of Saturn’s dusky ring. This
+telescope was surpassed, after fifteen years, through the
+energy and genius of Alvan Clark, the famous self-taught
+American optician, originally a portrait-painter at Cambridgeport,
+Massachusetts. Before it had left the workshop, an
+eighteen-inch achromatic, now the leading instrument at the
+Dearborn Observatory, Evanston, Illinois, won maiden honours
+by disclosing to Alvan G. Clark, one of the maker’s sons,
+January 31, 1862, the dim companion of Sirius, which, before
+being seen, had made itself <i>felt</i> by gravitational disturbances
+of its radiant primary. The Washington twenty-six-inch, by
+the same firm, was rendered illustrious by Professor Hall’s
+discovery, in 1877, of a pair of Martian moons; the Lick
+thirty-six-inch, by bringing within the range of Professor
+Barnard’s keen eyesight, September 9, 1892, Jupiter’s tiny
+“fifth satellite.” The diploma performance of the Yerkes
+forty-inch, mounted in 1896 at the Chicago University
+Observatory, is yet to come. Meanwhile, several very perfect
+refractors, up to thirty-two inches of aperture, have been
+built on this side of the Atlantic by Sir Howard Grubb of
+Dublin, and the MM. Henry of Paris; and a twenty-five-inch,
+finished so long ago as 1868, and at the cost of his life
+through the labours which it entailed, by Thomas Cooke of
+York, after having lain for upwards of a score of years choked
+by the fog and smoke of Gateshead, has recently begun a
+promising career at Cambridge, under the care of Mr. Frank
+Newall, son of the original owner.</p>
+
+<p class='c007'>And now we cannot but ask ourselves, has the <i>ne plus ultra</i>
+in telescopic magnitude been attained? There is no reason
+to suppose that it has, provided that due allowance be made
+for inexorable conditions. Climate is one of these. The
+largest instruments are those most readily crippled by
+atmospheric hindrances. The greater their powers, the fewer
+are the nights on which they are likely to be available. If
+they are to “shine in use,” and not “rust unburnished,” they
+must then be erected in exceptionally favourable localities,
+such as the summit of Mount Hamilton (the site of the Lick
+<span class='pageno' id='Page_27'>27</span>Observatory), or the Harvard College southern station at
+Arequipa in Peru. In South Africa, too, but “up country”—not
+in the Cape peninsula—splendid facilities for astronomical
+observation are to be found.</p>
+
+<p class='c007'>From Professor Keeler’s report it can readily be gathered,
+and he indeed explicitly states, that the Yerkes forty-inch
+marks the limit of useful size in equatorials. For the character
+of the star-images formed by it slightly change their
+character when it is directed to different parts of the sky; and
+this implies that its lenses become, as it moves, infinitesimally
+deformed through the effects of their own weight. No larger
+instrument, accordingly, can safely be permitted to swing in
+mid-air. The huge light-concentrating machines of the future
+will lie in wait for the objects to be observed, instead of
+pursuing them. They will either be supported horizontally,
+or mounted in the “Coudé” fashion invented by M. Loewy.
+In either case, the necessary movement will be performed
+vicariously by a plane mirror.</p>
+
+<p class='c007'>Thus, the optical and mechanical outlook is decidedly
+better than the atmospheric. The question, How to build
+giant telescopes? is more easily answered than the question,
+Where to place them when built? The ultimate barrier to
+seeing indefinitely far into space is the rigid circumstance that
+we live on an air-girt globe. The prospects of astronomy are
+deeply involved in the forecast of its hampering effects. The
+dependence of those prospects upon telescopic improvements
+became obvious when Herschel took the whole contents of the
+sphere “for his province.” These are indefinitely numerous,
+indefinitely far-off, indefinitely faint. The task of their correlation
+undertaken by Herschel, and inherited from him by
+modern astronomers, can at no time be more than approximately
+fulfilled; but for each successive approximation more
+light is needed. Those who would investigate the universe
+can never get enough of that too scarce commodity.</p>
+
+<p class='c007'>Until Herschel conceived the novel idea of a comprehensive
+science of the stars, they had been chiefly regarded as convenient
+sky-marks, by which to track the wanderings of our
+<span class='pageno' id='Page_28'>28</span>nearer neighbours in space. When it was perceived that the
+sky-marks were not fixed, it became necessary to determine
+their movements; and this was very roughly done for fifty-seven
+stars by Tobias Mayer of Göttingen, in 1757; and more
+accurately for thirty-six by Maskelyne, a third of a century
+later. But if the stars were travelling, the sun could not be
+supposed to stand still; and the possibility of laying down
+his line of march through space, by extricating a common element
+from the confused network of mutually-crossing stellar
+paths, occurred to Mayer, and was actually realised by
+Herschel in 1783. His inquiry, with the scanty materials
+then at command, was a wonderful stroke of audacity, which
+very nearly hit the mark; yet few believed in his result until
+it was confirmed by Argelander in 1837.</p>
+
+<p class='c007'>The various attempts made, prior to 1782, to measure the
+parallaxes of some of the brighter stars were instigated by
+the wish to find a demonstrative argument in favour of the
+Copernican theory of our system. They had no reference to
+sidereal structure. Herschel, however, took up the subject
+simply for the purpose of fixing the scale of that vast edifice.
+Before sounding the skies, he sought to ascertain the length
+of his fathom-line. He never ascertained it. To the end of
+his life, he could only make plausible assumptions as to the
+distances of the stars. Their real parallaxes were insensible
+with his instrumental means. But he fortunately chose for
+his experiments Galileo’s “double-star method.” This consisted
+in determining the relative positions of two close stars,
+one of which, taken to be indefinitely remote, was designed to
+serve as a standard of reference for the perspective shiftings
+of the other. It was thus that Herschel’s attention was
+directed to double stars. He found them to be astonishingly
+numerous—far more numerous than could have been anticipated
+by the doctrine of probabilities. In January, 1782, he
+presented to the Royal Society a catalogue of 269 star-pairs,
+and he had collected 434 more by December, 1784. From
+their abundance alone, the Rev. John Michell inferred their
+character of binary systems; and Herschel, after twenty years
+<span class='pageno' id='Page_29'>29</span>of observation, was able, in 1802, to announce the fact of their
+mutual revolutions. Thus was taken the second great step
+towards the unification of the Cosmos. Newton proved that
+terrestrial gravity dominates the solar system; Herschel
+showed that a law of attraction, presumably (and assuredly)
+identical in its mode of operation, extends through sidereal
+space.</p>
+
+<p class='c007'>One cannot reflect without amazement that the special
+life-task set himself by this struggling musician—originally a
+penniless deserter from the Hanoverian Guard—was nothing
+less than to search out the “construction of the heavens.”
+He did not accomplish it, for that was impossible; but he
+never relinquished, and, in grappling with it, laid deep and
+sure the foundations of sidereal science. No one before him
+had thought of approaching the subject otherwise than by
+way of speculation; he alone had the boldness to attack it
+experimentally. Having invented for the purpose an ingenious
+method of “star-gauging,” based upon the hypothesis
+that the stars are, on an average, scattered evenly through
+space, he concluded in 1784, from its application, that the
+Milky Way is the visual projection of a disc-shaped stellar
+aggregation, within which our sun is somewhat excentrically
+placed. The progress, however, of his telescopic studies convinced
+him that the continued action of a “clustering power”
+had long ago drawn the stars into many separate allotments,
+and annulled the original uniformity of their distribution. So
+the disc theory was given up, and the Milky Way came to be
+regarded as a collection of genuine clusters, arranged into an
+irregular ring encircling the solar system. This view, implicitly
+held by the elder Herschel from 1802, was explicitly
+stated by his son in 1847. The results that Herschel expected
+from star-gauging may, in the future, be derived from
+the more elaborate process of star-gauging by magnitudes,
+photographically executed; and the sky-charting work,
+rapidly progressing in all parts of the world, will at least
+supply ample materials for sounding the star-depths.</p>
+
+<p class='c007'>These are stored besides with the curious objects called
+<span class='pageno' id='Page_30'>30</span>“nebulæ.” They were little noticed until Herschel, on
+March 4, 1774, made</p>
+
+<div class='lg-container-b c015'>
+  <div class='linegroup'>
+    <div class='group'>
+      <div class='line in8'>“That marvellous round of milky light</div>
+      <div class='line'>Below Orion,”</div>
+    </div>
+  </div>
+</div>
+
+<p class='c016'>the subject of his earliest recorded observation. Except, indeed,
+as impediments to comet-hunting. Thus, Messier, one of the
+keenest sportsmen in that line who have ever scanned the
+sphere, tried to eliminate by enumerating them, and drew up
+in 1771 a list of 45 such misleading objects, enlarged in 1781
+to 103. And Lacaille, during an expedition to the Cape in
+1752–1755, picked up 42 more. So far this department of
+knowledge had been cultivated when Herschel began to
+“sweep the heavens.” To <i>sweep</i> them, be it remembered.
+Not merely to gaze at hap-hazard, or to look out for show
+specimens, but to gather in the celestial harvest methodically,
+zone by zone, so as to “leave no spot of the heavens unvisited.”
+The fruits were proportioned to his diligence. The
+nebulæ discovered by him amounted, in 1802, to 2,500. And
+he did not merely discover; he investigated them as well.
+He separated them into classes, noted the mode of their distribution,
+and searched out their relationships. To begin
+with, he believed them to be of a purely stellar nature—to be,
+in fact, independent galaxies. Miss Burney was informed by
+him in 1786 that he had “discovered fifteen hundred universes.”
+A few years later, however, he reasoned out for himself the
+gaseous nature of a great many nebulæ, such as that in Orion,
+and those of the “planetary” sort; and published in 1811 a
+complete theory, strikingly illustrated with examples taken
+from his telescopic experiences, of stellar development out of
+nebulous stuff. The supposition that they included the revelation
+of “exterior universes” was thus rendered, to say the
+least, superfluous; yet it was not perhaps, even by him,
+wholly abandoned. It was, moreover, revived in consequence
+of the performances of the great Rosse reflector, from 1845
+onwards, in resolving apparent nebulæ into “bee-like swarms”
+of stars. Meanwhile Sir John Herschel’s examination of
+<span class='pageno' id='Page_31'>31</span>those wonders of the southern heavens, the Magellanic Clouds,
+had virtually decided nebular standing. For they contain
+within a limited compass, as Dr. Whewell argued in 1853,
+“stars, clusters of stars, nebulæ, regular and irregular, and
+nebulous streaks and patches. These, then, are different
+kinds of things in themselves, not merely different to us.”
+That stars and nebulæ co-exist in every part of the heavens,
+has since been fully established; while the laws respectively
+governing their distribution over the sphere are related in
+such a manner as to leave no doubt that these two classes of
+sidereal objects unite to form the grand galactic whole.
+Hence, to all reasonable apprehension, “island universes”
+have vanished into the inane.</p>
+
+<p class='c007'>Sir John Herschel accomplished the unparalleled feat of
+sweeping the heavens from pole to pole. Having, within eight
+years from 1825, revised his father’s work at Slough, he conceived
+the noble idea of rounding it off in the southern hemisphere;
+and, in 1833–4, transported his instruments from
+Slough to Feldhausen near Cape Town. During the four
+years of his residence there, he not only executed his proposed
+survey, registering 1,790 nebulæ—300 of them for the first
+time—and discovering and measuring 2,100 double stars, but
+carried out a number of special researches. He catalogued
+the miscellaneous contents of the Magellanic Clouds—systems
+<i>sui generis</i>, as he justly termed them—made a detailed and
+laborious study of the Argo nebula, applied pretty extensively
+the paternal method of star-gauging, observed Halley’s comet
+at its second predicted return, measured the sun’s heat-emissions,
+carefully watched the spot-maximum of 1837, and
+finally, struck with a sudden rise in magnitude of η Argûs,
+brought to general knowledge that star’s extraordinary character.
+These varied results were embodied in a monumental
+volume, published in 1847.</p>
+
+<p class='c007'>One of the greatest triumphs of modern science has been
+the establishment of an “Astronomy of the Invisible.” It was
+primarily due to Bessel’s inquiries into the disturbed proper
+motions of the “Dog-stars,” Sirius and Procyon. They convinced
+<span class='pageno' id='Page_32'>32</span>him that each of these brilliant orbs is attended by a
+massive satellite, round which it revolves as it advances, its
+path in the sky being thus not straight but wavy. Telescopic
+verification of his forecast was, nevertheless, delayed
+until 1862 in the case of Sirius, until 1896 as regards Procyon.
+The earliest, and still the most memorable result in this line
+is the discovery of Neptune. Bessel knew that the thing was
+to be done, and in 1840 planned the doing of it. But his powers
+began, soon afterwards, to be crippled by deadly illness, to
+which he succumbed, March 17, 1846. <i>Uno avulso, non deficit
+alter.</i> Adams and Leverrier separately undertook the enterprise
+he had relinquished, and each with perfect success. It
+was a formidable one. The <i>direct</i> problem of perturbations
+taxes the highest mathematical resources; the <i>inverse</i> problem
+is not only more arduous, but was then untried. Laplace
+and Lagrange had shown how to determine the perturbations
+produced by a known disturbing body; it was left for Adams
+and Leverrier to find an unknown body through its disturbing
+effects. Irregularities in the movements of Uranus betrayed
+the presence of Neptune, and by the powerful analysis
+brought to bear upon them, were made to serve as an index to
+his actual place in the heavens at a given epoch. This was
+done by Adams in September, 1845; but his calculations, deposited
+at the Royal Observatory in the hope that they would
+incite to a telescopic search for the new planet, remained there
+buried in a drawer. Sir George Airy had no faith in them,
+and he unaccountably received no reply to a test-question addressed
+to their author. In the following June, however, he
+was roused by the intelligence of Leverrier’s advance towards
+the goal already attained by Adams, to arrange an exploratory
+campaign with the Cambridge “Northumberland
+equatorial.” But here again, disbelief—reinforced by the
+absence of a detailed star-map—stepped in to retard proceedings
+conducted by Professor Challis in so leisurely a fashion
+that the object “wanted” was found before he had sifted
+his observations, September 23, 1846, by Galle of Berlin, acting
+under Leverrier’s precise directions. It proved on inquiry
+<span class='pageno' id='Page_33'>33</span>to have been twice observed at Cambridge during the previous
+couple of months.</p>
+
+<p class='c007'>Gravitational astronomy won its crowning distinction by
+the discovery of Neptune. It afforded the first instance of a
+body made known as an unseen power previously to being
+visually detected. Many stellar systems, however, have since
+then been ascertained to include members which can only be
+<i>felt</i>, owing to their partial, if not total obscurity. Again, the
+spectroscope tells of the existence of others entirely beyond
+the range of direct vision with the most powerful optical
+appliances; not because they do not shine (although this is
+sometimes also the case), but because they revolve so close to
+their primaries as to form with them single and indissoluble
+telescopic objects.</p>
+
+<p class='c007'>The spectroscope and the photographic camera have been
+mentioned as aids to astronomy. Their adoption has profoundly
+modified the science, widening its borders, inviting it
+to undertake novel tasks, endowing it with previously
+undreamt-of powers. Realms of knowledge deemed inaccessible
+to human faculties have, as if at the touch of a magician’s
+wand, been thrown open; and of the many paths leading into
+the interior, only a few have yet been pursued, and that for a
+short distance. The prospects of exploration are hence
+unlimited, and of bewildering variety.</p>
+
+<p class='c007'>Spectrum analysis is essentially a chemical method. It
+depends upon the principle firmly established in 1859 by
+Kirchhoff and Bunsen, two professors at the university of
+Heidelberg, that different kinds of glowing vapour give out
+distinctive rays of variously coloured light, commonly called
+“lines,” simply because, for the purpose of getting rid of overlapping
+images, and for convenience of measurement, they are
+transmitted through a narrow slit. Thus, the presence of a
+familiar, and almost ubiquitous deep-yellow line, named by
+Fraunhofer “D,” and shown by a moderately powerful
+apparatus to be double, <i>infallibly</i> testifies to the presence of
+sodium; iron, rendered gaseous by heat, gives out several
+thousand lines ranging from end to end of the spectrum, not
+<span class='pageno' id='Page_34'>34</span>one of which is common to any other substance; hydrogen
+shows a radiant sequence exclusively its own; and so of all
+the remaining elements. To apply this mode of detection,
+the light from the source to be studied must be analysed, or
+dispersed into its various component colours through the
+unequal action upon them of a prism, or train of prisms.
+Dispersion can also be effected by “diffraction”; and since
+the spectrum thus produced is “normal,” or dependent wholly
+upon wave-length, it is always employed where a high degree
+of exactitude is aimed at. The coloured fringes of shadows
+originate in this way, through the interference of ethereal undulations;
+while the rainbow is a prismatic phenomenon,
+drops of water performing the refractive office of actual
+prisms.</p>
+
+<p class='c007'>The rainbow exemplifies too—although less perfectly than
+the electric light—what is called a “continuous spectrum.”
+Its tints merge one into the other insensibly, without any
+sensible dark interruption. Now, incandescent liquids and
+solids of every kind and quality give rainbow-like spectra;
+they emit light which <i>rolls out</i> into an unbroken band of
+colour. Hence there is nothing characteristic about them.
+They are to the chemical enquirer absolutely uncommunicative.
+Vapours and gases alone can be induced to show
+the <i>badge</i> of their particular nature.</p>
+
+<p class='c007'>Celestial spectrum analysis began with the sun. The solar
+spectrum is furrowed transversely by a multitude of fine dark
+lines, known as “Fraunhofer lines,” because Fraunhofer brought
+them within scientific cognisance by carefully mapping and
+measuring them. Their significance remained a standing
+puzzle until Kirchhoff, in 1859, furnished the key to it, by
+demonstrating the correlation of radiation and absorption. In
+other words, vapours and gases have the faculty of arresting
+those precise rays of light which they are in a condition to
+emit. Hence, the ignited, although relatively cool vaporous
+envelope of a white-hot body like the sun, or the carbons of
+the electric arc, acts predominantly as an intercepting medium,
+stopping more than it sends out of its peculiar rays. There
+<span class='pageno' id='Page_35'>35</span>results a continuous spectrum crossed by dark lines of the
+same chemical significance as if they were bright. They
+would, in fact, show as bright if the brilliant background,
+upon which they are seen projected, could be withdrawn.
+The interpretation, upon this principle, of the Fraunhofer
+lines, proved the sun to be surrounded by hydrogen in vast
+quantities, by incandescent sodium, magnesium, iron, calcium,
+and a number of other metals. Spectrum analysis in this
+way assumed a double aspect. The hieroglyphics of coloured
+light were rendered legible, whether positively or negatively
+written. And the spectra of the heavenly bodies are actually
+found to be inscribed, some in one way, some in the other;
+not unfrequently, in both combined.</p>
+
+<p class='c007'>The new and marvellous power of investigation thus acquired
+was in 1864 applied to the stars by Dr. Huggins and
+his coadjutor, Professor W. A. Miller. They ascertained the
+presence in the atmospheres of Aldebaran and Betelgeuse, of
+nine or ten terrestrial elements, thereby setting on foot the
+science of stellar chemistry. Moreover, on August 29, in the
+same year, Dr. Huggins made the signal discovery of gaseous
+nebulæ. Admitting the dim rays of a “planetary” in Draco
+through the slit of his spectroscope, he perceived it to be composed
+of three bright green lines, one of them Fraunhofer’s
+“F”—an emanation of hydrogen. This one observation
+verified after seventy-three years Herschel’s inference of the
+existence in the heavens of a “fiery haze,” destined, according
+to his long forecast of creative processes, eventually to
+“subside into stars.”</p>
+
+<p class='c007'>By the discovery of celestial spectrum analysis, a third
+stadium of progress towards the unification of the sciences
+was reached. The first step was taken with the demonstration
+that the force retaining the planets in their orbits is no
+other than that which causes rivers to flow, and apples to fall
+upon the earth. The extension of the same law to the stellar
+universe through the discovery of binary stars, showing that
+matter, wherever existing, possesses at least one unchanging
+quality, constituted the second. It was now learned that the
+<span class='pageno' id='Page_36'>36</span>sun and stars were composed of the identical <i>species</i> of matter
+scattered in the dust of the earth, dug up from its bowels, condensed
+to make its oceans, entering into the very framework
+of our own bodies. An universal chemistry was established,
+based upon the relations of light to material molecules, and of
+material molecules to the ether filling space; and, as an inevitable
+consequence, the new branch of knowledge, termed
+“astrophysics,” made its ardently welcomed advent. By it
+astronomy has entered into close alliance with the rest of the
+sciences. No laboratory experiment is any longer indifferent
+to her; and laboratory experiments, on the other hand, derive
+from the connexion vastly augmented importance. The youth
+of learning seems renewed. Secrets of nature, formerly believed
+to lie beyond the scope of investigation, have been
+penetrated; <i>nil desperandum</i> is the motto which astro-physicists
+have earned the title to adopt as their own.</p>
+
+<p class='c007'>The old art of direct observation has, during the latter half
+of the present century, developed in sundry novel directions.
+By the use of auxiliary appliances, the telescope has gained a
+wonderful increase of subtlety and power. Modern astronomical
+work may be divided into four classes:—telescopic,
+spectroscopic, photographic, and spectrographic or spectrophotographic.
+Daguerre’s invention was almost immediately
+tried with the sun and moon; J. W. Draper and the two Bonds
+in America, Foucault and Fizeau in France, and Warren de
+la Rue in this country, being among the pioneers of celestial
+photography. But it was not until after the introduction of
+the collodion process that really useful results were obtained.
+With the regular employment at Kew, from 1858 onwards,
+of De la Rue’s “photoheliograph,” began the daily selfregistration
+of sun-spots, suggested by Sir John Herschel in
+1847; and pictures of the eclipsed sun, obtained with the same
+instrument at Rivabellosa in Spain, July 18, 1860, terminated
+a prolonged dispute as to the nature of the red prominences
+by exhibiting them as undeniably solar appendages.
+Lunar photography was meanwhile successfully
+prosecuted, and Henry Draper’s picture, of September 3,
+<span class='pageno' id='Page_37'>37</span>1863, remained unsurpassed for a quarter of a century.
+Star-prints were first secured at Harvard College, under the
+direction of W. C. Bond in 1850; and his son, G. P. Bond,
+made, in 1857, a most promising start with double-star
+measurements on sensitive plates, his subject being the well-known
+pair in the Tail of the Great Bear. The competence
+of the new method to meet the stringent requirements of
+exact astronomy was still more decisively shown in 1866 by
+Dr. Gould’s determination from his plates of nearly fifty stars
+in the Pleiades. Their comparison with Bessel’s places for
+the same objects proved that the lapse of a score of years had
+made no sensible difference in the configuration of that immemorial
+cluster; and Professor Jacoby’s recent measures of
+Rutherfurd’s photographs, taken in 1872 and 1874, enforced
+the same conclusion. To the “collodion period” also belongs
+the earliest spectrograph, taken by Dr. Huggins in 1863; but
+the analysed light of Sirius left an uncharacteristic, although a
+strong impression. No lines were visible in it; a “virgin
+page” was presented. Before prosecuting the subject, fresh
+developments had to be awaited.</p>
+
+<p class='c007'>The invention of gelatine dry plates was the decisive event
+in the history of celestial photography. Dr. Huggins turned
+it to account with marked success for depicting the spectrum
+of Vega, December 21, 1876, and was able, three years later,
+to exhibit to the Royal Society photographs of the spectra of
+six white, or Sirian stars, stamped with the ultra-violet series
+of hydrogen lines, then for the first time recognised, whether
+on the earth, or in the sky. The uses of the camera have
+since then multiplied at a prodigious rate. Its versatility
+appears unbounded. There are very few departments of
+astronomy left in which the eye has the advantage over it.
+A volume might be written on its successes; its comparative
+failures would scarcely fill a page. Its extraordinary power
+of penetrating space would have amazed and delighted
+William Herschel. This is due to the indefinitely prolonged
+exposures rendered practicable by the employment of dry
+plates; and these exposures can be interrupted and resumed
+<span class='pageno' id='Page_38'>38</span>at pleasure. Three-night photographs are now quite commonly
+taken, following the example given by Dr. Roberts
+in 1889. Now every additional minute of exposure brings
+intelligence from further and further sky-depths, owing
+to the happy faculty of sensitive plates for accumulating
+impressions. The eye sees at once, or not at all; the
+chemical retina sees by degrees, storing up insensible effects
+until they become sensible, and this without definable limit.
+This is its most essential prerogative. For the portrayal of
+nebulæ and comets, it is inestimable; and by its means
+the boundaries of the sidereal system may be laid down
+before the twentieth century is far on its way. A picture of
+the great comet of 1882, standing out from a richly spangled
+background, taken at the Cape Observatory under Dr. Gill’s
+direction, was the object-lesson by which the advantages of
+photographic star-charting were effectually learnt. They have
+been practically illustrated in the <cite>Cape Durchmusterung</cite>, a
+southern continuation, by photographic means, of Argelander’s
+corresponding telescopic work at Bonn; and are being turned
+to account on a magnified scale, in the International Survey of
+the heavens, now in progress at seventeen observatories scattered
+over the face of the globe. Special problems have,
+meanwhile, been investigated with striking success, by the
+chemical method, and its fresh applications are innumerable.
+Hitherto, performance has usually outrun promise; but promise
+has now so quickened its pace as to make the issue of
+the race dubious. We can only be sure that the future will
+be full of surprises.</p>
+
+<div><span class='pageno' id='Page_39'>39</span></div>
+<div class='chapter ph1'>
+
+<div class='nf-center-c0'>
+<div class='nf-center c001'>
+    <div>ASTRONOMY</div>
+  </div>
+</div>
+
+<span class='pageno' id='Page_40'>40</span>
+
+</div>
+
+<div id='i_040'  class='figcenter id001'>
+<img src='images/i_040.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p>THE LICK REFRACTOR OF THIRTY-SIX INCHES APERTURE.</p>
+</div>
+</div>
+
+<div class='chapter'>
+  <span class='pageno' id='Page_41'>41</span>
+  <h2 class='c006'>SECTION II.—GEOMETRICAL ASTRONOMY AND ASTRONOMICAL INSTRUMENTS.</h2>
+</div>
+
+<div class='nf-center-c0'>
+<div class='nf-center c002'>
+    <div><span class='xlarge'><span class='sc'>By A. Fowler</span>, A.R.C.S., F.R.A.S.</span></div>
+  </div>
+</div>
+
+<h3 class='c013'>CHAPTER I.<br> <span class='c014'>THE EARTH AND ITS ROTATION.</span></h3>
+
+<p class='c003'>It is a common remark that we are creatures of circumstances,
+and in no sense is this truer than in its application to the
+conditions under which we view the heavenly bodies. At the
+commencement of a study of astronomy it is accordingly
+important to first ascertain as far as possible the nature of the
+earth on which we are situated, and to determine in what
+way our observations are affected by our local conditions.</p>
+
+<p class='c007'><span class='sc'>The Horizon.</span>—When we look at the sky we see a vast
+hemispherical vault of which we seem to occupy the centre.
+If we are at sea, the water and sky appear to meet at a
+certain distance, in whatever direction we look. Where these
+meet we have what is called the visible horizon. On land,
+the horizon is usually broken up by terrestrial objects, such
+as hills, buildings, or vegetation, but otherwise the appearances
+are the same as at sea.</p>
+
+<p class='c007'><span class='sc'>Shape of THE Earth.</span>—When we observe the horizon,
+whether from land or sea, our eyes are at a certain elevation
+above the level of the ground or water, as the case may be,
+and the higher we are situated, the greater is the distance of
+the visible horizon, although the circular outline is retained.
+No matter where we may be, the same appearances are noted,
+<span class='pageno' id='Page_42'>42</span>and we are thus led to infer that the earth is a globe, as no
+other shape could appear circular from all points of view.</p>
+
+<p class='c007'>There are other considerations which lead to the same
+conclusion with regard to the shape of the earth. One of the
+most familiar proofs that the earth cannot be flat is found in
+the aspects of a ship putting out to sea or coming into port,
+when observed from a somewhat elevated position on shore.
+A ship does not become visible in its entirety, as it would if
+diminishing distance were the only cause affecting its visibility;
+the masts are seen first, and then the lower parts of
+the vessel gradually make their appearance. This finds a
+simple explanation in the curvature of the surface of the sea,
+and as similar appearances can be seen in all parts of the
+world, a globular form is indicated.</p>
+
+<p class='c007'>The fact that one may continue to travel westward and yet
+return to the point of starting, is quite in harmony with the
+supposition that the earth is globular, but it does not furnish
+a proof. This facility would evidently be equally afforded by
+a cylindrical earth, or even by a flat earth of which the Pole
+occupied the centre.</p>
+
+<p class='c007'>Still another indication of the rotundity of the earth is given
+by the phenomena of an eclipse of the moon. On these
+occasions, as will appear later, the moon passes through the
+shadow of the earth, and as this shadow is always circular,
+nothing but a spherical, or nearly spherical, body can be in
+question.</p>
+
+<p class='c007'><span class='sc'>Size of the Earth Roughly Measured.</span>—Granting
+then that the earth is spherical, a measurement of its curvature
+will enable us to determine its size. To do this it is necessary
+to measure the distance of the visible horizon from the eye at
+a known elevation. Then it can be shown that if the height
+of the eye is only a small fraction of the diameter of the
+earth, the diameter is as many times larger than the distance
+of the horizon as that distance is greater than the height of
+the eye. Thus, to an observer whose eye is 5 feet above sea
+level, the horizon is 2¾ miles distant, while from the top of a
+lighthouse 66 feet high the sky would appear to meet the sea
+<span class='pageno' id='Page_43'>43</span>at a distance of 10 miles. One way in which an approximate
+measurement may be made is illustrated in Fig. 1. Three posts
+are placed in line, with their tops at the same height above
+the surface of some calm stretch of water such as is afforded
+by a canal. A telescope fixed to the first post, so that its
+centre is at the top, is directed to the upper end of the third
+post, and it is seen to sight the middle one at some distance
+from the top. When the posts are a mile apart, the line joining
+the two extremes turns out to be 8 inches below the top
+of the middle one.</p>
+
+<div id='i_043'  class='figcenter id003'>
+<img src='images/i_043.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 1.</span>—<i>Rough Measurement of Earth’s Diameter.</i></p>
+</div>
+</div>
+
+<p class='c007'>In our diagram this 8 inches is represented by the distance
+<i>b d</i>, and if we imagine an arc of a circle <i>d e</i> concentric with
+the surface of the water, the part which it intersects on the
+end post, namely <i>a e</i>, will also be 8 inches. This means that
+to an eye at <i>a</i>, 8 inches above the surface represented by <i>d e</i>,
+the visible horizon at <i>d</i> would be a mile distant. Applying
+the proportion named above, it results that the earth is 7,920
+miles in diameter.</p>
+
+<p class='c007'>Owing to various causes, this method only furnishes a rough
+indication of the dimensions of our globe; but, if we had no
+other evidence, the result would suffice to explain that the
+irregularities of the earth’s surface, though seeming so large
+to us who dwell upon it, are not inconsistent with the idea
+that the surface forms part of a sphere. The highest mountains
+with which we are acquainted do not exceed 5½ miles in
+height, and this is only ¹⁄₁₄₀₀th part of the earth’s diameter.
+On a globe 14 inches in diameter, representing the earth, the
+highest mountains would be less than a hundredth of an inch
+<span class='pageno' id='Page_44'>44</span>on the same scale; so that, taking the earth generally, it is
+practically a smooth globe.</p>
+
+<p class='c007'><span class='sc'>Different Horizons at Different Places.</span>—So far
+then we have learned that the earth is a globe about 8,000
+miles in diameter. This enables us to understand that
+persons in different parts of the earth will see the sky in
+different ways. At any given place we can see only what is
+above our horizon, and it results from the spherical form of
+the earth that no two observers have precisely the same
+horizon. If we consider
+a section of
+the earth, such as
+is shown in Fig. 2,
+an observer at the
+point <i>a</i> will have
+a horizon represented
+in section
+by the line <i>b c</i>,
+while the horizon
+of an observer at
+<i>d</i> will be represented
+be <i>e f</i>. It
+is clear then that an external distant object, such as the sun
+or a star, which may appear on the horizon in the direction
+<i>a b</i>, as seen from the point <i>a</i>, will be at a considerable angle
+above the horizon when seen from the point <i>d</i>.</p>
+
+<div id='i_044'  class='figcenter id001'>
+<img src='images/i_044.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 2.</span>—<i>Horizons at Two Places on the Earth.</i></p>
+</div>
+</div>
+
+<p class='c007'><span class='sc'>Sensible and Rational Horizon.</span>—Having this conception
+of the horizon as a thing terrestrial, we may consider
+its astronomical relationships a little further. If we imagine
+the plane of the horizon prolonged until it cuts the distant
+sphere on which the stars and other celestial bodies seem to
+lie, it will meet that sphere in what is called the <i>sensible
+horizon</i>. A parallel plane passing through the centre of the
+earth is called the <i>rational horizon</i>, but as the starry sphere is
+at an almost infinite distance, the rational and sensible
+horizons coalesce into one celestial horizon.</p>
+
+<p class='c007'>Closely associated with the horizon is the point vertically
+<span class='pageno' id='Page_45'>45</span>overhead which is called the <i>zenith</i>, and the point vertically
+below which is called the <i>nadir</i>. As the plane of the horizon
+is tangential to the earth’s surface at the point of observation,
+the zenith is simply the prolongation into space of the line
+joining the centre of the earth with the place of observation;
+at the point <i>a</i> in Fig. 2, for example, the zenith is in the
+direction <i>o a z</i>.</p>
+
+<p class='c007'>The zenith as thus defined, however, is not the astronomical
+zenith, but what is called the geocentric zenith. As will appear
+later, the earth is not truly spherical, so that the direction
+of gravity does not pass exactly through the earth’s
+centre, and the astronomical zenith is overhead in the direction
+of gravity.</p>
+
+<p class='c007'><span class='sc'>Diurnal Motion of the Heavens.</span>—In the day-time,
+when the sky is clear, we see the sun; at night, we sometimes
+see the moon, always some stars, and occasionally a comet.
+If we continue our observations, even for a few hours, we
+begin to recognise that the heavenly bodies have an apparent
+movement towards the west, very similar to the daily motion
+of the sun, with which everyone must have been familiar from
+childhood.</p>
+
+<p class='c007'>Continuing such observations, it is found that the great
+majority of the stars do not appear to change their positions
+relatively to each other, although their apparent places in the
+sky are different at different times. These have consequently
+been called the “fixed stars,” but in the light of our present
+knowledge, the name is not to be taken too strictly. On
+account of this seeming fixity, the stars have been divided
+from very remote times into <i>constellations</i>, or groups, which
+enable us to name and identify individual members of the
+starry host. Other bright objects having the appearance of
+stars, when they are viewed merely by the naked eye, may be
+seen to change their positions with regard to the stars in that
+part of the sky in which they appear. These are the <i>planets</i>—the
+“wandering stars” of the ancients, to whom five were
+known, namely, Mercury, Venus, Mars, Jupiter, and Saturn.</p>
+
+<p class='c007'>Comets also are seen to share in the general westward
+<span class='pageno' id='Page_46'>46</span>movement of the heavenly bodies, but, in addition, they have
+another movement relative to the stars situated in the same
+part of the sky.</p>
+
+<p class='c007'>If we closely observe the stars in Europe, we shall find
+some of them rising due east, and setting due west; others,
+again, will be found to rise in the north-east, and to travel
+nearly overhead; still others will be seen to rise south of
+east, attain only a small elevation above the horizon, and pass
+from our view as far south of west as they rise south of east.
+One point in the heavens appears stationary, and all the stars
+seem to traverse their daily courses round this as a centre.
+This stationary point is the north <i>celestial pole</i>. It is marked
+by no star, but a fairly conspicuous star is at present only
+about a degree and a half removed from it The name given
+to this star is the Pole Star, or Polaris. As seen from London,
+stars within 51½° from the celestial pole never set, and such
+stars are said to be <i>circumpolar</i>.</p>
+
+<p class='c007'>When our place of observation is changed from one of
+middle latitude to one very near the Equator, these appearances
+are modified. We still see the stars rising and setting
+daily, but there will be <i>two</i> points which do not seem to
+move, one on the northern and the other on the southern
+horizon. One of these stationary points is identical with that
+seen from higher latitudes, and the other, which is called the
+<i>south celestial pole</i>, is diametrically opposite to it What is
+more, stars which were not visible at all at our first place of
+observation will be seen in the south. All the stars will rise
+and set, and will alike be above the horizon for twelve hours.</p>
+
+<p class='c007'>If we could see the stars from the North Pole, the Pole Star,
+which is on the horizon of places at the Equator, would be
+found overhead, and all the stars visible to us would be ever
+above the horizon. Not only this, not one of the glittering stars
+which adorns the southern heavens would ever be seen at all.</p>
+
+<p class='c007'>In place of the rising and setting of stars, which lends such
+a great interest to their observation in other parts of the
+world, as seen from the poles the stars will simply travel round
+and round in circles parallel to the horizon.</p>
+
+<p class='c007'><span class='pageno' id='Page_47'>47</span>To produce the apparent daily revolution of the heavens,
+and the changes in the appearances observed at different
+places, one of two causes must be at work; either the celestial
+bodies themselves must be performing a daily majestic movement
+from east to west round a motionless earth, or the earth
+itself must be whirling round from west to east, and so changing
+the situation of the observer’s horizon with regard to
+external bodies. In the early days of astronomical observations
+this observed revolution of the heavens was thought to
+be real, but, with our present knowledge, we are no longer
+justified in regarding the earth as occupying a place of any
+such importance as that of the centre of the universe. By
+the earth’s rotation, an observer, unless situated exactly at
+the North or South Pole, is carried round in a circle, and his
+horizon is gradually swept round so that on one side stars are
+setting and on the other side rising. The appearances at
+different places find a simple and sufficient explanation in
+the varying inclination of the observer’s horizon to the
+earth’s axis of rotation as the place of observation is
+changed.</p>
+
+<p class='c007'>A very simple experiment will assist one to comprehend
+the varying position of the horizon in different latitudes, and
+its effect upon the apparent diurnal movement of the heavens.
+Through the middle of an orange pass a knitting-needle, so
+that the two together may be taken to represent the earth
+and its axis. A circular piece of thin card pushed on to the
+needle at one end will represent the polar horizon, and, if the
+orange be rotated, it will be at once realised that such movement
+produces no change in the plane of this horizon,
+although different points on the visible horizon will be
+successively brought in line with different groups of stars or
+other external bodies.</p>
+
+<p class='c007'>Another piece of card should next be fixed on the orange
+by means of a pin at a point corresponding to the Equator.
+Again spinning the model earth on its axis, this horizon will
+be seen to constantly change its plane with regard to outside
+objects, and in a manner which perfectly accounts for the
+<span class='pageno' id='Page_48'>48</span>apparent movement of the heavens as observed from a point
+on the Equator.</p>
+
+<p class='c007'>A third piece of card touching the surface of the orange at
+an intermediate place will have an oblique movement, and as
+referred to this plane, the stars appear to traverse their daily
+rounds in oblique circles.</p>
+
+<p class='c007'><span class='sc'>Experimental Proofs of Rotation.</span>—Not only does
+a supposed rotation of the earth accord perfectly with all that
+we can glean from observations of the heavens, but actual
+demonstrations of the reality of this movement are forthcoming.
+Sir Isaac Newton suggested one experimental
+method of setting the matter at rest. The further a thing is
+removed from the centre of the earth, the greater is the circle
+which it describes in a day, and the greater, consequently, the
+speed with which it must travel. Thus the top of a high
+tower moves more quickly than its base, and the surface of a
+mine than the bottom of the shaft. A stone let fall from the
+top of a tower thus starts with a greater forward velocity than
+that of objects at the base, and when it reaches the earth’s
+surface, it will be a little east of the point where a plumb-line
+let down from its starting-point reaches the surface. This
+experiment has been tried, but there are so many disturbing
+causes affecting the movement of the falling stone that the
+results are not very satisfactory, although generally confirming
+the earths rotation from west to east. Evidently this
+method would fail at the Pole, and would be most effective at
+the Equator.</p>
+
+<p class='c007'>A much more beautiful and perfect proof is furnished by
+the celebrated Foucault’s pendulum experiment. Again
+fancying ourselves at the North Pole, let us imagine a long
+and heavy pendulum, suspended in such a manner that the
+plane in which it swings is not affected by the earth’s rotation.
+The trace of such a pendulum on a bed of sand placed
+beneath it would remain in a constant position if the earth
+were at rest. As the earth rotates, the bed of sand is twisted
+round, and the path of the pendulum apparently changes.
+The experiment was first actually carried out by Foucault in
+<span class='pageno' id='Page_49'>49</span>1851, at the Pantheon in Paris, and it created a widespread
+interest. Since then, pendulums have been erected in various
+parts of the world, and all agree in essential results. The
+experiment can be seen in actual operation in the science
+section of the South Kensington Museum. The pendulum
+bob is a very heavy one, and before commencing the experiment,
+it is held out of the vertical by a loose band, which is
+fixed to the wall by a piece of string. On burning the string,
+the band falls off, and the pendulum starts its swing with little
+or no movement out of a plane. The pendulum bob is suspended
+by a long piano wire which is attached to a bracket
+carrying a conical pivot. The pivot rests on an agate plate
+at the end of a
+beam, and the
+weight of the
+bracket is compensated
+by
+an adjustable
+weight (Fig. 3).
+When swinging,
+the pendulum
+has a
+constant tendency
+to remain
+in one plane, and
+the turning of
+the beam beneath
+the pivot
+has no effect on
+the absolute
+direction of the
+plane of swing.
+Beneath the pendulum
+is a table
+divided into degrees, and the hourly apparent movement of
+the plane of swing at Kensington is observed to be nearly 12°.</p>
+
+<div id='i_049'  class='figcenter id001'>
+<img src='images/i_049.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 3.</span>—<i>Foucault’s Pendulum Experiment.</i></p>
+</div>
+</div>
+
+<p class='c007'>If the experiment could be performed at the North Pole, the
+<span class='pageno' id='Page_50'>50</span>pendulum plane would apparently rotate from east to west,
+making a complete rotation once a day. At the South Pole
+the direction of movement would be reversed, but the rate
+would be the same as at the North Pole. The experiment,
+however, fails altogether at the Equator, while at places between
+the Poles and Equator the rate of movement varies
+with the latitude.</p>
+
+<p class='c007'>A more compact piece of apparatus for demonstrating the
+earth’s rotation is the gyroscope, which we also owe to
+Foucault’s ingenuity. The principle is exactly the same as in
+the case of the pendulum. A heavy disc is set in very rapid
+rotation, and is suspended in such a way that its points of
+support may be turned round without disturbing its plane of
+rotation. The results obtained with this instrument substantiate
+those derived from pendulums.</p>
+
+<p class='c007'>These experimental proofs of the rotation of the earth
+further teach us the same fact that we learn from observations
+of the stars, namely, that the earth makes a complete turn
+on its axis once a day.</p>
+
+<p class='c007'><span class='sc'>Latitude and Longitude.</span>—Having thus arrived at the
+conclusion that the earth is a globe turning on an axis once
+in twenty-four hours, the <i>North and South Poles</i> may be defined
+as the points where the axis of rotation meets the surface,
+while the <i>Equator</i> is the circle passing through places midway
+between the Poles. Imaginary circles passing round the earth
+through the Poles are called <i>meridians</i>, while circles parallel to
+the Equator are called <i>parallels</i>. These conceptions enable us
+to define very precisely the situation of any particular place
+upon the terrestrial sphere. We measure its angular distance
+from the Equator, as seen from the centre of the earth, and
+call this its <i>latitude</i>; London, for instance, is 51½° north of the
+Equator, and this is abbreviated to lat. 51½°N. All places on
+the same parallel have the same latitude, so that another
+measurement is required to designate the exact location of
+any one place. For this purpose the meridian passing
+through some place is agreed upon as a start-point, and we
+can then say that the place in question is so many degrees
+<span class='pageno' id='Page_51'>51</span>east or west; such a measurement represents the <i>longitude</i> of
+the place. At present there is no universal agreement as to
+the initial meridian, but in all British maps the meridian
+passing through the centre of the transit instrument at the
+Royal Observatory, Greenwich, is taken as the start-point.
+Longitudes are reckoned up to 180° E. and 180° W. New
+York, for example, is in long. 73° 58′ W., and Berlin in long.
+13° 24′ E.</p>
+
+<p class='c007'><span class='sc'>The Cardinal Points.</span>—For general convenience in expressing
+the situation of an object, it is usual to say that it is
+towards the north, south, south-west, etc., as the case may be.
+A north or south line at any place, or a <i>meridian line</i>, as it is
+called, is in the direction of the terrestrial meridian passing
+through the place. The north point of the horizon is thus the
+point in which the meridian line meets the horizon towards
+the North Pole. The opposite point is south; while the east
+and west points lie in the directions at right angles. There
+are various ways in which a meridian line may be drawn.
+One of the simplest is to erect a vertical rod and to observe
+when its shadow thrown by the sun is shortest; at that
+moment the shadow marks the direction of north and south.
+This method is not very exact, as it is so difficult to tell when the
+shadow is shortest. A more accurate result may be obtained
+by drawing a circle round the stick as centre, and noting the
+points on this circle reached by the end of the shadow before
+and after noon; the point midway between these, marks the
+position of the shadow when shortest. By taking the average
+result of observations made with more than one circle, a good
+approximation can be obtained.</p>
+
+<p class='c007'>For a somewhat rough determination of the direction of the
+cardinal points, a watch showing the correct time may be
+utilised. Directing the hour hand to the sun, the south point
+will lie midway between that and XII. In the case of a
+watch having a dial marked up to XXIV., and reading XII.
+at mid-day, the latter figure would always point to the south
+when the hand indicating the hour was directed towards the
+sun. This will be easily understood if it be remembered that
+<span class='pageno' id='Page_52'>52</span>the sun is in the south at intervals of (approximately) twenty-four
+hours.</p>
+
+<div id='i_052'  class='figcenter id001'>
+<img src='images/i_052.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 4.</span>—<i>Day and Night.</i></p>
+</div>
+</div>
+
+<p class='c007'><span class='sc'>Day and Night.</span>—The succession of days and nights by
+which our daily arrangements are regulated is at once explained
+by the
+fact that the
+earth is round,
+and turns on its
+axis once a day.
+At any particular
+instant of time
+the sun can only
+shine on that
+half of the earth
+which is turned
+towards it. At
+all places included
+in the illuminated part the sun will be above the
+horizon, and it will be day. One half of the earth will be
+turned away from the sun, and to all places in that part it
+will be night. Under the conditions represented in Fig. 4, to
+a person situated at the point P it will be midnight; he will,
+however, be carried by the earth’s rotation along the circle
+P Q R; when he arrives at a point on <i>a b</i>, the sun will be rising
+to him, and his day will commence. On reaching the point
+R the sun will be on the spectator’s meridian, and it will be
+noon. After another interval he will arrive at the boundary
+of light and shade, and his night will commence.</p>
+
+<p class='c007'><span class='sc'>Atmospheric Refraction.</span>—In common with other substances
+through which light can pass, the atmosphere by
+which the earth is surrounded has the effect of bending rays
+of light out of their courses, and on account of this we do not
+see the heavenly bodies in their true positions. If the air
+were of uniform density the effect of this refraction would be
+as illustrated to the left in Fig. 5. The light from a star S will
+reach the observer at O after striking the atmospheric shell at
+<i>a</i> and being refracted along the line <i>a</i> O; consequently the
+<span class='pageno' id='Page_53'>53</span>observer will see it in the direction O S′, and not in the
+direction O S, which it would have if the air were absent. As
+a matter of fact, the atmosphere becomes less dense in passing
+upwards, so that the rays of light are subjected to a succession
+of small deviations; two such refractions are illustrated at the
+right of Fig. 5. When a star is overhead there is no refraction,
+and the greatest displacements of a star’s positions are produced
+on the horizon, where the light has to pass through a
+great thickness of atmosphere.</p>
+
+<p class='c007'>Refraction always makes the heavenly bodies appear higher
+in the sky than they otherwise would be, and some very
+curious effects can be traced to it. Thus the sun becomes
+visible on account of refraction some time before it has
+actually risen, and remains visible for a little while after it
+has really descended below the horizon. The amount of
+refraction varies with the temperature and pressure of the
+air, but the average amounts for different elevations above the
+horizon are as follows:</p>
+
+<table class='table1'>
+  <tr><th class='c008' colspan='2'><span class='sc'>Table of Mean Refractions.</span></th></tr>
+  <tr>
+ <th class='btt bbt blt c017'>Altitude.</th>
+ <th class='btt bbt blt brt c017'>Refraction.</th>
+  </tr>
+  <tr>
+ <td class='blt c018'>0°</td>
+ <td class='blt brt c018'>34′ 54″</td>
+  </tr>
+  <tr>
+ <td class='blt c018'>2°</td>
+ <td class='blt brt c018'>18′ &#8196;9″</td>
+  </tr>
+  <tr>
+ <td class='blt c018'>4°</td>
+ <td class='blt brt c018'>11′ 39″</td>
+  </tr>
+  <tr>
+ <td class='blt c018'>6°</td>
+ <td class='blt brt c018'>8′ 23″</td>
+  </tr>
+  <tr>
+ <td class='blt c018'>8°</td>
+ <td class='blt brt c018'>6′ 29″</td>
+  </tr>
+  <tr>
+ <td class='blt c018'>10°</td>
+ <td class='blt brt c018'>5′ 15″</td>
+  </tr>
+  <tr>
+ <td class='blt c018'>12°</td>
+ <td class='blt brt c018'>4′ 23″</td>
+  </tr>
+  <tr>
+ <td class='blt c018'>14°</td>
+ <td class='blt brt c018'>3′ 45″</td>
+  </tr>
+  <tr>
+ <td class='blt c018'>16°</td>
+ <td class='blt brt c018'>3′ 17″</td>
+  </tr>
+  <tr>
+ <td class='blt c018'>18°</td>
+ <td class='blt brt c018'>2′ 54″</td>
+  </tr>
+  <tr>
+ <td class='blt c018'>20°</td>
+ <td class='blt brt c018'>2′ 35″</td>
+  </tr>
+  <tr>
+ <td class='blt c018'>25°</td>
+ <td class='blt brt c018'>2′ &#8196;2″</td>
+  </tr>
+  <tr>
+ <td class='blt c018'>30°</td>
+ <td class='blt brt c018'>1′ 38″</td>
+  </tr>
+  <tr>
+ <td class='blt c018'>40°</td>
+ <td class='blt brt c018'>1′ &#8196;8″</td>
+  </tr>
+  <tr>
+ <td class='blt c018'>50°</td>
+ <td class='blt brt c018'>0′ 48″</td>
+  </tr>
+  <tr>
+ <td class='blt c018'>60°</td>
+ <td class='blt brt c018'>0′ 33″</td>
+  </tr>
+  <tr>
+ <td class='blt c018'>70°</td>
+ <td class='blt brt c018'>0′ 21″</td>
+  </tr>
+  <tr>
+ <td class='bbt blt c018'>90°</td>
+ <td class='bbt blt brt c018'>0′ &#8196;0″</td>
+  </tr>
+</table>
+
+<p class='c007'>Refraction is responsible, among other things, for the
+curiously distorted appearances of the sun and moon, when
+they are very near the horizon.</p>
+
+<p class='c007'><span class='sc'>Twilight.</span>—The atmosphere, or rather the solid and liquid
+particles which it always contains, has the property of reflecting
+light, and hence it does not suddenly become dark when
+<span class='pageno' id='Page_54'>54</span>the sun has set. Even until the sun has descended 18° below
+the horizon, the upper parts of the air continue to reflect his
+beams, and this is the origin of <i>twilight</i>. In the tropics the
+sun sets almost vertically, so that it gets below the twilight
+limit comparatively quickly, and this explains the short
+twilight which is remarked by all who have visited a tropical
+country. In our own country the sun has an apparent oblique
+motion, and a relatively long period elapses before twilight
+ends. The increase in the duration of twilight is, indeed, very
+noticeable in merely travelling from London to the north of
+Scotland in summer-time.</p>
+
+<div id='i_054'  class='figcenter id003'>
+<img src='images/i_054.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 5.</span>—<i>Atmospheric Refraction.</i></p>
+</div>
+</div>
+
+<p class='c007'>Within the Arctic Circle, at places where the sun itself is
+never visible for months together, its reflected beams in the
+form of twilight may be seen for months.</p>
+
+<div>
+  <span class='pageno' id='Page_55'>55</span>
+  <h3 class='c004'>CHAPTER II.<br> <span class='c014'>THE EARTH’S REVOLUTION ROUND THE SUN.</span></h3>
+</div>
+
+<p class='c003'><span class='sc'>Apparent Movements of the Sun.</span>—During any day on
+which we may observe the sun, it will be seen to rise at a
+certain place on the horizon, gradually ascend into the
+heavens to a certain point, then as steadily sink towards the
+west until it disappears at some point on the western horizon.
+If we watch the sun about the 20th of March, we shall find it
+to rise due east, and set due west; it will be above the horizon
+for exactly twelve hours, and below for the same length of
+time. When this happens, we have the <i>vernal</i> or <i>spring
+equinox</i>, as the nights are then equal in all parts of the world.
+From this time to the third week in June, we shall find the
+sun to rise more and more to the north of east, and to set
+gradually further north of west. This is accompanied by a
+daily increase in the apparent height of the sun at noon, and
+by increasing length of day and reduction of night. For
+some days before the 21st of June the change of the sun’s
+place of rising and setting is very slow, and after this day the
+places of rising and setting begin to recede to the south.
+We then have the <i>summer solstice</i>, so-called because the sun
+seems to stand still, in so far as its northward travel is concerned.
+The point of rising or setting of the sun goes on
+moving nearer to the south point of the horizon, until about
+September 22, we again have the sun above the horizon for
+twelve hours, and below the horizon for an equal period; this
+is the <i>autumnal equinox</i>. The southward movement is continued
+until December 21, after which the rising begins to
+take place further towards the north. When furthest south,
+we have the <i>winter solstice</i> in the Northern Hemisphere, the
+sun being above the horizon for only a short time, and reaching
+only a small altitude at noon. From December 21 to
+March 20, the sun rises further to the north, at first very
+<span class='pageno' id='Page_56'>56</span>gradually, and afterwards more rapidly.  These varying
+amounts of sunshine
+correspond
+to the short days
+of winter, and the
+long days of
+summer. A diagrammatic
+representation
+of the
+apparent path of
+the sun at the
+solstices and equinoxes
+for some
+place, such as London, is given in Fig. 6.</p>
+
+<div id='i_056'  class='figcenter id001'>
+<img src='images/i_056.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 6.</span>—<i>Apparent Paths of Sun at Equinoxes and Solstices.</i></p>
+</div>
+</div>
+
+<p class='c007'>It is clear, then, that our relations to the sun are very different
+from our relation to the stars, inasmuch as the apparent
+position of the sun, as projected upon the sky, is constantly
+changing, but returns to similar conditions at the end of a
+year. If our place of observation is changed, the apparent
+diurnal movement of the sun is affected in the same way as
+that of the stars.</p>
+
+<p class='c007'>To explain these annual changes of the sun, with regard to
+an observer’s horizon, it is only necessary to suppose that the
+sun marches northwards towards the celestial pole from the
+winter to the summer of the Northern Hemisphere, and southwards
+from summer to winter. It is not to be imagined,
+however, that this apparent movement towards or from the
+north celestial pole is necessarily a real movement of the sun;
+we shall, in fact, very shortly see that it is only an apparent
+movement due to the changing situation of the earth with
+respect to the sun.</p>
+
+<p class='c007'><span class='sc'>The Ecliptic.</span>—A very small amount of actual observation,
+without the aid of instruments, suffices to show that the
+changes in the sun’s relation to any observers horizon at
+different parts of the year are associated with a change in its
+situation among the stars. If we direct our gaze towards the
+south at midnight, we are looking towards that part of space
+<span class='pageno' id='Page_57'>57</span>which is directly opposite to the sun, as will be evident from
+Fig. 4, and if the sun’s apparent movement were only in a
+polar direction, we should always see the same stars in the
+same part of the sky at the same hour. Such, however, is not
+the case. The stars are found more and more towards the
+west at the same hour as the year advances. Sirius, for
+instance, is due south about midnight on December 31; but
+at the end of January it will pass through the south point
+shortly before ten <span class='fss'>P.M.</span> Similar changes are noted in the
+case of all the stars, and they indicate either an easterly
+movement of the sun among the stars, or a westerly motion
+of the stars with regard to the sun. If it were possible to see
+the stars in the immediate neighbourhood of the sun, this
+relative motion could be directly observed; but under the
+actual circumstances, the apparent track of the sun amongst
+the stars must be determined indirectly. When we make
+observations at midnight, we know that the sun is opposite
+to stars which are due south at that moment; and the
+height which it reaches above the horizon at noon indicates
+its angular distance from the celestial pole. It is thus possible
+to trace the sun’s apparent path on a map of the stars, or upon
+a celestial globe; this is called the <i>ecliptic</i>, and it is found to
+be a great circle of the celestial sphere—that is, it is a circle
+contained in a plane which passes through the centre of the
+sphere.</p>
+
+<p class='c007'>The observed movement of the sun among the stars might
+be produced either by a revolution of the sun round the earth
+in a year, or by a revolution of the earth round the sun in the
+same period, the stars being supposed at rest at a greater distance
+than the sun. There are many phenomena which indicate
+that it is the earth which moves round the sun, but the
+most direct proof is found in what is known to astronomers
+as the aberration of light.</p>
+
+<p class='c007'><span class='sc'>Aberration as a Proof of the Earth’s Revolution.</span>—While
+engaged on an observation having for its
+object the determination of the distance of a star, Dr. Bradley
+made a discovery of very great interest and importance to
+<span class='pageno' id='Page_58'>58</span>astronomers. What he found practically amounts to this,
+that in order to see a star exactly at the centre of the field of
+view of a telescope we must direct the optical axis of the
+instrument at a small angle to the line joining the earth and
+star, irrespective of other deviations, such as that produced by
+refraction. The direction of this displacement is constantly
+changing throughout the year, but it is common to all the
+stars, and the fact that the original apparent position is regained
+at the end of a year at once associates aberration with
+a revolution of the earth round the sun.</p>
+
+<div id='i_058'  class='figcenter id003'>
+<img src='images/i_058.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 7.</span>—<i>Aberrational Orbit of a Star.</i></p>
+</div>
+</div>
+
+<p class='c007'>In Fig. 7 we have a perspective view of the earth’s orbit
+with the sun at S. A star <i>s</i> would appear in the direction
+A <i>s</i> when seen from the earth, supposed at rest at the point
+A; actually it is seen at <i>a</i>, ahead of its place, and in the course
+of a year it describes the <i>aberrational orbit</i>, <i>a b c d</i>, these
+points corresponding to positions A B C D of the earth in
+its annual path.</p>
+
+<p class='c007'>As a result of aberration, then, each star appears to revolve
+once a year in a small elliptic path about its average position.
+<span class='pageno' id='Page_59'>59</span>The breadths of these ellipses vary according to their angular
+distances from the ecliptic, but all have precisely the same
+length of about 41″. Half the length of the ellipses, which
+amounts to 20″.5, is accordingly called the <i>constant</i> of
+<i>aberration</i>.</p>
+
+<p class='c007'>The fact that the earth’s velocity in its orbit forms a
+sensible fraction of the velocity of light is the cause of
+aberration. If we let an object fall down the middle of a
+tube which is at rest, it will fall to the bottom without touching
+the side if the tube be held vertically. When the tube
+has a forward movement, however, it must be inclined at an
+angle in order that the falling body may pass clear to the
+bottom, and the greater the speed of the tube the more it
+must be inclined. So it is with light which comes from a
+star and traverses the tube of a telescope situated on a
+moving earth; the tube must be inclined to the actual path
+of the light rays.</p>
+
+<p class='c007'>Other proofs that it is the earth which moves round the
+sun are furnished by the parallaxes of the stars, and by spectroscopic
+measures of the earth’s velocity.</p>
+
+<p class='c007'><span class='sc'>Approximate Scale of Earth’s Orbit.</span>—A very
+beautiful application of the constant of aberration is in the
+measurement of the distance of the earth from the sun. We
+have only to bear in mind that the apparent size of the sun
+does not change very much, in order to realise that the path
+of the earth must be very nearly a circle; if the distance
+changed very much there would be a correspondingly great
+change in the sun’s apparent diameter. Now the constant of
+aberration is a measure of the relative velocity of the earth in
+its orbit and the velocity of light. There are several ways of
+determining the velocity of light, and it is known to be very
+nearly 186,300 miles per second. In a right-angled triangle
+having one angle equal to the constant of aberration, the side
+opposite to this angle would represent the velocity of the
+earth, if the longer side represented that of light. In such a
+triangle the proportion between these sides would be nearly as
+1 to 10,000. That is, the velocity of light is about 10,000
+<span class='pageno' id='Page_60'>60</span>times that of the earth in its orbit. The earth’s velocity is
+thus found to be about 18½ miles per second, so that the
+distance which it traverses in a year is found by a simple
+multiplication. In this way the circumference of the earth’s
+orbit is obtained, and it is easily deduced that the radius of
+the orbit, which is nothing more than the sun’s distance, is not
+far from 93,000,000 miles.</p>
+
+<p class='c007'><span class='sc'>The Zodiac.</span>—The space about 8° above and below the
+ecliptic constitutes what is called the <i>zodiac</i>. The zodiac is
+of very great antiquity, and marks out the region traversed by
+the sun and all the planets known to the ancients. It is
+divided into twelve parts of 30° each, called signs of
+the zodiac, from the supposed outlines of animals, etc.,
+marked out by the stars. The names of these signs are probably
+familiar to everyone from the well-known rhyme:</p>
+
+<div class='lg-container-b c015'>
+  <div class='linegroup'>
+    <div class='group'>
+      <div class='line'>“<i>The Ram, the Bull, the Heavenly Twins,</i></div>
+      <div class='line'><i>And next the Crab the Lion shines,</i></div>
+      <div class='line in2'><i>The Virgin, and the Scales,</i></div>
+      <div class='line'><i>The Scorpion, Archer, and the Goat,</i></div>
+      <div class='line'><i>The man that bears the Watering-Pot,</i></div>
+      <div class='line in2'><i>And Fish with glittering tails.</i>”</div>
+    </div>
+  </div>
+</div>
+
+<p class='c007'>The astronomical names and symbols corresponding to
+these are as follows:—</p>
+
+<table class='table2'>
+  <tr>
+    <td class='c019'>♈︎</td>
+    <td class='c010'>Aries,</td>
+    <td class='c020'>The Ram.</td>
+  </tr>
+  <tr>
+    <td class='c019'>♉︎</td>
+    <td class='c010'>Taurus,</td>
+    <td class='c020'>The Bull.</td>
+  </tr>
+  <tr>
+    <td class='c019'>♊︎</td>
+    <td class='c010'>Gemini,</td>
+    <td class='c020'>The Twins.</td>
+  </tr>
+  <tr>
+    <td class='c019'>♋︎</td>
+    <td class='c010'>Cancer,</td>
+    <td class='c020'>The Crab.</td>
+  </tr>
+  <tr>
+    <td class='c019'>♌︎</td>
+    <td class='c010'>Leo,</td>
+    <td class='c020'>The Lion.</td>
+  </tr>
+  <tr>
+    <td class='c019'>♍︎</td>
+    <td class='c010'>Virgo,</td>
+    <td class='c020'>The Virgin.</td>
+  </tr>
+  <tr>
+    <td class='c019'>♎︎</td>
+    <td class='c010'>Libra,</td>
+    <td class='c020'>The Balance.</td>
+  </tr>
+  <tr>
+    <td class='c019'>♏︎</td>
+    <td class='c010'>Scorpio,</td>
+    <td class='c020'>The Scorpion.</td>
+  </tr>
+  <tr>
+    <td class='c019'>♐︎</td>
+    <td class='c010'>Sagittarius,</td>
+    <td class='c020'>The Archer.</td>
+  </tr>
+  <tr>
+    <td class='c019'>♑︎</td>
+    <td class='c010'>Capricornus,</td>
+    <td class='c020'>The Goat.</td>
+  </tr>
+  <tr>
+    <td class='c019'>♒︎</td>
+    <td class='c010'>Aquarius,</td>
+    <td class='c020'>The Water-Bearer.</td>
+  </tr>
+  <tr>
+    <td class='c019'>♓︎</td>
+    <td class='c010'>Pisces,</td>
+    <td class='c020'>The Fishes.</td>
+  </tr>
+</table>
+
+<p class='c007'><span class='pageno' id='Page_61'>61</span>The sun enters the sign Aries at the vernal equinox in
+March, and the others in successive months. On account of
+the precession of the equinoxes (see p. 69), however, the
+sun no longer enters the <i>constellation</i> Aries at the vernal
+equinox, but it is still said to enter the <i>sign</i> Aries.</p>
+
+<p class='c007'><span class='sc'>Inclination of the Earth’s Axis.</span>—The revolution of
+the earth round the sun provides us with a very satisfactory
+explanation of the apparent easterly movement of the sun
+among the stars. There is, however, another very important
+point. We have seen that during a year the sun has a movement
+towards and from the Pole, as well as an easterly
+movement. The plane of the earth’s orbit, therefore, cannot
+be coincident with the plane of the Equator; if it were, the
+sun would have the same apparent movement every day—it
+would always rise due east, and set due west, in all parts of
+the earth. The ecliptic, moreover, would be coincident with
+the celestial equator. When the ecliptic is determined by
+observations in the way already explained (p. 57), it is
+found to intersect the celestial equator in two points, and the
+plane containing it is inclined at an angle of very nearly
+23½° to the equatorial plane. This inclination of the Equator
+to the ecliptic, or “obliquity of the ecliptic,” indicates that
+the earth’s axis of rotation is inclined to the plane in which
+the revolution round the sun is performed, the actual inclination
+being about r66½°.</p>
+
+<p class='c007'>Further, the axis of rotation must remain parallel to itself
+during the revolution of the earth. Otherwise, the situation
+of the celestial pole would be seen to change, and the Pole
+Star would no longer serve to show us which way lies
+north.</p>
+
+<p class='c007'>It is precisely this inclination of the earth’s axis which
+brings about the varying lengths of days and nights which we
+associate with different seasons.</p>
+
+<p class='c007'><span class='sc'>The Seasons.</span>—Let us in the first place contrast the
+conditions in summer with those which obtain in winter.
+Imagine that we can view the sun and earth from a very
+distant point lying in the plane of the ecliptic, and situated so
+<span class='pageno' id='Page_62'>62</span>that a line joining it with the sun is perpendicular to the line
+joining the sun and earth in summer or winter.</p>
+
+<div id='i_062'  class='figcenter id003'>
+<img src='images/i_062.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 8.</span>—<i>The Sun’s Altitude in Summer and Winter.</i></p>
+</div>
+</div>
+
+<p class='c007'>The sun will thus appear in some position represented by
+O in Fig. 8; in the summer of the Northern Hemisphere the
+earth will be in the position S, and in winter in the position
+W, since it travels half way round its orbit in six months’
+time. An observer situated at London will be 38½° from the
+North Pole, and he is represented by the point A in our
+diagram. The horizon at noon of such an observer is represented
+by the line H R, tangential to the surface of the sphere
+at the point A. At noon, then, the altitude of the sun is
+equal to the angle O A H. When it is winter in the Northern
+Hemisphere, the earth’s axis is inclined away from the sun,
+and our observer at London is so situated that at noon his
+horizon is the line H′ R′, while the sun’s altitude is the angle
+O A′ R′, which is no less than 47° smaller than in summer.
+People who dwell in the Southern Hemisphere enjoy the long
+days of summer at the time when our own days are shortest,
+and <i>vice versâ</i>, and the reason for this is clearly that when the
+position of the earth’s axis presents the greatest part of the
+Northern Hemisphere towards the sun, the greater part of
+the southern half of our globe is turned away from the sun.</p>
+
+<p class='c007'>At the equinoxes, which occur very nearly midway between
+the solstices, the earth’s axis is directed neither towards nor
+away from the great source of light and heat, so that both
+<span class='pageno' id='Page_63'>63</span>hemispheres are presented to the sun under exactly the same
+conditions. This state of affairs is shown diagrammatically in
+Fig. 9. The sun’s altitude at noon at the commencement of
+spring is equal to that at the beginning of autumn, and
+depends only upon the observer’s latitude. The half of our
+globe which is then flooded with the sun’s rays comprises
+both the North and South Poles, and it is evident that as the
+earth turns round, every place upon it, whether in Arctic or
+equatorial regions, receives the benefit of twelve hours sunshine,
+and at the same time has a night of twelve hours
+duration.</p>
+
+<div id='i_063'  class='figcenter id003'>
+<img src='images/i_063.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 9.</span>—<i>The Sun’s Altitude at the Equinoxes.</i></p>
+</div>
+</div>
+
+<p class='c007'><span class='sc'>The Midnight Sun.</span>—The facilities which are now offered
+for foreign travel have induced many people to pay a visit to
+the north of Norway, one of the objects in view frequently
+being to witness the so-called “midnight sun.” It seems
+somewhat paradoxical to speak of night when the sun is
+above the horizon, but it simply means that in high latitudes
+the sun may be seen over the northern horizon when it is
+midnight at places further south which have the same longitude.
+We have seen that in our own country there are certain stars
+which never set, and when we get to the Pole itself, all the
+stars which are there visible will present this peculiarity.</p>
+
+<p class='c007'>In order to see the sun at midnight, then, what we have to
+do is to travel towards the Pole until we reach a latitude
+<span class='pageno' id='Page_64'>64</span>where the sun itself becomes circumpolar. At the Pole this
+would be the state of things during the whole of the northern
+summer, when the sun is north of the Equator, and since the
+sun never travels northward more than 23½°, it can only be
+circumpolar at places within that angular distance from the
+Pole, that is, within the Arctic Circle.</p>
+
+<div id='i_064'  class='figcenter id003'>
+<img src='images/i_064.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 10.</span>—<i>The Midnight Sun.</i></p>
+</div>
+</div>
+
+<p class='c007'>Let A in Fig. 10 be such a place, the sun being to the left.
+At noon the horizon of A is represented by H R, and the
+sun will appear in the south at a certain altitude, S A H. At
+midnight the earth’s rotation will change the observers
+position to A′ and his horizon to H′ R′, but it will not have
+taken him out of sunshine. The sun will then appear due north,
+but, except at the Pole, its altitude, S A′ H′, will be lower than
+at noon. At a place situated on the Arctic circle, latitude 66½°,
+the midnight sun would only be visible for one night at the
+summer solstice, were it not that refraction causes it to
+appear above the horizon when it is geometrically more than
+its own apparent diameter below.</p>
+
+<p class='c007'>At Tromsö the midnight sun is visible from May 19 to
+July 22, and at the North Cape from May 12 to July 29.</p>
+
+<p class='c007'><span class='pageno' id='Page_65'>65</span>Nature, however, exacts compensation for this lavish share
+of summer sunshine in high latitudes, and there is a correspondingly
+number of dreary days in winter when the sun
+does not rise at all.</p>
+
+<h3 class='c004'>CHAPTER III.<br> <span class='c014'>HOW THE POSITIONS OF THE HEAVENLY BODIES ARE DEFINED.</span></h3>
+
+<p class='c003'><span class='sc'>Two Measurements Requisite.</span>—In order to make a more
+precise study of the movements of the heavenly bodies, it is
+essential that we should have some very definite means of
+specifying their positions upon the celestial sphere. To define
+the position of any object, at least two measurements are required.
+If, for example, one wishes to draw attention to a
+particular letter on the page of a book, it is only necessary to
+say that it is so many lines from the top, and a certain number
+of letters from the end of the particular line on which it lies.
+In the same way, latitude and longitude sufficiently indicate
+the situation of a place on the surface of the earth, and
+similar measures can be employed to indicate the places of
+the heavenly bodies.</p>
+
+<p class='c007'><span class='sc'>Altitude and Azimuth.</span>—The horizon and zenith at
+any place—being in a constant position with reference to the
+earth—may be utilised for indicating the positions of external
+bodies. We may say, for instance, that at noon on June 24,
+the sun, as seen from London, is 62° above the horizon, or
+28° from the zenith. Technically, the former is called
+the <i>altitude</i> of the sun, being the angular distance above
+the horizon, while the latter measure is called the <i>zenith
+distance</i>.</p>
+
+<div id='i_066'  class='figcenter id001'>
+<span class='pageno' id='Page_66'>66</span>
+<img src='images/i_066.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 11.</span>—<i>Altitude and Azimuth.</i></p>
+</div>
+</div>
+
+<p class='c007'>We may next note that an object, besides having a certain
+altitude, is a certain number of degrees from the north, south,
+east, or west points, measured
+horizontally; if we
+reckon from the north point
+through E, S, and W, from
+0° to 360°, such a horizontal
+measurement is called
+<i>azimuth</i>; if reckoned north
+or south of the east or west
+points it is called the <i>amplitude</i>
+of the body. Fig. 11
+illustrates these terms. In this diagram the observer is placed
+at O, N S and E W respectively representing a north and
+south, and an east and west line in the horizon; the point Z
+is the zenith, and S a heavenly body. A vertical circle drawn
+from Z through S will meet the horizon at a point A. The
+azimuth of S is thus the angle N O A, and its amplitude is
+the angle E O A, while the altitude of S is simply the angle
+A O S. Measurements of altitude and azimuth are made by
+means of an instrument called the altazimuth, an account of
+which will be found on page <a href='#Page_202'>202</a>.</p>
+
+<p class='c007'><span class='sc'>Declination.</span>—Altitude and azimuth only specify the position
+of a star for a particular place at a particular time. A
+better system is evidently one which is independent of the
+observer’s situation on the earth. Of the two measurements
+required, one is readily decided upon; we can say that the
+sun, or star, or other heavenly body is a certain number of degrees
+from the north celestial pole; or, what is just as good,
+we can state the number of degrees north or south of the
+celestial equator, which lies midway between the poles. The
+former measurement gives what is called the <i>north polar
+distance</i> of the star, and the latter its <i>declination</i>.</p>
+
+<p class='c007'><span class='sc'>Right Ascension.</span>—Just as the latitude of a place on the
+earth does not tell whether it is in Europe or North America,
+so declination alone fails to locate a heavenly body. We must
+have some measurement equivalent to terrestrial longitude,
+<span class='pageno' id='Page_67'>67</span>and it is therefore necessary in the first instance to select a start-point,
+which shall do for stars what Greenwich does for our geographical
+maps. By universal consent the fundamental point
+for the stars is a point situated on the celestial equator where it
+is crossed by that part of the ecliptic occupied by the sun at the
+vernal equinox. This zero mark is called the <i>First Point of
+Aries</i>, and is frequently denoted by the symbol ♈︎ identical
+with that employed for the corresponding sign of the zodiac.</p>
+
+<p class='c007'>The location of this reference point being thus determined,
+the <i>right ascension</i> of a celestial body may be defined as its
+angular distance from the First Point of Aries, as measured
+along the celestial equator. Like terrestrial longitude, it may
+be stated in degrees, but it is more usually expressed in
+hours, minutes, and seconds of time, for the reason that in
+general the measurement of a right ascension consists of an
+observation of the time at which the body in question comes
+to a certain position.</p>
+
+<p class='c007'>The right ascensions and declinations of stars are best
+determined when they are on the meridian of the place of
+observation, and such measurements are made by means of a
+transit instrument. When a star is on the meridian, its
+declination is estimated by the angle at which the instrument
+is inclined to the celestial equator when directed to the star.
+The fact that the earth is turning on its axis furnishes us
+with a simple method of finding the right ascensions of the
+heavenly bodies. Imagine a plane passing through the
+observers position on the earth and through the earth’s axis.
+This, prolonged indefinitely, cuts the celestial sphere in his
+meridian, and it is evident that on account of the earth’s
+rotation it will turn completely round every twenty-four
+hours. It may therefore be regarded as the hour-hand of a
+clock, which is provided with figures ranging from I. to
+XXIV. When this gigantic clock hand sweeps past the First
+Point of Aries, all stars then seen in the plane—that is, all
+stars which are on the meridian—will have zero right ascension.
+After a complete rotation it will again sweep through the
+First Point of Aries.</p>
+
+<p class='c007'><span class='pageno' id='Page_68'>68</span><span class='sc'>Use of Star Time.</span>—Meanwhile, suppose we have a
+clock regulated so that it marks twenty-four hours between
+these two meridian passages of the First Point of Aries.
+Evidently, then, the time by this clock at which any object in
+the sky is seen on the meridian will depend upon its angular
+distance from the celestial meridian passing through the First
+Point of Aries. As the earth is rotating through 360° in
+twenty-four hours, reckoned by our clock, the meridian plane
+will travel at the rate of 15° per hour, so that, for example,
+a star 60° from the celestial meridian passing through
+the First Point of Aries, will appear to cross the observer’s
+meridian at IV. hours by the clock. A clock so regulated to
+keep time with the stars is called a sidereal clock, and the
+sidereal time at which a celestial body crosses the meridian, or
+“souths,” is the right ascension of that object. Such a time
+measurement can be converted into angular measure by
+allowing 15° per hour, 15′ per minute, and 15″ per second of
+time.</p>
+
+<p class='c007'><span class='sc'>Celestial Latitude and Longitude.</span>—In some astronomical
+questions it is often convenient to adopt a different
+system of co-ordinates to indicate the situation of a celestial
+body. Just as the earth’s equatorial plane serves as a basis
+for the measurement of declination, the earth’s plane of revolution—that
+is, the plane of the ecliptic—is used as the
+term of reference for <i>celestial latitude</i>, which may be defined
+as the angular distance of an object above or below the
+plane of the ecliptic. <i>Celestial longitude</i> is the angular
+distance from the First Point of Aries measured along the
+ecliptic.</p>
+
+<p class='c007'>A diagram such as that in Fig. 12 may assist the comprehension
+of these co-ordinates. Here the observer is supposed
+to be situated at the point O, at the centre of the celestial
+sphere. To him the north and south celestial poles will
+appear in some such positions as N and S, and the celestial
+equator will be represented by a great circle at right angles to
+the line joining these two points. The apparent path of the
+sun—the ecliptic—will be indicated by another great circle,
+<span class='pageno' id='Page_69'>69</span>which is inclined to the Equator; and the poles of the ecliptic
+will be represented by P and P′.</p>
+
+<p class='c007'>The Equator crosses the ecliptic at the First Point of Aries,
+marked ♈︎. Considering now a star which the observer sees
+in the direction of the line O S, its position would be reckoned
+as follows in the two systems:—</p>
+
+<table class='table2'>
+  <tr>
+    <td class='c010'>Right</td>
+    <td class='c010'>Ascension</td>
+    <td class='c019'>=</td>
+    <td class='c019'>Angle</td>
+    <td class='c019'>♈︎ O R</td>
+    <td class='c021'>}</td>
+  </tr>
+  <tr>
+    <td class='c010' colspan='2'>Declination</td>
+    <td class='c019'>=</td>
+    <td class='c019'>„</td>
+    <td class='c019'>S O R</td>
+    <td class='c021'>}</td>
+  </tr>
+  <tr>
+    <td class='c010'>&#160;</td>
+    <td class='c010'>&#160;</td>
+    <td class='c019'>&#160;</td>
+    <td class='c019'>&#160;</td>
+    <td class='c019'>&#160;</td>
+    <td class='c021'>&#160;</td>
+  </tr>
+  <tr>
+    <td class='c010'>Celestial</td>
+    <td class='c010'>Longitude</td>
+    <td class='c019'>=</td>
+    <td class='c019'>Angle</td>
+    <td class='c019'>♈︎ O L</td>
+    <td class='c021'>}</td>
+  </tr>
+  <tr>
+    <td class='c010'>„</td>
+    <td class='c010'>Latitude</td>
+    <td class='c019'>=</td>
+    <td class='c019'>„</td>
+    <td class='c019'>S O L</td>
+    <td class='c021'>}</td>
+  </tr>
+</table>
+
+<p class='c007'>Either pair of co-ordinates can, by a mathematical process,
+be expressed in terms of the other.</p>
+
+<div id='i_069'  class='figcenter id001'>
+<img src='images/i_069.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 12.</span>—<i>Right Ascension, Declination, Celestial Latitude, and Celestial Longitude.</i></p>
+</div>
+</div>
+
+<p class='c007'><span class='sc'>Precession of the Equinoxes.</span>—It is not too early to
+remark that the First Point of Aries is not absolutely a fixed
+point on the celestial equator. This is on account of the precession
+of the equinoxes, which consists of a backward movement
+<span class='pageno' id='Page_70'>70</span>of the First Point, due to a change in the position of the
+earth’s equator. As a point common to the ecliptic and
+equator, it is conveniently retained as the starting-point of
+right ascensions and celestial longitudes, but in consequence
+of precession, these co-ordinates are subject to a constant
+change. The amount of precession for a point on the
+Equator is 50″·2 per annum, and this movement requires
+25,800 years for a complete revolution.</p>
+
+<p class='c007'><span class='sc'>Geocentric and Heliocentric Positions.</span>—When
+observing objects at a very great distance, they will appear in
+the same direction to a spectator on the earth as they would
+if he could by some means be transferred so as to be able to
+see them from the sun. If, for instance, one sees the Peak of
+Teneriffe from a distant ship, its apparent direction will be
+very slightly affected by a change of a mile in the ship’s
+position. But a similar change of place would produce a
+greater difference of direction when a nearer body was under
+observation. If an object is relatively near to the sun and
+earth, its direction, and, therefore, its apparent position on the
+celestial sphere, will be different, as seen from the earth and
+sun. Such will be the case with planets and other bodies
+which lie in our immediate neighbourhood, speaking astronomically.
+Hence, it is often convenient to distinguish
+between the <i>geocentric</i> position of a celestial body—referring
+it to the position it would occupy if it could be seen from the
+centre of the earth—and the <i>heliocentric</i> position, representing
+it as it would appear to an observer occupying the centre of
+the sun. We thus have geocentric and heliocentric latitudes
+and longitudes of the nearer heavenly bodies.</p>
+
+<p class='c007'><span class='sc'>Star Catalogues.</span>—The problem of constructing catalogues
+showing the positions of the stars is one of considerable
+practical value, as well as one of great scientific importance.
+In the first instance, such catalogues were of necessity
+compiled from data acquired by naked eye observations, so
+that the ancient catalogues comprise only a small number of
+stars.</p>
+
+<p class='c007'>As far back as 295 <span class='fss'>B.C.</span>, the positions of stars were determined
+<span class='pageno' id='Page_71'>71</span>by Timocharis with sufficient accuracy to lead
+Hipparchus to his great discovery of the precession of the
+equinoxes about 170 years later. From observations at
+Rhodes, Hipparchus drew up a catalogue of 1,022 stars, giving
+their latitudes and longitudes; this is preserved for us in
+Ptolemy’s “Almagest,” where the positions are corrected for
+precession, and reduced to the epoch 150 <span class='fss'>A.D.</span> The next
+catalogue of importance was due to the industry of Tycho
+Brahé (1546–1601), who gave the positions of 1,005 stars with
+greater accuracy than had been previously obtained; indeed,
+notwithstanding his want of optical aid, it has been estimated
+that the probable errors of his measures were not more than
+24″ and 25″ in right ascension and declination respectively.
+The last of the naked eye catalogues is that of Hevelius,
+giving the positions of 1,553 stars.</p>
+
+<p class='c007'>Coming to more recent times, in which the employment of
+telescopes has vastly increased the power of accurate observation,
+there are the catalogues of Flamsteed, Halley, Lacaille,
+Lalande, Argelander, the British Association, and catalogues
+of the stars in particular parts of the sky which have been
+published by all the leading national observatories. Eighteen
+observatories are now taking part in the construction of an
+international star catalogue by means of photography, and
+this is intended to record with great accuracy the positions of
+nearly 3,000,000 stars. A modern star catalogue usually
+places the stars in the order of their right ascensions, and,
+in addition to the two co-ordinates, furnishes the necessary
+data for determining the exact situations of the stars at any
+particular time.</p>
+
+<div>
+  <span class='pageno' id='Page_72'>72</span>
+  <h3 class='c004'>CHAPTER IV.<br> <span class='c014'>THE EARTH’S ORBIT.</span></h3>
+</div>
+
+<p class='c003'><span class='sc'>Exact Shape of the Orbit.</span>—It will be clear that if we
+made our annual journey in a circle we should always be at
+the same distance from the sun, and the apparent size of that
+luminary would never vary. This, however, is not the case.
+Exact measurements, which are best made by means of the
+transit instrument, indicate variations which, though not perceptible
+to the unassisted eye, establish a want of circularity.
+The observations bearing on this point consist of a measurement
+of the time required for the sun to cross the meridian—the
+larger its apparent diameter, the longer it will obviously
+be in passing the meridian. An observation of the sidereal
+time at which the centre of the sun passes the meridian
+determines the right ascension, and from this one can calculate
+the sun’s longitude.</p>
+
+<div id='i_072'  class='figcenter id001'>
+<img src='images/i_072.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 13.</span>—<i>Elliptic Form of Earth’s Orbit.</i></p>
+</div>
+</div>
+
+<p class='c007'>If such observations be made at intervals during a year, we
+can utilise them for determining the shape of the earths orbit
+independently of a knowledge of the actual size. In Fig. 13
+let us suppose the sun to be situated at the point S; from S
+we draw a line,
+S A, representing
+the line joining
+the earth and sun
+at the vernal equinox
+when the sun’s
+longitude is zero.
+If our observations
+include a measure
+of the sun’s diameter
+on that day,
+let S A be drawn
+on some convenient
+scale. To plot the observations for other days, we must
+<span class='pageno' id='Page_73'>73</span>draw S F, S E, etc., at angles A S F, A S E, etc., equal to the
+sun’s longitude, and make the lengths inversely proportional to
+the apparent diameters, on the same scale as S A. The other
+observations can be plotted in the same way, and the earths
+orbit is then found to be an ellipse with the sun in one of its
+foci. Actually, the earth’s orbit is much more nearly circular
+than is shown in Fig. 13, and in illustration of this the following
+numerical data may be given:—</p>
+
+<table class='table2'>
+  <tr>
+    <td class='c019'>1896.</td>
+    <td class='c022'>Jan. 1</td>
+    <td class='c022'>Greatest</td>
+    <td class='c019'>apparent diameter of sun</td>
+    <td class='c022'>= 32′ 35″·2</td>
+    <td class='c019'>in long.</td>
+    <td class='c023'>281°</td>
+  </tr>
+  <tr>
+    <td class='c019'>&#160;</td>
+    <td class='c022'>July 3</td>
+    <td class='c022'>Least</td>
+    <td class='c019'>„        „       „</td>
+    <td class='c022'>= 31′ 30″·6</td>
+    <td class='c019'>„</td>
+    <td class='c023'>102°</td>
+  </tr>
+  <tr>
+    <td class='c019'>&#160;</td>
+    <td class='c022'>March 29</td>
+    <td class='c022'>Mean</td>
+    <td class='c019'>„        „       „</td>
+    <td class='c022'>= 32′ 4″</td>
+    <td class='c019'>„</td>
+    <td class='c023'>9°</td>
+  </tr>
+  <tr>
+    <td class='c019'>&#160;</td>
+    <td class='c022'>Oct. 5</td>
+    <td class='c019'>„</td>
+    <td class='c019'>„        „       „</td>
+    <td class='c022'>= 32′ 4″</td>
+    <td class='c019'>„</td>
+    <td class='c023'>193°</td>
+  </tr>
+</table>
+
+<p class='c007'>It thus appears that in 1896 we were nearest to the sun on
+January 1, as on that day the sun’s apparent diameter was
+greatest, while we were furthest removed on July 3.</p>
+
+<div id='i_073'  class='figcenter id003'>
+<img src='images/i_073.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 14.</span>—<i>The Ellipse.</i></p>
+</div>
+</div>
+
+<p class='c007'>The ellipse is a curve of such importance in astronomy that
+an understanding of some of its properties is essential for
+further progress. This beautiful closed curve lies in one
+plane, and its figure is such that the sum of the distances of
+any point upon it from two fixed points within the curve is
+<span class='pageno' id='Page_74'>74</span>constant. These two fixed points, F F′ (Fig. 14), are called
+the foci of the ellipse, and we have, for example, the sum of
+the lengths P F and P F′, equal to the sum of P′ F and
+P′ F′. The line A B passing through the foci is the
+greatest distance across the ellipse, and is called the
+major axis; at right angles to this is the minor axis C D.</p>
+
+<p class='c007'>Following our definition of the ellipse, we see that as B is
+a point upon its circumference, B F + B F′ must be equal to
+the sum of the distances of any point P from the foci. But
+since B F is of the same length as A F′, the sum of the
+distances of the point B from the foci, and therefore of all
+other points, is equal to the major axis. Hence the average
+or mean distance of the focus F from all points on the ellipse
+is half the length of the major axis. It follows also that
+C F is equal to the semi-major axis O B.</p>
+
+<p class='c007'>At the point O, where the axes intercept each other, we have
+the centre of the ellipse, and the ratio between the distance
+from the centre to either of the foci and the semi-major axis,
+<i>i.e.</i>, (O F)/(O B) is called the eccentricity of the ellipse. Thus, in
+an ellipse of eccentricity 0·5, the foci would lie midway
+between the centre of the ellipse and the extremities of the
+major axis. The eccentricity is always less than unity; if it
+become unity, the two foci merge together, and the curve
+becomes a circle.</p>
+
+<div id='i_074'  class='figcenter id001'>
+<img src='images/i_074.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 15.</span>—<i>How to draw an Ellipse.</i></p>
+</div>
+</div>
+
+<p class='c007'>To draw an ellipse, two pins may be stuck into a
+piece of paper at the points intended as foci. A loop
+of thread is then made and
+thrown over the pins. A
+pencil placed inside the
+loop, so as to stretch it, and
+traced completely round,
+will outline an ellipse. The
+size and shape of the ellipse
+may be varied by changing
+the length of the thread
+and the distance between
+the pins. Such, then, is
+<span class='pageno' id='Page_75'>75</span>the curve in which our earth performs its annual journey
+round the sun, the sun being relatively fixed in one of the
+foci.</p>
+
+<p class='c007'><span class='sc'>Aphelion and Perihelion.</span>—When the earth is in that
+part of its orbit where it makes its nearest approach to the
+sun, it is said to be in <i>perihelion</i>; when at the point furthest
+removed from the sun it is in <i>aphelion</i>. The line joining these
+two points is obviously the major axis of the earth’s orbit, and
+when this is imagined to be prolonged indefinitely into space
+it is called the <i>line of apsides</i>, or <i>apse line</i>. When the earth is
+in perihelion, the sun’s apparent diameter will be the greatest
+possible, and when in aphelion it will be at a minimum. A
+knowledge of these limiting values of the apparent solar
+diameter enables us to determine the eccentricity of the orbit
+of the earth. The sun’s apparent diameter when the earth is
+in perihelion amounts to 32′ 35″·2, and to 31′ 30″·6, when the
+earth is in aphelion, from which it results that the value of e
+is 0·0167.</p>
+
+<p class='c007'><span class='sc'>Unequal Speed of the Earth.</span>—The observations by
+which we are enabled to determine the true form of the
+earth’s orbit are not quite exhausted of their usefulness; we
+can utilise them still further for studying the varying rate of
+the earth’s motion. If the earth moved through equal angles
+every day, the apparent movement of the sun would always
+be uniform, and in that case the sun’s daily increase of longitude
+would be constant.</p>
+
+<p class='c007'>The following figures, however, prove that this uniformity
+does not exist:—</p>
+
+<table class='table2'>
+  <tr>
+    <th class='c010'>1896.</th>
+    <th class='c021'>Sun’s daily motion in longitude.</th>
+  </tr>
+  <tr>
+    <td class='c010'>Jan. 1</td>
+    <td class='c021'>1°   &#8196;1′   &#8196;8″·5</td>
+  </tr>
+  <tr>
+    <td class='c010'>Mar. 29</td>
+    <td class='c021'>1°   &#8196;0′   &#8196;6″·7</td>
+  </tr>
+  <tr>
+    <td class='c010'>July 3</td>
+    <td class='c021'>0°   57′   12″·1</td>
+  </tr>
+</table>
+
+<p class='c007'>Facts such as these led Kepler in 1609 to the discovery of
+his famous second law of planetary motion, namely, that the
+<span class='pageno' id='Page_76'>76</span>radius vector (the line joining
+the sun and earth in the case
+of the earth’s orbit) describes
+equal areas in equal times.
+For the sake of clearness,
+imagine the earth’s orbit to be
+represented by the elongated
+ellipse in Fig. 16, with the sun
+in the focus <i>S</i>. When the earth is near perihelion, it will move
+over a certain distance, <i>a b</i>, in a given time; some time afterwards
+it will be in another part of the orbit, and in the same
+interval as before it will traverse the distance <i>c d</i>; again, in
+another equal interval of time, it will move from the point <i>e</i> to
+the point <i>f</i>. The law tells that the areas <i>S a b</i>, <i>S c d</i>, and <i>S e f</i>,
+are equal so long as equal times are in question; in different
+parts of its path, then, the earth’s rate of motion must vary,
+<i>c d</i>, for example, being smaller than <i>a b</i>. It will be seen that
+the motion is most rapid when the earth is in perihelion, and
+least rapid when in aphelion.</p>
+
+<div id='i_076'  class='figcenter id001'>
+<img src='images/i_076.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 16.</span>—<i>Illustrating Kepler’s Second Law.</i></p>
+</div>
+</div>
+
+<p class='c007'><span class='sc'>Changes in the Earth’s Orbit.</span>—Owing to disturbances
+caused by the proximity of other bodies, the earth’s
+orbit is not always of the same shape. The eccentricity is
+steadily diminishing, and in about 24,000 years the orbit will
+be very nearly a circle; it will afterwards become more
+elliptical again, until in another 40,000 years or so the
+eccentricity will be about 0·02. So far as our knowledge
+goes, the eccentricity will never exceed 0·07.</p>
+
+<p class='c007'>The direction of the major axis of the earth’s orbit, that is,
+the line of apsides, moves forward at the rate of about 11″ per
+annum, so that at this speed a whole revolution will be made
+in a period of 108,000 years.</p>
+
+<p class='c007'>On account of precession, the equinox moves backwards
+along the orbit at the rate of 50″·2 per annum, so that the
+movement of the apse line with regard to the equinox is 61′
+in a year; or, in other words, the perihelion point of the
+earth’s orbit makes a complete revolution with respect to the
+equinoctial point in a little over 20,000 years. The earth at
+<span class='pageno' id='Page_77'>77</span>present passes through perihelion in our northern winter, but
+owing to this motion of the apse line it will in 10,000 years
+time be at aphelion in winter. Northern winters will then be
+somewhat colder than at present. The plane of the orbit itself
+is subject to changes, with the result that the obliquity of
+the ecliptic is variable in amount. In the course of ages the
+obliquity may oscillate between the limits 24° 35′ 58″ and 21°
+58′ 36″. The mean value during 1896 was 23° 27′ 9″·9.</p>
+
+<p class='c007'><span class='sc'>The Earth’s Real Path.</span>—In this and preceding
+chapters, we have had occasion to consider various features of
+the earth’s orbit, but it must now be pointed out that what
+we call the orbit of the earth is not quite the same thing as
+the earth’s actual path in space. The earth, as we know, is
+accompanied by the moon, and these two bodies are bound
+together in such a way that it is really the centre of gravity
+of the earth and moon which describes an elliptic orbit round
+the sun; the moon is so small in relation to the earth that
+the centre of gravity of the two companions lies within the
+earth’s surface, but, nevertheless, an oscillatory displacement
+of the earth’s centre in space is produced by the moon’s
+monthly circuit round the earth. We judge of the earth’s
+movement by the apparent movement of the sun, and we
+actually find a monthly inequality in the sun’s apparent
+motion. A very good illustration of this may be found in
+the varying celestial latitude of the sun. It will be clear that
+if the earth always moved in the plane of the ecliptic, the
+sun’s latitude would always be zero. If, on the other hand,
+the earth has a motion round the common centre of gravity,
+it will be above the ecliptic when the moon is below, and
+<i>vice versâ</i>; the sun will, therefore, not always appear to be in
+the ecliptic, and its latitude will depend upon that of the
+moon. The following figures from the “Nautical Almanac”
+will illustrate this point:</p>
+
+<table class='table2'>
+  <tr>
+    <th class='c019'></th>
+    <th class='c019'>&#160;</th>
+    <th class='c009'>&#160;</th>
+    <th class='c019'>Sun’s apparent latitude.</th>
+    <th class='c021'>Moon’s latitude.</th>
+  </tr>
+  <tr>
+    <td class='c019'>1896,</td>
+    <td class='c019'>April</td>
+    <td class='c009'>1</td>
+    <td class='c019'>0″·70 S.</td>
+    <td class='c021'>5° &#8196;9′ S.</td>
+  </tr>
+  <tr>
+    <td class='c019'>&#160;</td>
+    <td class='c019'>„</td>
+    <td class='c009'>10</td>
+    <td class='c019'>0″·01 N.</td>
+    <td class='c021'>1° 41′ N.</td>
+  </tr>
+  <tr>
+    <td class='c019'>&#160;</td>
+    <td class='c019'>„</td>
+    <td class='c009'>16</td>
+    <td class='c019'>0″·39 N.</td>
+    <td class='c021'>5° &#8196;6′ N.</td>
+  </tr>
+  <tr>
+    <td class='c019'>&#160;</td>
+    <td class='c019'>„</td>
+    <td class='c009'>22</td>
+    <td class='c019'>0″·07 S.</td>
+    <td class='c021'>0° 48′ N.</td>
+  </tr>
+  <tr>
+    <td class='c019'>&#160;</td>
+    <td class='c019'>„</td>
+    <td class='c009'>29</td>
+    <td class='c019'>0″·74 S.</td>
+    <td class='c021'>5° &#8196;1′ S.</td>
+  </tr>
+</table>
+
+<p class='c007'><span class='pageno' id='Page_78'>78</span>The displacement in right ascension amounts to a little
+over 6″, and is, therefore, large enough to be directly
+measurable.</p>
+
+<p class='c007'>On account of this association with her satellite, the earth’s
+centre moves some hundreds of miles above and below the
+plane of the ecliptic.</p>
+
+<p class='c007'>The so-called “perturbations,” or disturbing effects of the
+other planets, also cause the earth to depart more or less from
+the plane of the ecliptic and from a geometrical elliptic path.
+Nevertheless, these disturbances can be calculated and
+allowed for, so that when we speak of the earth’s orbit we
+really mean the path which the centre of gravity of the earth
+and moon would traverse if subject only to the influence of
+the sun.</p>
+
+<h3 class='c004'>CHAPTER V.<br> <span class='c014'>MEAN SOLAR TIME.</span></h3>
+
+<p class='c003'><span class='sc'>Sun-Dial Time.</span>—The changing directions of shadows
+thrown by the sun have been utilised from very remote
+periods for the measurement of time, the instrument usually
+employed being a sun-dial. On account of the varying
+declination of the sun, it is necessary to employ as a time-measurer
+the shadow of a line which lies parallel to the earth’s
+axis, that is, if we wish the same hour marks to be permanently
+useful. Such a rod must lie in the plane of the
+meridian, and be inclined to the horizon at an angle equal to
+the latitude of the place. If the shadow be received on a
+horizontal dial, hours may be marked upon it corresponding
+to the duration of the longest day at the place where it is set
+up. Sometimes, as on old churches, one sees a vertical sun-dial,
+the rod, or <i>style</i>, as it is called, being still parallel to the
+<span class='pageno' id='Page_79'>79</span>earth’s axis, but as a dial facing the south is only serviceable
+for twelve hours, another on the north wall is necessary
+for times before six in the morning and after six in the
+evening. As indicated by the sun-dial, it will always be
+noon when the sun is on the meridian, that is, when it is due
+south.</p>
+
+<p class='c007'>The time indicated by sun-dials is distinguished astronomically
+as <i>apparent time</i>, and an <i>apparent solar day</i> is the
+time which elapses between two successive southings of the
+sun. It is longer than the sidereal day, for the reason that
+the sun moves eastward among the stars.</p>
+
+<p class='c007'><span class='sc'>Necessity for Mean Time.</span>—The varying speed of the
+earth in its orbit, or what comes to the same thing, the variable
+rate of the sun’s apparent eastward movement, prepares
+us for the discovery that the intervals between successive
+noons as indicated by sun-dials are unequal. That is, the
+apparent solar day is not of uniform length, and our clocks
+could not be regulated to indicate noon at the same moments
+as the sun-dial unless they were rated afresh every day. All
+our daily actions are regulated by the sun, and our time-keepers
+must also be controlled by its movement if they are
+to be as convenient as is necessary for purposes of everyday
+life. Our clocks and watches are therefore regulated to
+measure twenty-four hours in the time corresponding to the
+average duration of the apparent solar day throughout a year.
+In other words, they are controlled by the movements of an
+imaginary sun, called the <i>mean sun</i>, which is supposed to
+come to the meridian after equal intervals, and in order that
+it may do this while having a uniform motion, it must of
+necessity move along the celestial equator. In this way the
+time shown by our clocks and watches never departs very
+greatly from that shown by sun-dials, the maximum discrepancy
+being little more than a quarter of an hour. A
+<i>mean solar day</i> is thus the average length of the apparent
+solar days throughout a year.</p>
+
+<p class='c007'><span class='sc'>The Equation of Time.</span>—The difference between apparent
+and mean solar time is called the <i>equation of time</i>, and a
+<span class='pageno' id='Page_80'>80</span>knowledge of its amount enables us to determine mean time
+from an observation of apparent time.</p>
+
+<p class='c007'>One of the causes of this difference we have already seen
+to be the varying speed of the earth in its orbital movement;
+this produces a correspondingly irregular motion of the sun
+amongst the stars, and in consequence the true sun comes to
+the meridian after unequal intervals. Neglecting for a
+moment another cause of the varying length of the day, the
+relation of the apparent and mean solar days would be somewhat
+as follows:—Let us suppose that when the earth is at
+perihelion, we set our clocks to the same time as the sun-dial.
+In the interval which elapses before noon next day the
+true sun will have moved faster than the mean sun, because
+the earth, which produces the apparent eastward movement
+of the sun, is then travelling at its greatest speed. Consequently,
+our meridian will overtake the mean sun before it
+comes up to the true sun, and mean noon will occur before
+apparent noon; the difference will be the equation of time
+for the day, and it must evidently be added to apparent
+time in order to give mean time. This will go on for a
+certain period, when, in consequence of the reduced rate of
+the earth’s orbital velocity, the suns eastward motion will be
+less than that of the mean sun, and the two will again come
+to the meridian at the same time when the earth reaches its
+aphelion point; clocks and sun-dials would then give identical
+times. After aphelion passage, the earth is moving slowly,
+and the apparent eastward velocity of the true sun will be
+less than that of the mean; our meridian will therefore come
+to the true sun before it overtakes the mean sun, so that
+apparent noon will precede mean noon, and the equation of
+time will have to be subtracted from apparent time to give
+mean time. The two suns would again come together when
+the earth reached perihelion, and the equation of time, so far
+as this cause was concerned, would vanish. As the earth’s
+orbit is only slightly elliptical, the equation of time due to
+this cause alone would never amount to more than seven
+minutes.</p>
+
+<p class='c007'><span class='pageno' id='Page_81'>81</span>This, however, is by no means the whole cause of the equation
+of time; a still greater source of variation is the obliquity
+of the ecliptic. To investigate the part played by this
+inclination of the fundamental planes, let us now suppose that
+the true sun has a uniform angular motion in the ecliptic,
+while the mean sun moves uniformly along the Equator.
+Both these fictitious suns would have the same rate of movement
+along their respective paths, since they come back to
+the same places after the lapse of a year. If, then, these two
+suns start together at the equinox, both would indicate noon
+at that time, and there would be no equation of time. The
+“ecliptic sun” would then be moving at an angle of 23½° to
+the Equator, as along <i>a b</i> in Fig. 17. If the distance <i>a b</i>
+represents the average
+daily movement of the
+“ecliptic” sun, and <i>d c</i>
+the equal movement of
+the mean sun, it is clear
+that our meridian will
+overtake the true sun at
+<i>b</i> before the mean sun
+at <i>c</i>, so that apparent
+noon will precede mean
+noon, and the equation of time must be subtracted from
+apparent time to give mean time. The difference becomes
+greater up to a certain limit, and then since both suns will
+traverse 90° in the same time, they will pass the meridian
+together at the solstice.</p>
+
+<div id='i_081'  class='figcenter id001'>
+<img src='images/i_081.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 17.</span>—<i>Effect of Obliquity of Ecliptic upon the Equation of Time.</i></p>
+</div>
+</div>
+
+<p class='c007'>In the next quarter of a revolution, from solstice to equinox
+the difference is similar, but in the opposite direction, and
+the same applies to successive quadrants described throughout
+the year.</p>
+
+<p class='c007'>The net amount of the equation of time at any moment
+is thus the added effects due to two causes.</p>
+
+<p class='c007'>In 1896 the greatest and least values of the equation of
+time at Greenwich mean noon were as follows:—</p>
+
+<table class='table2'>
+  <tr><td class='c008' colspan='4'><span class='pageno' id='Page_82'>82</span></td></tr>
+  <tr>
+    <th class='c022'></th>
+    <th class='c009'>M.</th>
+    <th class='c009'>S.</th>
+    <th class='c020'>&#160;</th>
+  </tr>
+  <tr>
+    <td class='c022'>Feb. 11</td>
+    <td class='c009'>14</td>
+    <td class='c009'>27</td>
+    <td class='c020'>to be added to apparent time.</td>
+  </tr>
+  <tr>
+    <td class='c022'>April 14</td>
+    <td class='c009'>0</td>
+    <td class='c009'>7</td>
+    <td class='c021'>„         „        „</td>
+  </tr>
+  <tr>
+    <td class='c022'>May 13</td>
+    <td class='c009'>3</td>
+    <td class='c009'>50</td>
+    <td class='c020'>to be subtracted from apparent time.</td>
+  </tr>
+  <tr>
+    <td class='c022'>June 13</td>
+    <td class='c009'>0</td>
+    <td class='c009'>6</td>
+    <td class='c021'>„         „        „</td>
+  </tr>
+  <tr>
+    <td class='c022'>July 25</td>
+    <td class='c009'>6</td>
+    <td class='c009'>17</td>
+    <td class='c020'>to be added to apparent time.</td>
+  </tr>
+  <tr>
+    <td class='c022'>August 31</td>
+    <td class='c009'>0</td>
+    <td class='c009'>0</td>
+    <td class='c021'>„         „        „</td>
+  </tr>
+  <tr>
+    <td class='c022'>Nov. 2</td>
+    <td class='c009'>16</td>
+    <td class='c009'>20</td>
+    <td class='c020'>to be subtracted from apparent time.</td>
+  </tr>
+  <tr>
+    <td class='c022'>Dec. 24</td>
+    <td class='c009'>0</td>
+    <td class='c009'>7</td>
+    <td class='c020'>to be added to apparent time.</td>
+  </tr>
+</table>
+
+<p class='c007'>A somewhat notable effect, owing its origin to the equation
+of time, is seen in the times of sunrise and sunset given in our
+almanacs. On November 8, for example, the sun rises at Greenwich
+at 6h. 58m., and sets at 4h. 31m., thus apparently making
+the afternoon about half an hour longer than the morning. As
+reckoned by the sun-dial, however, the morning and afternoon
+would differ only by a few seconds, and the peculiarity noted
+arises simply from the fact that our clocks keep time with
+the mean, and not with the true sun.</p>
+
+<p class='c007'><span class='sc'>Determination of Time.</span>—Although the sun-dial may be
+used to indicate the time of day with sufficient accuracy for
+some purposes, its use is limited by the fact that it can only
+be employed when the sun is visible at the place of observation.
+Other modes of measuring the flow of time have,
+therefore, long been adopted. In early days, the rate at which
+a candle burned, or at which water or sand escaped through a
+small aperture, was employed as a time-measurer. Coming
+to more recent times, clocks and watches serve a similar purpose,
+but from what has already been stated, it is evidently
+necessary to regulate them according to the results of astronomical
+observations.</p>
+
+<p class='c007'>The most precise determinations of time are made by means
+of a transit instrument, that is, an instrument by which the
+exact moment at which a celestial body passes the meridian
+can be observed. The positions of certain fundamental stars
+called “clock stars” have been determined with great accuracy,
+and it is therefore known to within a very small fraction of a
+second at what sidereal time one of these stars will pass the
+<span class='pageno' id='Page_83'>83</span>meridian. If the sidereal clock does not indicate this time
+when the star is observed on the meridian, its error can be
+noted and corrected. In this way the sidereal time is ascertained,
+and its equivalent in mean solar time is only a matter
+of simple calculation.</p>
+
+<p class='c007'>Another method is to observe, by means of a sextant, or
+an altazimuth, the time, by a clock, at which the sun or a star
+has a certain altitude before noon, and the time at which it
+has the same altitude after noon. Midway between these
+times marks the time at which the body passed the meridian;
+the true sidereal time of passage is furnished by the known
+right ascension, and the corresponding mean time can therefore
+be calculated.</p>
+
+<p class='c007'>At sea, time is most frequently determined by observing
+the altitude of the sun in the morning or evening, when it is
+nearly in an east or west direction. The time by the chronometer
+corresponding to a certain altitude of the sun is noted,
+and by spherical trigonometry the apparent solar time is
+deduced; mean solar time is then obtained by correcting for
+the equation of time. The nearer the sun is to due east or
+west, the more accurate are the results obtained by this method.</p>
+
+<p class='c007'><span class='sc'>Time at Different Places.</span>—In all these methods of
+finding the time, <i>local time</i> is alone determined, whether it be
+sidereal or solar. When solar time is in question, we have
+seen that mean noon is determined by the passage of the
+mean sun across the meridian. All places on the same
+meridian will thus have equal times; but at places on different
+meridians, the local times will be different. When it
+is noon at Greenwich, it will be something before noon at
+places to the west of Greenwich (for the reason that the sun
+has not yet crossed their meridians), while at places to the
+cast it will be afternoon, because the sun has already passed
+the meridian. As the earth rotates through 360° in a day, it
+will turn 15° in an hour, or 1° in four minutes. Hence at
+places 15° east of Greenwich the time will be an hour in
+advance of Greenwich time, while at places 15° west it will be
+an hour earlier. For places in other longitudes, the difference
+<span class='pageno' id='Page_84'>84</span>of time is in the same proportion. The following are the
+local times at several places when it is noon at Greenwich:—</p>
+
+<table class='table2'>
+  <tr>
+    <th class='c010'></th>
+    <th class='c019'>A.M.</th>
+    <th class='c010'>&#160;</th>
+    <th class='c021'>P.M.</th>
+  </tr>
+  <tr>
+    <td class='c010'>Dublin</td>
+    <td class='c009'>11.35</td>
+    <td class='c010'>Paris</td>
+    <td class='c023'>0.9</td>
+  </tr>
+  <tr>
+    <td class='c010'>New York</td>
+    <td class='c009'>7.4</td>
+    <td class='c010'>Berlin</td>
+    <td class='c023'>0.54</td>
+  </tr>
+  <tr>
+    <td class='c010'>Toronto</td>
+    <td class='c009'>6.42</td>
+    <td class='c010'>Calcutta</td>
+    <td class='c023'>5.53</td>
+  </tr>
+  <tr>
+    <td class='c010'>Vancouver</td>
+    <td class='c009'>3.38</td>
+    <td class='c010'>Melbourne</td>
+    <td class='c023'>9.40</td>
+  </tr>
+</table>
+
+<p class='c007'>Throughout the whole of England and Scotland, Greenwich
+mean time is exclusively employed in preference to local
+times. This has the very practical advantage of uniformity;
+and as in no case does local time differ more than half an hour
+from Greenwich time, there is little inconvenience in regard
+to the beginning and end of day.</p>
+
+<p class='c007'>Until recently, the time systems of other countries have
+been mainly based on the times corresponding to their various
+national observatories. At present, what is called “zone
+time,” in which the hours alone differ from Greenwich time,
+has been adopted in several European states, as well as in
+other parts of the world.</p>
+
+<p class='c007'>The present state of time reckoning on this much improved
+plan is indicated by the following table:—</p>
+
+<table class='table1'>
+  <tr>
+ <th class='bbt c017'><i>Country.</i></th>
+ <th class='bbt blt c024'><i>Standard time.</i></th>
+  </tr>
+  <tr>
+ <td class='bbt c025'>England<br>Belgium<br>Holland</td>
+ <td class='bbt blt c026'>Greenwich time.</td>
+  </tr>
+  <tr>
+ <td class='bbt c025'>Denmark<br>Germany<br>Italy<br>Switzerland<br>Norway and Sweden</td>
+ <td class='bbt blt c026'>Mid-European time, 1 hour fast on Greenwich.</td>
+  </tr>
+  <tr>
+ <td class='bbt c025'>Colony of Natal</td>
+ <td class='bbt blt c026'>2 hours fast on Greenwich.</td>
+  </tr>
+  <tr>
+ <td class='bbt c025'>United States<br>Canada</td>
+ <td class='bbt blt c026'>4, 5, 6, 7, or 8 hours slow on Greenwich, according to longitude.</td>
+  </tr>
+  <tr>
+ <td class='bbt c025'>Japan</td>
+ <td class='bbt blt c026'>9 hours fast on Greenwich.</td>
+  </tr>
+  <tr>
+ <td class='bbt c025'>Western Australia</td>
+ <td class='bbt blt c026'>8 „ „ „</td>
+  </tr>
+  <tr>
+ <td class='bbt c025'>South Australia</td>
+ <td class='bbt blt c026'>9 „ „ „</td>
+  </tr>
+  <tr>
+ <td class='bbt c025'>Victoria, New South Wales, Queensland, and Tasmania</td>
+ <td class='bbt blt c026'>10 „ „ „</td>
+  </tr>
+</table>
+
+<p class='c007'><span class='sc'>Telegraphing Time.</span>—An important part of the work of
+<span class='pageno' id='Page_85'>85</span>the chief national observatories is the determination of correct
+time, and its communication to the public at large. Railways
+have especially created a demand for a uniform and accurate
+system of time reckoning, and to meet this need there is
+usually an organised service providing an automatic distribution
+of time-signals by means of the electric telegraph. The
+transmission of such time-signals was first established on a
+large scale in connection with Greenwich Observatory, and at
+the present time signals are sent to the General Post Office,
+whence they are distributed automatically to post offices and
+subscribers throughout the kingdom. In addition, signals are
+sent direct to Westminster for the regulation of the great clock
+on the Houses of Parliament, and time-balls are dropped at
+certain hours at Greenwich and Deal, in order that navigators
+may have the opportunity of rectifying their chronometers.</p>
+
+<p class='c007'><span class='sc'>The Year.</span>—The day is too small an interval of time to be
+conveniently employed as a unit for chronological purposes,
+so that at present the count of time by days is practically
+limited to the number of days in a month. A greater unit,
+but still too small, is supplied by the month, and the necessity
+for a more serviceable unit early led to the adoption of the
+length of the year. This is at once a natural division of time,
+corresponding to the recurrence of the seasons, and sufficiently
+answers all requirements for measuring extended intervals.</p>
+
+<p class='c007'>If we determine the exact time required by the sun to pass
+from one fixed point in the heavens to the same point again,
+we shall find the time in which the earth makes a complete
+revolution round the sun, that is, the time in which a line
+joining the earth and sun sweeps through an angle of 360°.
+This interval, which is called the <i>sidereal year</i>, amounts to
+365 days 6 hours 9 minutes 9 seconds of mean solar time. It
+will be clear, however, that the most useful year is that which
+will give us the same day of the month at the same season in
+all years. If there were no precession of the equinoxes, this
+would be of the same length as the sidereal year, but on
+account of precession the passage of the sun from the vernal
+equinox to the same equinox again occupies less than a
+<span class='pageno' id='Page_86'>86</span>sidereal year. In fact, this equinoctial, or <i>tropical year</i>
+amounts to 365 days 5 hours 48 minutes 46 seconds; that is,
+about 20 minutes less than the sidereal year. This is the
+year which is always understood, unless it is otherwise stated.
+If our calendars were regulated according to the sidereal
+year, the same day of the month would in time run through
+all possible changes of seasons, the 25th of December, for
+instance, occurring at one time in winter, and gradually
+changing through spring, summer, and autumn.</p>
+
+<p class='c007'><span class='sc'>The Calendar.</span>—The earlier calendars with which
+history acquaints us were mainly based on the lunar month
+of about 29½ days, twelve of which made up a lunar year of
+354 days. The calendar year was thus more than 11 days
+shorter than the actual year, and in order to bring the dates
+into agreement with the seasons, arbitrary intercalations were
+occasionally made by the authorities.</p>
+
+<p class='c007'>In the year 45 <span class='fss'>B.C.</span> a great reform was introduced by Julius
+Cæsar; 365¼ days was adopted as the length of the year, and
+it was prescribed that ordinary years should be reckoned as
+consisting of 365 days, while every fourth year divisible by
+4 without remainder should be a <i>leap year</i> of 366 days.
+Matters were so much simplified by this arrangement that
+the Julian calendar remained unaltered until 1582, and is
+even now retained throughout Russia.</p>
+
+<p class='c007'>The tropical year, as we have seen, is less than 365¼ days,
+so that the Julian calendar does not quite keep course with
+the seasons. Although the difference is only 11¼ minutes, it
+amounts to an entire day in 128 years, so that if the vernal
+equinox occurred on the 21st of March at one time it would
+occur on the 20th after 128 years. If, then, it be desired to
+bring the existing dates of any particular year into agreement
+with dates at a previous period, as regards the seasons, a
+correction in addition to that ordained by Cæsar must be
+introduced. In the time of Pope Gregory, in the year 1582,
+the vernal equinox fell on the 11th of March, and the
+necessity of a new calendar came to be recognised. The
+astronomer Clavius, with the authority of the Pope, devised
+<span class='pageno' id='Page_87'>87</span>our present “Gregorian” calendar. This arrangement, first
+of all, altered the actual date of the equinox from the 10th to
+the 21st of March, that is, to the day on which it occurred in
+the year of the great Council of the Church at Nicæa, 325 <span class='fss'>A.D.</span>
+To bring about this alteration it was necessary to drop 10
+days from the calendar, and it was therefore decided that the
+day following the 4th of October, 1582, should be called the
+15th instead of the 5th. To prevent subsequent changes in
+the date of the equinox the Julian rule for leap year was
+slightly modified. If the date number of a year is divisible
+by 4 without remainder it is still to be a leap year, unless it
+be a century year, in which case it must be divisible by 400
+without remainder if it is to be called a leap year.</p>
+
+<p class='c007'>It was not until 1752 that the Gregorian calendar was
+adopted in England, and as 1700 was a leap year according
+to the Julian rule the old style date was 11 days behind the
+Gregorian date. An Act of Parliament decreed that the day
+following September 2, 1752, should be called the 14th. The
+Act was carefully planned so as to prevent injustice in the
+collection of rents and the like, but it was only accepted after
+considerable opposition.</p>
+
+<p class='c007'>It has lately been pointed out that if we wish to make the
+day of the year correspond with the seasons for all time, a
+modification of the Gregorian calendar must be adopted. By
+the Gregorian rule, three leap years are omitted every four
+centuries; but Mr. W. T. Lynn has drawn attention to the fact
+that if one were dropped every 128 years instead, the calendar
+would be sensibly perfect, and the seasons would always
+commence on the same dates.</p>
+
+<h3 class='c004'>CHAPTER VI.<br> <span class='c014'>THE MOVEMENTS OF THE MOON.</span></h3>
+
+<p class='c003'><span class='sc'>The Moon’s Revolution.</span>—Apart from the changes in
+the appearance of the moon due to the ever-varying phases,
+<span class='pageno' id='Page_88'>88</span>the first fact which strikes the attentive observer is that the
+moon has an eastward movement among the stars, and that
+this motion is much more rapid than that of the sun. Indeed,
+the moon gains a whole revolution upon the sun in a period
+of about 29½ days, this being the interval between two
+successive new or full moons. As referred to the stars,
+however, it is found that the moon and any particular star
+which cross the meridian together at a certain time will again
+do so after the lapse of only 27⅓ days. Besides this eastward
+movement among the stars, the moon moves towards and
+away from the Pole; the full moon, for instance, is sometimes
+seen high in the heavens at midnight, and at other times very
+low. Indeed, the moon’s apparent movements resemble in a
+very general way those of the sun, but they cannot be
+attributed to a revolution of the earth round the moon, as
+those of the sun are to a real movement of the earth
+round the sun. We have seen that there are direct proofs of
+the earth’s revolution round the sun, and a revolution round
+the moon, even in a smaller orbit, would not be consistent
+with the observed movements of the greater luminary. Being
+convinced of the reality of the moon’s movements around the
+earth, we can next proceed to investigate the circumstances
+of its varied motions.</p>
+
+<p class='c007'>Just as we learn the conditions of the earth’s movements
+by observations of the sun’s apparent movements which
+are their natural consequence, we can determine the moon’s
+motions by studying its varying situations with regard to the
+much more distant stars. We can measure the moon’s right
+ascension and declination at different times with the transit
+instrument, and, if desired, we can mark out the apparent
+path on our star charts or celestial globes. In this way
+it is found that the moon moves in a plane which
+is inclined at 5° 9′ to the plane of the ecliptic. As to
+the shape of the orbit, we have only to observe the
+changes in the moon’s apparent size; when it is nearest to
+us it will appear largest, and when furthest removed its
+apparent diameter will be least. Actual observations show
+<span class='pageno' id='Page_89'>89</span>that, like the orbit of the earth, the moon’s orbit is an ellipse,
+with the earth in one focus. Owing to various causes, the
+orbit is somewhat variable in shape, and its eccentricity ranges
+from 0·07 to 0·045. When the moon is at the point of its
+orbit nearest to the earth, it is said to be in <i>perigee</i>; and when
+at the most distant part of its orbit, in <i>apogee</i>.</p>
+
+<p class='c007'>The earth’s orbit, as we shall see by and by, is very small
+as compared with stellar distances, and the moon’s apparent
+movement, with regard to the stars, is not affected by the
+revolution of the earth and moon round the sun; consequently
+the interval between its passing a star and overtaking the
+same star again is a measure of the time in which the moon’s
+movement round the earth is performed—this is 27 days,
+7 hours, 43 minutes, and is called the moon’s <i>sidereal period</i>.
+The direction of the moon’s motion is opposite to that of
+the hands of a clock, a movement which is said to be <i>direct</i>
+(motion in the reverse direction would be <i>retrograde</i>).</p>
+
+<p class='c007'><span class='sc'>Phases.</span>—Two circumstances lead us to suppose that the
+light of the moon is borrowed from the vast store thrown out
+into space by the sun. First, the fact that it puts on <i>phases</i>,
+for if it were a body shining by its own light we should
+always see a full moon. Second, the fact that the phase we
+see depends absolutely on the moon’s situation with regard
+to the sun and earth.</p>
+
+<p class='c007'>There is every reason to suppose that the moon is a dark
+globular body, so that the sun can only illuminate that
+hemisphere which is turned towards it. At new moon the
+illuminated part is turned directly away from us, and we
+are thus led to infer that when new the moon lies directly
+between the earth and sun. At full moon, on the contrary,
+the whole of the illuminated part is presented to us, and we
+therefore conclude that at this time the earth lies between the
+sun and moon. On account of the inclination of the moon’s
+orbit to that of the earth, the sun, earth, and moon do not
+always come exactly in a straight line at new or full moon;
+when they do, the interesting phenomena of solar and lunar
+eclipses occur. (Chapter VIII.)</p>
+
+<p class='c007'><span class='pageno' id='Page_90'>90</span>A diagram will help to elucidate the production of the
+moon’s intermediate phases. Supposing the sun’s rays to
+proceed from the left, the earth being at O, the moon will be
+at A when new. Proceeding towards B, a small portion of
+the illuminated side will be turned towards us, and the moon
+will be a crescent. On reaching the point C, exactly half of
+the sunlit hemisphere will be visible to us, and we have the
+moon’s <i>first quarter</i>. Passing to the point D we see more
+than half of the bright part of our satellite, and it appears
+gibbous in form, until it reaches E, where it becomes full.
+Similar phases occur in inverse order during the movement
+along the other part of the orbit.</p>
+
+<div id='i_090'  class='figcenter id003'>
+<img src='images/i_090.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 18.</span>—<i>The Moon’s Phases.</i></p>
+</div>
+</div>
+
+<p class='c007'>Such would be the conditions as to the phases of the moon,
+if the earth were at rest.</p>
+
+<p class='c007'><span class='sc'>The Month.</span>—If the earth were fixed in space with
+regard to the sun, the moon’s phases would be repeated in
+the time corresponding to its period of revolution round the
+earth. This is 27 days 7 hours 43 minutes, and measures the
+length of a sidereal month.</p>
+
+<p class='c007'>It is much more useful, however, to refer the month to the
+phases actually observed. If in Fig. 19 we have the sun,
+<span class='pageno' id='Page_91'>91</span>earth, and moon represented at a full moon by S, E, and M
+respectively, the next full moon will not occur until the three
+bodies occupy the positions S, E′, and M′, the earth having
+travelled about 30° along its orbit. Between two full moons,
+then, the moon must make a complete revolution round the
+earth, and through an additional angle, A E′ M′, which will
+be equal to the earth’s angular motion in the interval. This
+movement of the moon occupies 29 days 12 hours 44 minutes,
+and is the duration of a <i>lunar month</i>. It also determines the
+<i>synodic period</i> of our satellite, a term which, taken generally,
+signifies the period in which a planet or satellite recovers the
+same position with respect to the sun when observed from
+the earth.</p>
+
+<div id='i_091'  class='figcenter id001'>
+<img src='images/i_091.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 19.</span>—<i>The Lunar Month.</i></p>
+</div>
+</div>
+
+<p class='c007'>A calendar month, of which there are twelve in a year,
+must of necessity consist of a whole number of days, and the
+average duration of such a month is longer than that of a
+lunar month.</p>
+
+<p class='c007'>A remarkable relation exists between the synodic month and
+<span class='pageno' id='Page_92'>92</span>the length of the year. In 19 Julian years of 365¼ days there
+are almost exactly 235 synodic months, so that after the completion
+of this period full moons again occur on the same
+days of the month. The discovery of this cycle is usually
+ascribed to Meton, a Greek astronomer, 433 <span class='fss'>B.C.</span> It is
+accordingly known as the <i>Metonic Cycle</i>, and is still used in
+the calculation of the moveable festival of Easter.<a id='r1'></a><a href='#f1' class='c027'><sup>[1]</sup></a></p>
+
+<p class='c007'><span class='sc'>Rotation and Librations.</span>—Even observations made
+without instrumental assistance show that the surface of our
+satellite always presents the same face to us, and without
+further inquiry one might suppose that it had no axial movement
+corresponding to that of its primary. If there were no
+rotation, however, we should in turn see all parts of the moon,
+and the observed circumstances indicate that it must rotate
+on an axis, in the same direction as that of its orbital movement,
+and in the same time. In Fig. 20 let E represent the
+earth, and <i>a b c</i> the part of the moon which is turned towards
+us when it is at M. When the moon arrives at M′, observations
+show us that the same part is presented to our view, so
+that the part corresponding to that we saw in position M is
+represented by <i>a′ b′ c′</i>. Now, if the moon had not rotated in
+the interval, the line joining <i>a</i> and <i>c</i> would have retained the
+same direction, and would have been in the position <i>d e</i>; the
+part <i>c′ e</i> would thus have been carried out of sight, while
+another part which was not seen when the moon was at M
+would have come into view. In order that we may see the
+same part of the moon in two different positions, M and M′,
+the dividing-line <i>a c</i> between the visible and invisible portions
+must turn through an angle equal to that between the lines
+<i>d e</i> and <i>a′ c′</i>; and since this angle is equal to that described
+by the moon in the same time, the period of the moon’s
+rotation on its axis must be equal to that of its revolution
+round the earth.</p>
+
+<p class='c007'>On account of the elliptical form of its orbit, the angular
+movement of the moon is not quite uniform; like the earth, it
+<span class='pageno' id='Page_93'>93</span>is subject to the law of areas. Hence, as the rotation is
+equable, the foregoing explanation does not strictly hold.
+In fact, this varying velocity results in a <i>libration in longitude</i>,
+which means that we sometimes see a little more of the
+western edge and sometimes of the eastern edge. There is
+also a <i>libration in latitude</i> on account of the fact that the
+moon’s axis is inclined to the plane of its orbit, so that at
+different times we see more of the North or South Pole, as
+the case may be; in this respect the moon behaves to the
+earth somewhat as the earth does to the sun in regard to the
+seasons, but the inclination is not so great.</p>
+
+<div id='i_093'  class='figcenter id001'>
+<img src='images/i_093.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 20.</span>—<i>The Moon’s Rotation.</i></p>
+</div>
+</div>
+
+<p class='c007'>The moon is so near to us that the portion of it which we
+see depends to a slight extent upon our terrestrial location.
+When the moon is rising we see a little more of its western
+edge than will be seen by an observer to the east of us, where
+the moon is in the south, and more than we ourselves shall
+see when it has come to our own meridian. Just before the
+time of setting we get to see a little beyond the eastern edge.
+This is called the <i>diurnal libration</i>, and never amounts to
+more than a degree.</p>
+
+<p class='c007'><span class='pageno' id='Page_94'>94</span>Thanks to these librations, we are enabled to make telescopic
+observations of 9 per cent. of the moon’s surface which
+would not otherwise be open to our investigations.</p>
+
+<p class='c007'><span class='sc'>Changes of the Moon’s Orbit.</span>—The moon’s orbit is by
+no means to be regarded as a hard and fast geometrical
+figure. Indeed, it is subject to such great distortions in consequence
+of “perturbations” that the computation of the
+moon’s position at any future time is one of great complexity.
+One of the most easily recognised changes in the orbit is the
+revolution of its <i>nodes</i>, that is, of the points where it crosses
+the plane of the ecliptic.</p>
+
+<div id='i_094'  class='figcenter id003'>
+<img src='images/i_094.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 21.</span>—<i>The Moon’s Nodes.</i></p>
+</div>
+</div>
+
+<p class='c007'>The latter being a plane of indefinite extent, to which the
+moon’s orbit is inclined at 5° 9′, the moon will be alternately
+above and below the ecliptic for about half its period of revolution.
+The point where it passes from south to north of
+the ecliptic, A in Fig. 21, is the <i>ascending node</i>, and the
+corresponding point on its southward path is the <i>descending
+node</i> of the orbit. Connecting these two points is the line of
+nodes (A B), and by observations of the points where the
+moon’s path intersects the ecliptic at different times it is
+found that the line of nodes <i>regredes</i> or moves backwards.
+<span class='pageno' id='Page_95'>95</span>The rate of this revolution of the moon’s nodes is very
+irregular, but a whole revolution is made in 18·6 years.</p>
+
+<p class='c007'>This retrogression of the moon’s nodes may be well
+illustrated by the following heliocentric longitudes of the
+ascending node as given in recent “Nautical Almanacs”:</p>
+
+<table class='table2'>
+  <tr>
+    <td class='c019'>1892</td>
+    <td class='c019'>January 1</td>
+    <td class='c023'>53° 51′·56.</td>
+  </tr>
+  <tr>
+    <td class='c019'>1893</td>
+    <td class='c019'>„</td>
+    <td class='c023'>34° 28′·69.</td>
+  </tr>
+  <tr>
+    <td class='c019'>1894</td>
+    <td class='c019'>„</td>
+    <td class='c023'>15° 19′·00.</td>
+  </tr>
+  <tr>
+    <td class='c019'>1895</td>
+    <td class='c019'>„</td>
+    <td class='c023'>355° 49′·31.</td>
+  </tr>
+  <tr>
+    <td class='c019'>1896</td>
+    <td class='c019'>„</td>
+    <td class='c023'>336° 29′·61.</td>
+  </tr>
+</table>
+
+<p class='c007'>The line of apsides of the moon’s orbit joins the perigee
+and apogee; the direction of this line in space changes in a
+very variable manner, but in the long run it makes a complete
+revolution in 8·9 years.</p>
+
+<p class='c007'>When the sun is passing through the moon’s line of apsides
+it temporarily increases the eccentricity of the orbit; when
+at right angles to this line, the orbit becomes more nearly
+circular. This disturbance of the moon has accordingly a
+period equal to that required for two successive passages of
+the sun over the apse line of the moon’s orbit.</p>
+
+<p class='c007'>Such are a few of the movements which come within the
+province of the <i>lunar theory</i>, a fuller treatment of which is
+beyond our scope.</p>
+
+<p class='c007'><span class='sc'>The Harvest Moon.</span>—The full moon which occurs
+nearest to the autumnal equinox is called the <i>harvest moon</i>,
+for the reason that it rises very nearly at the same hour for
+several nights together, and so gives us a greater share of
+moonlight, by which harvest operations may be extended.
+At that time the sun will be at the autumnal equinoctial
+point, and when it is setting in the west, the vernal equinoctial
+point, and the moon with it, must be rising due east.
+The part of the ecliptic then above the horizon will extend
+from the east to the west point, but will lie wholly below the
+celestial equator (Fig. 22). As the moon’s path is very slightly
+inclined to the ecliptic, its movement will thus make only a
+<span class='pageno' id='Page_96'>96</span>small angle with the horizon, and for several nights together
+it will rise at nearly the same time.</p>
+
+<p class='c007'>In March, when the sun is near the vernal equinox, the
+full moon will be near the autumnal equinoctial point; when
+the sun is setting, the moon will be rising as before, but in
+this case the part of the ecliptic which is above the horizon
+lies wholly above the celestial equator. The ecliptic is thus
+inclined at an angle to the horizon greater by 47° than when
+the vernal equinox is rising in autumn; the moons path
+being near the ecliptic, its movement during a day will at this
+time carry it a long way below the Equator, and it will rise
+much later the following day.</p>
+
+<div id='i_096'  class='figcenter id003'>
+<img src='images/i_096.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 22.</span>—<i>Position of Ecliptic at Sunset at Vernal Equinox</i> (E A W) <i>and Autumnal Equinox</i> (E B W).</p>
+</div>
+</div>
+
+<p class='c007'>In the Southern Hemisphere, the conditions are reversed,
+the harvest moon occurring at our vernal equinox, which,
+however, is the commencement of the southern autumn quarter.</p>
+
+<p class='c007'>The phenomena of the harvest moon recur, but are not so
+marked, in the month of October, and it is then called the
+hunter’s moon.</p>
+
+<p class='c007'>It is important to bear in mind that this rising of the moon
+at nearly the same hour for several days occurs every month,
+<span class='pageno' id='Page_97'>97</span>but as the risings then occur either in daylight or after midnight,
+and the moon is not full, no special attention is drawn
+to them.</p>
+
+<p class='c007'>Again, since the phenomenon of the harvest moon depends
+upon the small inclination of the path of the full moon to the
+horizon when it is at the equinoctial point, the circumstances
+will be modified by the latitude of the place of observation.
+At the Equator, for example, there will be no harvest moon,
+as there the ecliptic is always greatly inclined to the horizon;
+in fact, it will be inclined at the same angle in spring as in
+autumn.</p>
+
+<p class='c007'>The moon’s path being inclined to the ecliptic, the conditions
+as to the harvest moon will depend to a small extent
+upon the position of the moon’s nodes, which, as we have
+seen, revolve in a period of a little less than 19 years. At
+times, then, the moon’s path will be inclined 5° more, and 9
+years afterwards 5° less, than is the plane of ecliptic, and
+under the latter conditions the harvest moon will be most
+pronounced.</p>
+
+<p class='c007'><span class='sc'>High and Low Moons.</span>—At the time of full moon, the
+moon is in the opposite part of the heavens to that occupied
+by the sun, sometimes being 5° above and other times 5°
+below. Manifestly, then, if the sun be high in the heavens at
+mid-day, it will be only a little below the northern horizon at
+midnight, and the moon, consequently, will be only a small
+distance above the southern horizon. In summer, then, quite
+apart from the fact that the nights are shorter, there is less
+moonlight. In winter, on the other hand, the sun descends
+far below the northern horizon at midnight, and the full moon
+has a high elevation in the southern part of the sky. By this
+happy arrangement, the full moon is longest above the
+horizon when its light is of greatest benefit to mankind.</p>
+
+<div>
+  <span class='pageno' id='Page_98'>98</span>
+  <h3 class='c004'>CHAPTER VII.<br> <span class='c014'>MOVEMENTS OF PLANETS, SATELLITES, AND COMETS.</span></h3>
+</div>
+
+<p class='c003'><span class='sc'>Apparent Movements of Planets.</span>—It has already been
+pointed out that like the sun and moon, the planets also
+have an apparent movement with respect to the more distant
+stars. Mercury and Venus are never seen very far from the
+sun, while other planets, among which are Mars, Jupiter, and
+Saturn, may be seen in the part of the heavens opposite to
+the sun.</p>
+
+<p class='c007'>One point, and that a very important one, which we notice
+from our observations is that the planets never depart very
+far from the ecliptic, so that the planes in which they perform
+their movements are nearly coincident with the plane in
+which our own annual journey round the sun is performed.
+The apparent movements of the planets are such that it is
+quite impossible to regard these bodies as circulating in
+regular orbits round the earth itself. If they revolve round
+any other body it is manifest that their apparent or geocentric
+motions will be compounded of the real movements of the
+planets and that of the earth. It is not necessary here to
+trace the steps by which it has been determined that the
+planets revolve in regular orbits around the sun. Suffice it to
+say that their observed movements are simply and sufficiently
+explained by supposing that, like the earth, which may now
+be regarded as a planet, they travel in elliptic orbits with the
+sun at one of the foci. Besides this revolution, the planets
+have a rotatory motion about their axes, but this question
+cannot be studied apart from the telescopic features, and will
+therefore be treated in Section III. of the present work.</p>
+
+<p class='c007'>The circumstance that the planets Mercury and Venus are
+never seen long after sunset or before sunrise, indicates that
+their orbits must lie between us and the sun. Hence, they
+are distinguished as the <i>interior planets</i>, while those outside
+the earth’s orbit are called the <i>exterior planets</i>.</p>
+
+<p class='c007'><span class='pageno' id='Page_99'>99</span><span class='sc'>Movements of Interior Planets.</span>—Let us consider
+briefly the conditions under which we observe the interior
+planets. If such a planet be represented by M in Fig. 23,
+while the earth is represented
+by E traversing a larger orbit,
+the planet is said to be in <i>inferior
+conjunction</i> with the sun,
+when it lies directly between
+the sun and earth. The actual
+movements of the planets
+being direct—that is, anticlockwise—the
+planet at M has
+an apparent westerly motion
+as seen by an observer situated
+on the earth, and from
+this we gather that it moves
+more rapidly than the earth.
+For simplicity let us regard the earth as being at rest
+at the point E. Then, as the planet reaches the position
+M′, where it is as far as possible to the west of the sun,
+it is said to be at its <i>greatest western elongation</i>. Proceeding
+in its orbit, the planet’s apparent movement is direct,
+and it eventually comes in line with the sun on the further
+side as seen from the earth; it is then said to be in <i>superior
+conjunction</i>. From this point the planet moves to the east of
+the sun until it comes to the point M, after which the
+motion becomes retrograde, and the planet proceeds to
+inferior conjunction again. When at its greatest distance
+to the east of the sun, as at M‴, the planet is said to be at its
+<i>greatest eastern elongation</i>. Taking the term <i>elongation</i> in
+general, it may be regarded as a measure of the angular
+distance of a planet from the sun as observed from the earth.</p>
+
+<div id='i_099'  class='figcenter id001'>
+<img src='images/i_099.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 23.</span>—<i>Movement of an Interior Planet.</i></p>
+</div>
+</div>
+
+<p class='c007'>If the orbits of the planets were perfect circles, the greatest
+elongation distances of an interior planet would always be
+the same; sometimes, however, we are nearer to the sun than
+at the other times, and the apparent separation of the planet
+from the sun would seem greater than at other times, even if
+<span class='pageno' id='Page_100'>100</span>there were no other cause at work. The variations of the
+elongation distances are greater than can be accounted for by
+our own varying distance, and are naturally attributed to the
+elliptical form of the orbits of the interior planets themselves.
+Mercury, for example, sometimes only departs 18° from the
+sun, while at other times it reaches as far as 28° east or
+west.</p>
+
+<p class='c007'>When we take account of the fact that the earth has also
+a movement along its orbit, it will be seen that the same
+conditions hold good with regard to elongations and conjunctions,
+except that the intervals between them will be
+longer.</p>
+
+<div id='i_100'  class='figcenter id001'>
+<img src='images/i_100.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 24.</span>—<i>Morning and Evening Stars.</i></p>
+</div>
+</div>
+
+<p class='c007'><span class='sc'>Morning and Evening Stars.</span>—From superior to inferior
+conjunction an interior planet is to the east of the sun.
+It then rises
+after the sun,
+and sets after
+the sun, so that
+it is visible for
+a short time in
+the early evening;
+in other
+words, it is an
+<i>evening star</i>
+during this part
+of its path. Between
+inferior
+and superior
+conjunctions,
+the planet is
+conversely a
+<i>morning star</i>.
+This is illustrated
+in Fig.
+24, where the
+position of an
+observer towards whom the sun is rising is shown at A. An
+<span class='pageno' id='Page_101'>101</span>interior planet at P is above the horizon at sunrise, but will be
+below at sunset, the observer having been carried to A′ by the
+earths rotation; it will thus be a morning star. When the
+planet occupies the position P′ it is below the horizon at sunrise,
+but will remain in sight after the sun has set in the
+evening, the observer then having been transferred to A′ by
+the earth’s rotation.</p>
+
+<p class='c007'><span class='sc'>Phases of Interior Planets.</span>—From the conditions
+which have been stated with regard to the movements of the
+interior planets, one is not surprised to find that telescopic
+examination reveals that these bodies put on phases similar
+to those of the moon. At superior conjunction the planets
+exhibit a fully illuminated disc, at greatest elongations they
+appear as a half moon, while at inferior conjunction their
+dark sides alone are presented to us. The apparent sizes of
+the planets, as measured with the aid of a telescope, are also
+found to vary according to their positions; when at inferior
+conjunction, the planet is much nearer to us than at other
+times, and it consequently appears larger. The apparent
+brightness of an interior planet also varies. At superior conjunction
+the whole of the disc is illuminated, but the planet
+is then so far removed from us that its light is very feeble.
+On the other hand, at inferior conjunction, when it is nearest
+to us, the dark side of the planet is turned towards us. The
+greatest brightness thus occurs at some intermediate point.
+In the case of Venus this is between the greatest elongations
+and inferior conjunction, when it is 40° from the sun. It
+is then bright enough to be seen with the naked eye in
+full sunshine, and has sometimes, on such occasions, been
+erroneously regarded by ignorant persons as the Star of
+Bethlehem.</p>
+
+<p class='c007'><span class='sc'>Transit of Venus.</span>—If an inferior conjunction occurs
+when the planet is very near to a node—this term having the
+same significance as in the case of the moon (p. 94)—the planet,
+whether it be Mercury or Venus, will be seen projected as a
+dark spot upon the bright disc of the sun. Such an occurrence
+is called a <i>transit of Venus</i> or of Mercury, as the case
+<span class='pageno' id='Page_102'>102</span>may be. Just as we do not get an eclipse of the sun every
+month, so we do not get a transit of Venus every time the
+earth and that planet have the same heliocentric longitude,
+and for the same reason, namely, that the plane of the orbit
+is inclined to the ecliptic. As we shall see in another chapter,
+a transit of Venus has a most important application in the
+determination of one of the fundamental constants of astronomy—the
+sun’s distance. The conditions as to the recurrence
+of transits are of great interest. In the case of Venus,
+the <i>synodic</i> period is 584 days, this being the time which
+elapses between two successive inferior conjunctions. Five
+synodic periods are thus very nearly equal to eight years,
+and 152 synodic revolutions are even more nearly equal to
+243 years. As seen from the earth, the sun crosses the nodes
+of the orbit of Venus on June 5 and December 7, and since
+there can be no transit when the planet is more than 4½° from
+the node, the transits will all occur about these dates. A
+transit will be followed by another after the lapse of 8 years,
+if the planet is not too far from the node; but there can be no
+other transit with the planet at the same node until 243 years
+have elapsed. There are, however, transits occurring at similar
+intervals when the planet is at the other node. The following
+dates on which transits have occurred, or will occur, will
+illustrate the foregoing statements:—</p>
+
+<table class='table1'>
+  <tr>
+    <td class='c024 bbt' rowspan='2'>8 years</td>
+ <td class='blt c028'>December 7, 1631,</td>
+ <td class='blt c028 bbt' rowspan='3'>243 years.</td>
+ <td class='blt c028'><hr></td>
+  </tr>
+  <tr>
+
+ <td class='blt c028'>December 4, 1639,</td>
+
+ <td class='blt c028 bbt' rowspan='3'>243 years.</td>
+  </tr>
+  <tr>
+    <td class='c024 bbt' rowspan='2'>8 years</td>
+ <td class='blt c028'>December 9, 1874,</td>
+
+
+  </tr>
+  <tr>
+
+ <td class='blt c028'>December 6, 1882,</td>
+ <td class='blt c028'><hr></td>
+
+  </tr>
+  <tr>
+    <td class='c024 bbt' rowspan='2'>8 years</td>
+ <td class='blt c028'>June 5, 1761,</td>
+ <td class='blt c028 bbt' rowspan='3'>243 years.</td>
+ <td class='blt c028'><hr></td>
+  </tr>
+  <tr>
+
+ <td class='blt c028'>June 3, 1769,</td>
+
+ <td class='blt c028 bbt' rowspan='3'>243 years.</td>
+  </tr>
+  <tr>
+    <td class='c024 bbt' rowspan='2'>8 years</td>
+ <td class='blt c028'>June 8, 2004,</td>
+
+
+  </tr>
+  <tr>
+
+ <td class='blt c028'>June 6, 2012,</td>
+ <td class='blt c028'><hr></td>
+
+  </tr>
+</table>
+
+<div id='i_103'  class='figcenter id001'>
+<img src='images/i_103.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 25.</span>—<i>Movement of an Exterior Planet.</i></p>
+</div>
+</div>
+
+<p class='c007'><span class='sc'>Movements of Exterior Planets.</span>—The exterior
+<span class='pageno' id='Page_103'>103</span>planets are at once recognised as such by their occasional
+appearance in the part
+of the sky opposite to
+that of the sun. They
+are then said to be in
+<i>opposition</i>. When in the
+same line as the sun, and
+on the remote side of it,
+as at P′ in Fig. 25, the
+planet is in <i>conjunction</i>.
+The apparent movements
+of such a planet are very
+complex. Neglecting for
+a moment the earth’s
+motion, it is evident that
+the apparent rate of movement
+of the planet with reference to the stars will vary very
+considerably according as the planet is near opposition or
+near conjunction, the movement appearing to be most rapid
+when the planet is nearest to us. Upon this unequal rate of
+motion is superposed a varying direction of motion produced
+by the changing position of the earth. When the planet is
+at P, and the earth at E, both are moving in the same direction,
+but as the earth has the greater angular velocity, the
+apparent motion of the planet will be retrograde, that is, the
+planet will appear to go backwards in its path. If the earth
+be near the point E′, its orbital movement will be directed
+away from the planet, and will scarcely affect its apparent
+position; accordingly, about this time the planet has a direct
+movement in the heavens. Between these two points the
+direction of the apparent movement of the planet has
+changed, so that at some intermediate position it would seem
+to have suspended its wanderings; here we have a <i>stationary
+point</i>. For a certain time, before and after conjunction, the
+linear directions of movements of the earth and planet will be
+opposed to each other, and on this account the <i>direct</i> apparent
+motion of the planet will be accelerated. Presently, as the
+<span class='pageno' id='Page_104'>104</span>earth gains on the planet, another stationary point will be
+reached, and with the approach to opposition the planet will
+again retrograde.</p>
+
+<p class='c007'>If both orbits were in the same plane, these apparent movements
+would all be backwards and forwards along a great
+circle of the celestial sphere coincident with the ecliptic, the
+eastward movement predominating. The planes in which
+the planets perform their revolutions are, however, inclined to
+the ecliptic, and the result is that they appear to us to travel
+in loops, some of which are illustrated in Fig. 26.</p>
+
+<div id='i_104'  class='figcenter id003'>
+<img src='images/i_104.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 26.</span>—<i>Apparent Paths of Ceres, Pallas, Juno and Vesta, in 1896.</i></p>
+</div>
+</div>
+
+<p class='c007'>From the fact that we are constantly within the orbit of an
+outer planet, it is evident that we must always see more than
+half of the planetary hemisphere on which the sun is shining.
+Consequently, an exterior planet never puts on a crescent
+phase, or presents the appearance of a half moon. The
+nearer the planet the greater will be the dark area which it is
+possible for us to observe. In the case of Mars, for example,
+we sometimes see it gibbous like the moon about three days
+from full, but in the more distant planets this gibbosity is
+scarcely perceptible. The greatest phase of an exterior
+<span class='pageno' id='Page_105'>105</span>planet occurs when it is at <i>quadrature</i>, that is, when a line
+joining the earth and sun is perpendicular to one joining the
+earth with the planet.</p>
+
+<p class='c007'><span class='sc'>Favourable and Unfavourable Oppositions.</span>—A
+little consideration of Fig. 25 will make it perfectly clear that
+an exterior planet is very much nearer to us at a time of opposition
+than at a conjunction. We are, in fact, then, nearer to
+the planet by the diameter of the earth’s orbit, a matter of some
+186 millions of miles. Accordingly, the planets, more especially
+our neighbour Mars, are best studied in the telescope
+about a time of opposition. Now, if we had to deal with
+circular orbits, the distance of a planet at opposition would
+remain constant, and we should see the planet equally well at
+all oppositions. It is found, however, that this is not the
+case, and the ellipticity of the orbits of the earth and planets
+supplies a simple and sufficient explanation. Sir Robert Ball
+illustrates this in the case of Mars by a diagram similar to
+Fig. 27. It will be seen that, when the opposition occurs in
+August, the earth
+is much nearer to
+Mars than when
+it happens at
+other times. The
+least favourable
+oppositions are
+those which occur
+in February, the
+planet then being
+nearly twice as far
+removed from us
+as at the nearest
+approach during
+an August opposition.</p>
+
+<div id='i_105'  class='figcenter id001'>
+<img src='images/i_105.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 27.</span>—<i>Opposition of Mars.</i></p>
+</div>
+</div>
+
+<p class='c007'>As regards the
+more distant planets, the diameter of the earth’s orbit and
+the variations of opposition distance are of less importance,
+<span class='pageno' id='Page_106'>106</span>since they form a much smaller proportion of the distances of
+those planets from the sun.</p>
+
+<p class='c007'><span class='sc'>Elements of a Planetary Orbit.</span>—A complete study
+of the apparent movements of the planets with which we are
+acquainted shows that their real movements are performed
+round the sun in ellipses, the sun being placed at a focus.
+Each orbit, like that of the earth, has its perihelion and
+aphelion points, and its apse line; not being coincident with
+the ecliptic, it will have a line of nodes, and an ascending and
+descending node. Each planet will further have a particular
+inclination to the ecliptic, and a period of revolution peculiar
+to itself. Consequently, to systematise our knowledge of any
+particular orbit, certain conventions are adopted, and the
+seven things we must know, in order that we may specify the
+size of the orbit, its position in space, and the situation of the
+planet in its orbit, are as follows:—</p>
+
+  <dl class='dl_1'>
+    <dt><i>a</i> =</dt>
+    <dd>Semi axis major of elliptic orbit.
+    </dd>
+    <dt><i>e</i> =</dt>
+    <dd>Eccentricity.
+    </dd>
+    <dt><i>i</i> =</dt>
+    <dd>Inclination to ecliptic.
+    </dd>
+    <dt> Ω  =</dt>
+    <dd>Longitude of ascending node.
+    </dd>
+    <dt> π  =</dt>
+    <dd>Longitude of perihelion.
+    </dd>
+    <dt> P  =</dt>
+    <dd>Period of revolution. (<i>u</i>, the mean daily motion, sometimes         replaces P.)
+    </dd>
+    <dt> E  =</dt>
+    <dd>The epoch, giving the longitude of the planet at some         particular time.<a
+    id='r2'></a><a href='#f2' class='c027'><sup>[2]</sup></a>
+    </dd>
+    </dl>
+
+<p class='c007'>The first two quantities indicate the size and shape of the
+orbit, the next three its position with regard to the ecliptic,
+and the last two are required to determine the situation of
+the planet in its orbit. Some of the elements are illustrated
+in Fig. 28.</p>
+
+<div id='i_107'  class='figcenter id003'>
+<span class='pageno' id='Page_107'>107</span>
+<img src='images/i_107.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 28.</span>—<i>Elements of an Elliptic Orbit.</i></p>
+</div>
+</div>
+
+<p class='c007'><span class='sc'>Determination of a Planet’s Period.</span>—Observations
+enable us to determine the synodic period of a planet, and
+knowing that the earth’s period is a year, it is a simple matter
+to determine that of the planet. In the case of an exterior
+planet, the interval from opposition to opposition furnishes
+the best means of determining the synodic period. The
+exact moment of an opposition cannot usually be directly
+observed, and what one actually does is to measure the R.A.
+and declination of the sun on several days about the time of
+opposition, as also those of the planet; then, by reducing
+these co-ordinates to celestial longitude and latitude, it is not
+difficult to determine at what moment the longitudes differed
+by 180°, that is, the moment at which opposition took place.
+The problem of finding the planet’s sidereal period, then,
+amounts to this: at what rate must the planet be moving in
+order that the earth may make a complete revolution, and
+move, in addition, through the same angle as the planet? In
+other words, what must be the period of the planet in order
+that the earth may gain a whole revolution in the interval
+corresponding to the synodic period? The daily movement
+of the planet will be 360°/P, and that of the earth 360°/365¼, if P denote
+the number of days in the planet’s sidereal period. The
+earth’s gain per day will thus be the difference between
+<span class='pageno' id='Page_108'>108</span>these two quantities, and since a whole revolution is gained
+in the synodic period, the gain per day can be expressed
+as 360°/S, where S represents the synodic period; thus we get</p>
+
+<div class='center'>
+
+<p class='c007'><span class='fraction'><span class='under'>360°</span><br>365¼</span> − <span class='fraction'><span class='under'>360°</span><br>P</span> = <span class='fraction'><span class='under'>360°</span><br>S</span></p>
+
+<p class='c007'>or</p>
+
+<p class='c007'><span class='fraction'>1<br><span class='vincula'>365¼</span></span> − <span class='fraction'>1<br><span class='vincula'>P</span></span> = <span class='fraction'>1<br><span class='vincula'>S</span></span></p>
+
+</div>
+
+<p class='c007'>The synodic period of Mars is 780 days, and the application
+of the foregoing formula leads us to 687 days as the time
+of its revolution round the sun.</p>
+
+<p class='c007'>A single determination of a synodic period does not give
+precise results, for the reason that the orbits of the planets
+are elliptical, and the intervals consequently dependent upon
+whether the planet is near perihelion, or far removed from it
+when an opposition is observed. It is, therefore, necessary to
+determine the time of opposition at long intervals, and so
+reduce the errors in measuring the length of a single period.</p>
+
+<p class='c007'><span class='sc'>Movements of Satellites.</span>—Telescopic observations
+show that some of the planets are accompanied by <i>satellites</i>,
+which revolve round their primaries as the moon revolves
+round the earth. The apparent movements of these bodies,
+with regard to the planets, are very similar to those of the
+interior planets with regard to the sun, having similar points
+of greatest eastern and western elongations. The facts which
+have been collected show that each satellite, like our own
+moon, moves in an elliptical orbit, with the planet in one of
+its foci. With one exception, the satellites attending the
+planets of our system have a direct movement; those of
+Uranus, however, have apparently a movement in the same
+direction as the hands of a watch, but this can be regarded as
+direct, if we consider the plane of the orbit to be inclined
+more than 90° to the plane of the ecliptic.</p>
+
+<p class='c007'><span class='sc'>The Orbits of Comets.</span>—Another class of bodies which
+circulate round the sun now claims our attention. These are
+the <i>comets</i>, some of which are never seen without the aid of
+telescopes, while others have been brilliant enough to excite
+<span class='pageno' id='Page_109'>109</span>a widespread wonder and interest. They usually have a very
+rapid movement relatively to the stars; and to learn something
+as to their real motions, we commence by measuring
+their right ascensions and declinations as frequently as possible.
+When such observations are plotted, they give us the geocentric
+movement of a comet, which generally seems very irregular,
+and gives one the idea that it is subject to no law. Unlike
+the planets, comets do not usually keep near the ecliptic, but
+move in planes inclined at all angles to it. Their rates of
+apparent movement also change very rapidly.</p>
+
+<p class='c007'>When the effect of the earth’s movement upon that of a
+comet is eliminated, it is found that the movement of the
+comet is performed either in an ellipse, a parabola, or an
+hyperbola, the sun in each case occupying one of the foci.</p>
+
+<p class='c007'>From our definition of the eccentricity of an ellipse, it will
+be seen that, when the eccentricity is zero, we have a circle.
+When the eccentricity becomes unity, the ellipse becomes a
+parabola, so that the latter curve may be regarded as part of
+an ellipse, of which the foci are at an infinite distance apart.
+In the case of the hyperbola, the eccentricity is greater than
+unity.</p>
+
+<p class='c007'>Comets which move round the sun in ellipses are called
+<i>periodic comets</i>, for the reason that they return regularly into
+the sun’s neighbourhood. Those which traverse parabolic or
+hyperbolic paths will pass once round the sun and continue
+to journey into the depths of interstellar space until their
+movements are changed by the proximity of other bodies into
+the neighbourhood of which their wanderings may take them.</p>
+
+<p class='c007'>When a new comet is observed, one of the things which
+astronomers endeavour to do is to determine its orbit, so
+that its path may be predicted with sufficient accuracy to
+enable it to be picked up readily with a telescope when it
+becomes so feeble that it is no longer visible to the naked
+eye. In the first instance, the motion is assumed to be parabolic,
+and any deviation from such an orbit forms the subject
+of a rigorous calculation by means of which the precise form is
+determined.</p>
+
+<div>
+  <span class='pageno' id='Page_110'>110</span>
+  <h3 class='c004'>CHAPTER VIII.<br> <span class='c014'>ECLIPSES AND OCCULTATIONS.</span></h3>
+</div>
+
+<p class='c003'><span class='sc'>Eclipses of the Moon.</span>—As the various members of the
+solar system shine only by virtue of the light which they
+receive from the sun, they will cease to be visible if by any
+means they are deprived of the sun’s rays. Each planet or
+satellite must evidently cast a shadow which is turned directly
+away from the sun, and any other body passing wholly or
+partially within such a shadow will be proportionately
+debarred from receiving the direct light of the sun.</p>
+
+<div id='i_110'  class='figcenter id003'>
+<img src='images/i_110.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 29.</span>—<i>The Earth’s Shadow.</i></p>
+</div>
+</div>
+
+<p class='c007'>Were the sun a mere point of light these shadows would be
+parts of cones, the apex always being at the sun, and they
+would be prolonged indefinitely into space. As a matter of
+fact, every individual point upon the sun’s disc is competent
+to cast a conical shadow, and the net result is that
+only a relatively small space behind a planet or satellite is
+really in total darkness. This will be readily understood
+from Fig. 29, in which S is the sun, and E the earth. The
+total shadow now becomes a cone, with the apex turned
+directly away from the sun, but round this there is a region
+of partial shadow which is only illuminated by portions of the
+sun. If we imagine a section of the shadow across the line
+<i>a b</i>, we should find a central disc of total darkness called the
+<span class='pageno' id='Page_111'>111</span><i>umbra</i>, and surrounding this a ring of half shadow called the
+<i>penumbra</i>.</p>
+
+<p class='c007'>From the known dimensions of the sun and earth, and the
+distance between them, it is easy to calculate the size of the
+earth’s shadow-cone, and its length is found to be greater
+than the distance of the moon. The axis of this shadow
+will, of course, always be in the plane of the ecliptic. If, then,
+at the time of opposition, the moon is sufficiently near the plane
+of the ecliptic, it will pass through the shadow, and we shall
+have the phenomena of a <i>lunar eclipse</i>. When the moon is
+wholly immersed in the umbra, the eclipse is total, and if it
+further passes quite symmetrically through the shadow, the
+eclipse is said to be central. This would always be the state
+of affairs if the moon performed its monthly journey in the
+plane of the ecliptic, and a total eclipse would occur every
+month. The moon’s orbit, however, is inclined to the ecliptic,
+so that for a central eclipse, the moon must be simultaneously
+at opposition and at a node. If the moon be near the node
+when at opposition, a total eclipse may occur, but it cannot
+be central, and the duration of the total obscuration will be
+reduced. Still further from the node, the moon will be above
+or below the ecliptic, and will be only partially involved in the
+shadow-cone; such an eclipse is called a partial one. Beyond
+a certain distance from the node, the inclination of the moon’s
+orbit will take the moon entirely out of the umbral shadow,
+and no eclipse will be possible.</p>
+
+<p class='c007'>The circumstances of an eclipse of the moon thus vary very
+considerably, and there is still another reason why we may
+expect them to be different. We have seen that the earth’s
+distance from the sun changes throughout the year, and, in
+consequence, its shadow will be of varying length, and the
+diameter of the shadow at any specified distance will not be
+constant. The moon, again, is not always at the same distance
+from the earth, and it will, therefore, pass through
+varying depths of shadow in different eclipses, and with
+different velocities.</p>
+
+<p class='c007'>The breadth of the earth’s umbral shadow at the point
+<span class='pageno' id='Page_112'>112</span>where the moon passes through it is, on the average, about
+three times the moon’s diameter, and the time taken for the
+moon to traverse this distance is about two hours. The
+duration of totality in a central eclipse may, therefore, amount
+to two hours, while an additional two hours may be occupied
+by the partial phases.</p>
+
+<div id='i_112'  class='figcenter id003'>
+<img src='images/i_112.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 30.</span>—<i>The Lunar Ecliptic Limit.</i></p>
+</div>
+</div>
+
+<p class='c007'><span class='sc'>The Lunar Ecliptic Limit.</span>—The greatest distance
+of the moon from a node at which a partial eclipse
+is possible, is called the <i>lunar ecliptic limit</i>, and is
+very easily calculated. In Fig. 30, let E N represent
+a part of the ecliptic, N being the node of the moon’s
+orbit, and E the centre of the earth’s shadow. As the orbit
+of the moon is inclined about 5° 9′ to the ecliptic, it may be
+represented by the line N M, inclined at an angle to N E. If
+E A be the radius of the earth’s shadow, which, on the average,
+is about three-quarters of a degree, and M A the moon’s
+apparent semi-diameter (about a quarter a degree), it is clear
+that the point beyond which no eclipse is possible is that in
+which the line M E, perpendicular to N M, is equal to the
+sum of the semi-diameters. All the quantities for solving the
+triangle N E M are thus known, and it can be readily calculated
+that N M, the greatest distance of the moon from the
+node at which an eclipse would be possible, under average
+conditions is about 11°.</p>
+
+<p class='c007'>Taking into account the varying distances between the sun,
+earth, and moon, it is found that an eclipse must always
+occur if the moon is within 9° of the node, and may occur if it
+be 12° from the node. These figures refer to the passage of
+<span class='pageno' id='Page_113'>113</span>the moon through the umbra, as the effect of its entrance into
+the penumbra is too slight to be observed.</p>
+
+<p class='c007'>The entrance of the moon into the earth’s shadow is a definite
+phenomenon, which is independent of the observer’s
+position on the earth, and the phases of the eclipse are seen
+at exactly the same moment from all places where the moon is
+above the horizon. The computation of the circumstances at
+a given place is accordingly a simple one.</p>
+
+<p class='c007'>When a lunar eclipse is not total at any of its phases, it
+is usual to specify its <i>magnitude</i> by the ratio of the greatest
+measurement of the obscured part to the moon’s diameter.
+Thus the magnitude of the partial eclipse of February 28th,
+1896, is given in the “Nautical Almanac” as 0·870, the moon’s
+diameter being taken as unity.</p>
+
+<p class='c007'>The conditions of lunar eclipses which have been stated
+have reference to the moon’s passage through the earth’s
+geometrical shadow, but the actual conditions are greatly
+modified by the fact that the earth is surrounded by an
+atmosphere which refracts the suns light so much that the
+moon is seldom quite obscured during totality. The commencement
+of the total phase is also rendered difficult of
+observation by the somewhat indefinite boundary between
+the umbra and penumbra.</p>
+
+<p class='c007'><span class='sc'>Eclipses of the Sun.</span>—If the moon performed its revolution
+in the plane of the ecliptic, it is evident that it must
+always come between us and the sun once in each month.
+This it does not do, but occasionally it happens to be in the
+ecliptic when in conjunction, and the moon is then seen to
+be projected upon the sun. In other words, there is an
+eclipse of the sun. Let us consider the circumstances, in the
+first instance, to an observer placed at the centre of the earth.
+If the centres of the moon and sun appear in the same
+straight line, the eclipse will be <i>total</i> or <i>annular</i>, according
+as the moon or sun has the greater apparent diameter. Both
+these forms of eclipses are possible, on account of the varying
+apparent diameters of the sun and moon consequent upon
+their variable distances from the earth. If the moon appear
+<span class='pageno' id='Page_114'>114</span>the larger it will evidently cover up the whole of the sun, but
+when it is the smaller, a ring of sunlight will be visible round
+the dark holy of the moon, and the eclipse will be an
+annular one. These conditions are illustrated in Fig. 31,
+<i>a</i> and <i>b</i> representing a total and an annular eclipse respectively.
+If the moon and sun be not quite in the same
+straight line, the moon may still be seen partially projected
+on the sun’s disc, in which case there will be a <i>partial eclipse</i>
+of the sun, as in Fig. 31, <i>c</i>.</p>
+
+<div id='i_114'  class='figcenter id001'>
+<img src='images/i_114.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 31.</span>—<i>Eclipses of the Sun.</i> (<i>a</i>) <i>Total Eclipse</i>, (<i>b</i>) <i>Annular Eclipse</i>, (<i>c</i>) <i>Partial Eclipse.</i></p>
+</div>
+</div>
+
+<p class='c007'>In a total eclipse there are four so-called <i>contacts</i>: the first
+when the moon is seen to encroach upon the sun’s disc, the
+second when the advancing edge of the moon reaches the
+opposite limb, the third when the following edge of the moon
+first touches the sun’s boundary, and the fourth when the
+projected moon finally passes off the sun. The interval between
+the second and third contacts marks the duration of
+<span class='pageno' id='Page_115'>115</span>totality. As referred to our supposed observer at the centre
+of the earth, the duration evidently depends upon the apparent
+rate of the moon’s eastward movement as compared
+with that of the sun, as well as upon the differences of the apparent
+diameters of the two bodies.</p>
+
+<p class='c007'>The production of eclipses of the sun may also be considered
+as arising from the immersion of an observer in the
+shadow of the moon. This shadow has its axis turned from
+the sun, but is so short that it does not always reach the
+earth. If an observer comes near the axis of the conical
+shadow, and within the apex, the eclipse will be total; if he
+is in the axis, but outside the apex, the eclipse will be annular.</p>
+
+<div id='i_115'  class='figcenter id003'>
+<img src='images/i_115.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 32.</span>—<i>Duration of a Solar Eclipse.</i></p>
+</div>
+</div>
+
+<p class='c007'>The whole of the shadow of the moon is so small that only
+a few places on the earth’s surface can be simultaneously immersed
+in it, and when we come to discuss the conditions of
+an eclipse with regard to a particular observer, the problem
+becomes a complicated one. At some places the
+eclipse may be total, at others it will be only partial,
+while at others no eclipse will occur at all. These differences
+are due to the fact that the sun is scarcely appreciably
+displaced by the change of locality, while the apparent
+position of the moon may be affected to the extent of
+nearly a degree. Again, the observer situated on the earth’s
+surface has a movement of his own, produced by the earth’s
+rotation, and his rate of motion depends upon the latitude in
+which he is situated. The effect of this movement upon the
+conditions of the eclipse are very pronounced. Suppose for a
+moment that the sun, moon, and earth, are fixed along the
+same straight line S M E in Fig. 32, a terrestrial observer at
+<span class='pageno' id='Page_116'>116</span><i>a</i> on the earth’s Equator would see an eclipse at noon; if he
+were not in rotation, and the three bodies remained at rest,
+the eclipse would be a perpetual one. He is, however,
+carried onward by the earth’s rotation, and even if the moon
+were at rest, it would appear to him to pass over the sun
+in the reverse direction. This retardation of the moon
+will be less in amount for observers away from the Equator,
+and also for observers to whom the sun is not on the
+meridian when eclipsed. The effect of rotation on an observer
+at <i>b</i> (Fig. 32), for example, is to move him almost in
+the direction of the line joining the moon and sun, and the
+backward tendency of the moon due to rotation is very
+slight. On account of the earth’s rotation, then, the duration
+of a solar eclipse is lengthened, the greatest increase occurring
+at those places where the sun is on the meridian at the
+time of eclipse.</p>
+
+<p class='c007'>There is another source of gain of duration of an eclipse to
+the observer who sees the phenomenon about noon. The
+moon’s apparent diameter is then augmented by a greater
+amount than at other places, because the observer is then
+nearest to the moon; while the sun’s apparent diameter is
+not appreciably affected. The greater the difference in the
+apparent diameters of the sun and moon, the longer will
+totality last.</p>
+
+<p class='c007'>These and other circumstances have all to be taken into
+account in computing the conditions under which an eclipse
+will be seen at any given place.</p>
+
+<p class='c007'>According to an eminent authority, Professor Young,
+the greatest possible diameter of the moon’s shadow,
+where it strikes the earth, is 167 miles. It may, however,
+cover a larger space on the earth’s surface, because the latter
+does not pass perpendicularly through the shadow. To all
+persons within the shadow, the eclipse will be total, but to
+those on its outer boundary the duration of totality will be
+for an instant only. The penumbral shadow has a cross
+section about 4,500 miles in diameter, covering sometimes a
+space on the earth’s surface 6,000 miles across. To all
+<span class='pageno' id='Page_117'>117</span>persons within this area, but not in the central shadow, the
+eclipse will be partial. The shadow spot travels over the
+earth’s surface, because of the moon’s movement, but its
+track and speed are greatly modified by the earth’s rotation.
+The movement of the shadow, as affected by the earth’s
+rotation, would be along a parallel of latitude; but its ultimate
+direction of movement, though trending eastwards, depends
+upon this, combined with the direction of the moon’s movement
+at the time of the eclipse. Thus, a portion of the track
+of the total eclipse of April 16, 1893, is as that shown in
+Fig. 33.</p>
+
+<div id='i_117'  class='figcenter id001'>
+<img src='images/i_117.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 33.</span>—<i>Track of Eclipse of April 16, 1893.</i></p>
+</div>
+</div>
+
+<p class='c007'>These considerations will suffice to explain the necessity for
+very precise calculations as to the position of the central line
+of an eclipse, when observers are sent out for the purpose of
+recording the phenomena.</p>
+
+<p class='c007'>Under the most favourable combination of conditions, that
+is, when the eclipse occurs at noon at a place on the Equator,
+an eclipse cannot be total for more than 7 minutes 58 seconds,
+nor be annular for a longer time than 12 minutes 24 seconds.
+From first to last contact may occupy as much as 2 hours,
+when all the circumstances are similarly favourable. (Loomis.)</p>
+
+<p class='c007'><span class='pageno' id='Page_118'>118</span><span class='sc'>The Solar Ecliptic Limit.</span>—In order that an eclipse of
+the sun may occur, the moon must be so near the ecliptic that
+it can be seen projected on the sun, either wholly or partially,
+from some point on the earth. It must therefore not be very
+far from the node, and the distance it may be from the node,
+while still being seen upon the sun, is called the <i>solar ecliptic
+limit</i>. As in the case of lunar eclipses, this distance is
+determined by the inclination of the moon’s orbit, and the
+distances of the moon and sun from the earth. The latter
+being variable quantities, the limit is not always the same.
+It is calculated without much difficulty that an eclipse <i>must</i>
+occur if the new moon happens when it is within 15° 21′ of the
+node, and may occur within 18° 31′. These are called the
+minor and major ecliptic limits respectively. For total or
+annular eclipses, the limits are respectively 9° 55′ and 11° 50′.</p>
+
+<p class='c007'><span class='sc'>Number of Eclipses in a Year.</span>—If the moon’s nodes
+were fixed, the sun would pass through the line of nodes
+twice a year. At such times an eclipse of the sun must
+necessarily occur if the moon were within 15° 21′ of the node
+on either side. The sun requires more than a month to
+traverse this space of 30° 42′, and the moon must therefore
+pass through each node at least once while the sun is traversing
+these limits. It follows, then, that there must be at least
+two eclipses of the sun in a year. Since the line of nodes
+of the moon’s orbit revolves backwards in a period of about
+nineteen years, the sun returns to the same node after an
+interval of 346·6 days, and there must accordingly be two
+solar eclipses in this interval. If, then, there be an eclipse
+early in January, there will be another about the middle of
+the year, and another at the end of the year, so that on this
+ground alone there is a possibility of three solar eclipses in a
+year.</p>
+
+<p class='c007'>Again, while the sun is passing through the ecliptic limits,
+it may happen that an eclipse occurs on its entrance, and
+then another will occur before it gets beyond on the other
+side of the line of nodes. In this way two eclipses may
+occur in the region of each node passage, and if the first of
+<span class='pageno' id='Page_119'>119</span>the series occurs early in January, five eclipses of the sun
+may occur in a single year.</p>
+
+<p class='c007'>The sun, however, is not a month in traversing the lunar
+ecliptic limit. Consequently, a whole year may elapse without
+the moon being sufficiently near the node to pass within
+the earth’s shadow, and in many years there are accordingly
+no eclipses of the moon. Only one full moon can occur
+within the lunar ecliptic limits when the sun passes the node,
+but if there be an eclipse at one node, there may also be one
+six months later at the other node. As in the case of the
+solar eclipses, the “eclipse year” is one of 346·6 days, so that
+if there be an eclipse of the moon early in January, there may
+possibly be three altogether in the course of the year, but
+there could not be three lunar eclipses if the extra solar
+eclipse were possible. Altogether, then, there may be seven
+eclipses in the course of a year—five of the sun and two of the
+moon. Usually there are four or five, some particulars of which
+are furnished by all respectable almanacs. It will be observed
+that the number of solar eclipses is much larger than that of
+lunar ones, but as the latter are visible at all places having
+the moon above the horizon, while the former are restricted
+to small parts of the earth’s surface, more lunar than solar
+eclipses are visible at any specified place.</p>
+
+<p class='c007'><span class='sc'>Recurrence of Eclipses.</span>—We have seen that the sun
+requires only 346·6 days to travel from one of the moon’s
+nodes back to the same node again, in consequence of the
+regression of the nodes, while the moon requires 27·2 days.
+Suppose, then, that the moon and sun are at a node, and there
+is an eclipse at new moon; after 346·6 days the sun will return
+to the same node, but the moon will not be at the node, nor
+will it be exactly new. It will not be until the sun has
+returned nineteen times to the node that the moon is also
+very nearly new at the same node again. Nineteen returns of
+the sun to the moon’s nodes occupy a period of 6,585·78 days;
+223 intervals between successive new moons (synodic months)
+cover 6,585·32 days, while 242 node passages of the moon
+require 6,585·357 days. In this period of 18 years 11⅓ days
+<span class='pageno' id='Page_120'>120</span>(or 10⅓ days if there are five, and 12⅓ if there are three leap
+years in the interval), the sun and moon thus return to nearly
+the same conditions as affecting the possibility of eclipses.
+This period was called the <i>Saros</i> by the Chaldeans, by whom
+it was employed in the prediction of eclipses. The adjustment
+of periods, however, is not quite precise, so that predictions
+based upon the Saros are only approximations, which
+serve as a guide for more accurate computations.</p>
+
+<p class='c007'>This eclipse period is still more remarkable from the fact
+that it almost exactly represents 239 passages of the moon
+through perigee, so that after the lapse of 18 years 11⅓ days
+the moon is almost at the same distance from the earth, as
+well as nearly at the same phase and the same distance from
+a node.</p>
+
+<p class='c007'>As the Saros includes a fraction of a day, an eclipse is not
+necessarily repeated at the same place after the lapse of 18
+years 11⅓ days, for the reason that the eclipse will not occur
+at the same time of day, and the sun may be below the
+horizon. After three Saroses, however, the eclipse will be
+repeated nearly at the same hour, but even then it will not be
+seen under the same conditions, because the track of the
+shadow will be in different latitudes, for the reason that the
+moon does not return <i>exactly</i> to the node in the interval
+between 223 new or full moons, and eclipses can only occur
+when the moon is new or full.</p>
+
+<p class='c007'>Beginning as a partial eclipse, an eclipse of the moon will
+gradually become of greater magnitude at successive intervals
+of 18 years 11 days, until it becomes a total eclipse, and
+will again gradually become of smaller magnitude, until it
+ceases to be reproduced at all. Altogether, it would be repeated
+once in every 223 months for 865 years.</p>
+
+<p class='c007'>Since the solar ecliptic limit is greater than the lunar, a
+solar eclipse is repeated at similar intervals of 18 years for
+about 1200 years. Most of these eclipses would be partial,
+27 would be annular, and 18 total. During this period, the
+track of the central eclipse would shift northwards if the
+moon were at a descending node, and southwards if at an
+<span class='pageno' id='Page_121'>121</span>ascending node, until finally it passed altogether clear of the
+earth.</p>
+
+<p class='c007'>It must be remarked, however, that, in the period corresponding
+to a single Saros, about 28 eclipses of the moon, and
+43 of the sun, usually appear, so that altogether about 71
+series of eclipses are in progress. Of the solar eclipses which
+occur during a period of 18 years, about 12 are total at some
+places upon the earth.</p>
+
+<p class='c007'><span class='sc'>Occultations of Stars and Planets by the Moon.</span>—In
+its monthly round, the moon is constantly passing in front
+of some of the stars which lie in its apparent path, and these
+luminaries will, therefore, at times, be hidden temporarily by
+the moons disc. Occasionally a planet may appear in the same
+line of vision as the moon, and that also will pass from view
+until subsequent motion again removes the intercepting body.
+These disappearances are closely allied to the phenomena of
+eclipses, and receive the name of <i>occultations</i>. On account of
+the moon’s eastward movement, it is evident that the disappearance
+of stars or planets when occulted will take place
+on the eastern edge of the moon; but since the moon trends
+north or south in some parts of its orbit, the disappearance
+near the northern and southern edges may occur slightly
+on the western side of the north or south point of the moons
+limb. Similarly, the reappearance generally occurs on the
+western side of the moon, but occasionally may occur on the
+eastern side—that is, when the northern or southern edge of
+the moon does not much more than appear to graze the
+stars.</p>
+
+<p class='c007'>The calculation of the circumstances of an occultation is
+very similar to that involved in the computation of eclipses.
+(A simple graphical method for working out the conditions
+of an occultation is described by Major Grant, R.E., in the
+<cite>Geographical Journal</cite> for June, 1896.)</p>
+
+<p class='c007'><span class='sc'>Eclipses and Occultations of Satellites by
+Planets.</span>—Just as we find the moon eclipsed by passing
+through the earth’s shadow, we find the satellites of other
+planets to be at times invisible for a similar reason. We thus
+<span class='pageno' id='Page_122'>122</span>observe <i>eclipses</i> of the satellites. The satellites may also be
+invisible to us for the reason that they are behind the planet,
+and they are then said to be <i>occulted</i>. These satellite phenomena
+are especially remarked in the case of Jupiter, and their
+observation is one of great interest. When a satellite passes
+between the sun and the planet it throws a shadow on the
+surface of the planet similar to that of the moon upon the
+earth. This is visible to us as a dark spot, and from the
+centre of that dusky patch an inhabitant of Jupiter would undoubtedly
+see a total eclipse of the sun. To us on the earth
+the passage of such a shadow across the planet’s disc is but a
+“transit of the shadow” with its “ingress” and “egress.”</p>
+
+<p class='c007'>The times of all these appearances are computed from a
+knowledge of the movements of the satellites.</p>
+
+<h3 class='c004'>CHAPTER IX.<br> <span class='c014'>HOW TO FIND OUR SITUATION ON THE EARTH.</span></h3>
+
+<p class='c003'><span class='sc'>Determination of Latitude.</span>—In order that we may
+precisely define our situation upon the terrestrial sphere, we
+have seen that two measurements are necessary, namely,
+latitude and longitude. The first of these indicates the
+angular distance from the Equator, and the latter the angular
+distance east or west of an arbitrary initial meridian. It is
+necessary for us then to learn something of how these important
+co-ordinates can be determined.</p>
+
+<p class='c007'>In considering the apparent movements of the heavenly
+bodies in different latitudes, we have already seen that at
+places on the earth’s Equator the north celestial pole is on the
+horizon, while at the North Pole it is in the zenith, and in
+other latitudes is elevated at different angles. If one sails
+from England to the Cape, for example, the Pole Star is seen
+<span class='pageno' id='Page_123'>123</span>to gradually get lower and lower in the sky, until, on crossing
+the Equator, it descends below the northern horizon and
+is no longer visible. Sailing northward, as to Norway, the
+Pole Star is seen to get higher in the sky.</p>
+
+<p class='c007'>Now, although the Pole Star is not exactly at the north
+celestial pole, it is a convenient guide to the eye as to the
+location of that very important mathematical point, and what
+we learn from its behaviour as our latitude is changed is that
+the altitude of the Pole above the horizon is equal to the
+latitude of the place of observation.</p>
+
+<p class='c007'>One of the methods employed for finding the latitude of a
+place is accordingly to determine the altitude of the Pole.
+This can be obtained by an instrumental measurement of the
+altitude of the Pole Star, from which, if the time of observation
+be known, the altitude of the true Pole, which occupies the
+centre of the small diurnal circle traversed by the star, can be
+computed. Tables which save an immense amount of labour
+in the calculations involved are given in the “Nautical
+Almanac,” and in “Whitaker’s Almanac.”</p>
+
+<p class='c007'>Another method of finding the elevation of the Pole is to
+take advantage of the fact, that at intervals of twelve sidereal
+hours the Pole Star passes the meridian alternately above and
+below the Pole. If, then, one finds the altitudes at the upper
+and lower transits, and corrects them for refraction, the
+average of the readings is a measure of the altitude of the true
+Pole, and therefore of the latitude. Other stars which are
+circumpolar may be employed for the same purpose, and this
+method has the great advantage that a knowledge of the
+correct time, or of the exact position of the star observed, is
+superfluous. The disadvantage is that the correction for refraction,
+especially in low latitudes, cannot be made with the
+necessary degree of accuracy. It must be remembered that
+an error of only 1′ in latitude implies a mistake of a mile
+measured on the earth’s surface.</p>
+
+<p class='c007'>Other methods, however, are available. As we go southwards,
+not only does the Pole Star become lower in the sky,
+other stars in the southern part of the sky become higher at the
+<span class='pageno' id='Page_124'>124</span>same rate that the Pole Star descends. Other stars can
+therefore be utilised, and in order that refraction may affect
+the observations as little as possible, stars of known declination
+near the zenith are observed. Suppose an observer,
+situated at O (Fig. 34) on the earth’s surface, observing a star
+S on his meridian, O Z will represent
+his zenith, and O E, parallel
+to the Equator, will be the direction
+in which he will see the celestial
+equator where it crosses his meridian.
+The declination of the star,
+represented by the angle S O E,
+has been previously determined
+with great accuracy, and the angle
+S O Z, the zenith distance of
+the star, is the angle which he
+measures. In the case illustrated
+by the diagram, the difference between
+the declination and the zenith distance will give the
+angle Z O E, which is evidently equal to the latitude O C Q.
+To get rid of the ever troublesome refraction of our atmosphere,
+stars which pass as nearly as possible through the
+zenith are selected for observation, and stars both to north
+and south are observed.</p>
+
+<div id='i_124'  class='figcenter id001'>
+<img src='images/i_124.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 34.</span>—<i>Determination of Latitude.</i></p>
+</div>
+</div>
+
+<p class='c007'>Another way of determining the latitude, which is very
+commonly employed, is known as Talcott’s method. The
+observations are made with the aid of a zenith telescope.
+The latitude being approximately known, two stars are
+selected which transit nearly at the same time and nearly at
+the same distance from the zenith, one to the north and the
+other to the south. That which transits first is brought to the
+centre of the field of view, which is marked by a spider thread.
+The instrument is then reversed in its bearings so that it
+points at the same angle on the opposite side of the zenith.
+When the second star comes into the field, the telescope is
+kept fixed, and a moveable spider thread is made to coincide
+with the star passing through the field. The distance between
+<span class='pageno' id='Page_125'>125</span>the spider threads furnishes a measure of the difference in
+zenith distances. Half the sum of the declinations added to
+half the difference of zenith distances gives the latitude when
+this method is employed.</p>
+
+<p class='c007'>Various other methods have been devised for the precise
+determination of latitude, but the foregoing will sufficiently
+serve to illustrate the processes followed when the observations
+are made on land.</p>
+
+<p class='c007'>Before the invention of astronomical instruments, latitude
+was approximately measured by the lengths of shadows. At
+the summer solstice, at noon, the shadow of a vertical stick is
+at its shortest, while at the winter solstice it is longest.
+By measuring these
+lengths, a diagram
+can be made showing
+the altitude of
+the sun at noon on
+each occasion.
+Midway between
+these will be the
+altitude of the
+celestial equator
+where it crosses
+the meridian.
+Since the altitude
+of the Pole is equal to the latitude, the altitude of the Equator,
+subtracted from 90°, thus gives the latitude.</p>
+
+<div id='i_125'  class='figcenter id001'>
+<img src='images/i_125.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 35.</span>—<i>Ancient Mode of measuring Latitude.</i></p>
+</div>
+</div>
+
+<p class='c007'>It will be noted that this <i>gnomon</i> experiment also furnishes
+a measure of the obliquity of the ecliptic. The gnomon was
+in use by the ancient Chinese, and it is also believed that the
+Egyptian obelisks which are now embellishing various cities
+were originally erected for the same purpose.</p>
+
+<p class='c007'><span class='sc'>Determination of Longitude.</span>—As we have imagined
+an observer travelling in a north or south direction in connection
+with the measurement of latitude, let us consider
+what will happen to an observer who travels only in longitude—that
+is, east or west. At the starting-point, he will see the
+<span class='pageno' id='Page_126'>126</span>Pole at a certain altitude, and the stars will perform their
+diurnal revolutions at a certain inclination to the horizon depending
+upon his latitude. If he travels towards the east,
+the Pole will remain at the same angle above the horizon, and
+he will detect no difference in the apparent movements of the
+stars. What then is there to indicate that he has changed
+his place at all? The answer is simple; he will find that
+the sun and stars cross the meridian earlier, and if he be 15°
+east of his first station they will transit an hour sooner, because
+it takes the earth an hour to turn through that angle.
+If he travel westward in the same way, the earth must turn
+through a greater angle to bring him back to the same star,
+so that the stars will appear to cross the meridian later.</p>
+
+<p class='c007'>The determination of longitude is accordingly based upon
+a measurement of the difference in the times of transit of sun
+or stars at the place of observation, and the place from which
+longitude is reckoned.</p>
+
+<p class='c007'>Let us take Greenwich as the start-point for our longitudes,
+and suppose we are in Dublin. The sun, or a star, will cross
+the meridian of Dublin at a certain interval after it has
+passed that of Greenwich, and if we measure this interval,
+the angle turned through by the earth in that time will
+determine the longitude. With a transit instrument one can
+readily tell the exact moment when the star crosses the
+meridian of Dublin, but how is one to know the exact
+moment at which the star crossed the meridian of Greenwich
+without going there?</p>
+
+<p class='c007'>Looking at the question in another way, let us remember
+that the clocks in Dublin register local time, that is time
+reckoned from the passage of the sun over the meridian of
+Dublin, while the Greenwich clock indicates times based on
+the transit of the sun over the Greenwich meridian.
+Evidently the difference of these times is the difference of
+longitude, and our question becomes, how to find the time at
+Greenwich when stationed at the observatory in Dublin.</p>
+
+<p class='c007'>In all modern work, the telegraph is employed whenever it
+is available, the two stations being directly connected. An
+<span class='pageno' id='Page_127'>127</span>observer at Greenwich is thus enabled to transmit a signal to
+the observer in Dublin at the exact moment a star passes
+through the centre of his transit instrument, and the latter
+observer then notes the interval which elapses before the
+same star passes the central line of his own instrument. If
+the signals were transmitted instantaneously, the interval
+elapsed from the reception of the signal to the observed
+transit of the same star would give the longitude as reckoned
+in time.</p>
+
+<p class='c007'>Practically, what is done is for each observer to determine
+his local sidereal time very accurately, with the aid of his
+transit instrument, and in this way to find the error of his
+clock. It is then only necessary to compare the two clocks,
+and this is done in the following way: the clock at Greenwich
+has an attachment by which an electrical contact is made
+every second, and this is switched in to the telegraphic circuit,
+so that the Dublin observer receives a signal every
+second so long as the clock is connected. These signals are
+automatically recorded by a chronograph, together with
+similar signals from the Dublin clock, and the times to which
+each of them corresponds is easily identified. Immediately
+afterwards the Dublin clock is switched into the circuit, and
+records its beats on the chronograph sheet at Greenwich,
+alongside those sent by the Greenwich clock. In this way
+the differences between the clocks can be very accurately
+measured, and the longitude can then be reckoned in degrees
+and minutes by allowing 15° for each hour. Before the invention
+of the telegraph, less accurate methods were of
+necessity employed. Among others the entrance of the moon
+into the earth’s shadow during an eclipse was noted by an
+observer desiring to know his longitude. As we have already
+seen, this occurrence is independent of the observer’s position
+on the earth, so that if he records the local time of the observation
+and compares with the calculated Greenwich time
+of the commencement of the eclipse, he can find his longitude.
+Similarly, the eclipses of the satellites of Jupiter may be
+utilised to signal Greenwich time to an observer situated
+<span class='pageno' id='Page_128'>128</span>elsewhere. Unfortunately, the shadows are too ill-defined at
+the edges to permit very accurate determinations in this way.</p>
+
+<p class='c007'><span class='sc'>Methods Employed at Sea.</span>—One of the most important
+applications of astronomy to the needs of everyday life is in
+enabling the navigator on the open ocean to determine the
+situation of his ship. Without the help supplied by astronomical
+predictions the sea would be truly trackless, and commerce
+by sea would be almost impossible.</p>
+
+<p class='c007'>A sextant and two or three good chronometers, together
+with a copy of the current “Nautical Almanac,” furnish the
+means of ascertaining the geographical position of a ship.
+With the aid of the sextant, the sun’s greatest angular distance
+above the sea horizon—that is, its meridian altitude—is
+measured, and from the known declination of the sun at
+the time, the latitude is deduced in exactly the same way as
+in the case of an observation of a star (p. 124).</p>
+
+<p class='c007'>The sextant also enables the observer, by measuring the
+sun’s altitude in the early morning or evening, to determine
+the local time, as already explained (p. 83). Greenwich time
+is kept by the chronometers, and the difference between this
+and the local time is a measure of the longitude. More than
+one chronometer is carried by a ship, for fear that a single one
+might fail, through accident or other causes, to give correct
+readings. The rate of each has been previously very accurately
+gauged, and by taking the average indications,
+Greenwich time is known with considerable accuracy.</p>
+
+<p class='c007'>Should the chronometers fail, or any doubt be thrown upon
+their accuracy, there is another method by which the Greenwich
+time, and thence the longitude, can be ascertained. This
+is the <i>lunar method</i>, in which the heavens become the equivalent
+of the dial of a clock, while the moon, with its rapid
+easterly movement, plays the part of the hands.</p>
+
+<p class='c007'>In the words of Dr. Lardner, this is “a chronometer of
+unerring precision; a chronometer which can never go down,
+nor fall into disrepair; a chronometer which is exempt from
+the accidents of the deep; which is undisturbed by the
+agitation of the vessel; which will at all times be present and
+<span class='pageno' id='Page_129'>129</span>available to him wherever he may wander over the trackless
+and unexplored regions of the ocean.”</p>
+
+<p class='c007'>From the known movements of the moon, its position with
+regard to the sun, planets, or conspicuous stars, at definite
+Greenwich times, can be calculated in advance, and “lunar
+distances” are accordingly tabulated in our nautical almanacs.
+We find, for instance, that the apparent distances of the moon
+from the star Regulus, as they would appear from the earth’s
+centre, were as follows on Jan. 1, 1896:—</p>
+
+<table class='table2'>
+  <tr>
+    <td class='c009'>6 <span class='fss'>P.M.</span></td>
+    <td class='c019'>G.M.T.</td>
+    <td class='c029'>35° 50′ 22″</td>
+  </tr>
+  <tr>
+    <td class='c009'>9 <span class='fss'>P.M.</span></td>
+    <td class='c019'>„</td>
+    <td class='c029'>34° &#8196;3′ 23″</td>
+  </tr>
+  <tr>
+    <td class='c009'>12 <span class='fss'>P.M.</span></td>
+    <td class='c019'>„</td>
+    <td class='c029'>32° 16′ 12″</td>
+  </tr>
+</table>
+
+<p class='c007'>To utilise these predictions for the purpose in hand, the
+observer would measure with the sextant the apparent distance
+of the moon from Regulus at a known local time, and
+he would then compute what the apparent distance would
+have been if his observation had been made from the earth’s
+centre. From the tabulated distances, he would then be able
+to find the Greenwich time at which his observation was
+made; and, as we have seen, the difference between this and
+local time is a measure of the longitude.</p>
+
+<h3 class='c004'>CHAPTER X.<br> <span class='c014'>THE EXACT SIZE AND SHAPE OF THE EARTH.</span></h3>
+
+<p class='c003'><span class='sc'>Geodesy.</span>—We have already seen that the earth is a sphere,
+or of some form which differs but little from a sphere, and a
+rough method of determining its size, on this supposition, has
+been indicated. Now we have to inquire more minutely into
+<span class='pageno' id='Page_130'>130</span>the size and shape of our planet, for, as we shall see presently,
+a knowledge of these facts is essential to the adequate explanation
+of the various movements of the heavenly bodies,
+besides forming the basis of all our knowledge of the distances
+which separate us from the other bodies which people space.
+As an illustration of the importance of an exact knowledge of
+the size of the earth, it may be remarked that Newton’s grand
+law of gravitation was kept from the world for ten years,
+owing to an error in the generally accepted value of the
+earth’s radius, which was afterwards rectified by the labours of
+a French astronomer, Picard.</p>
+
+<p class='c007'>A great amount of labour has been expended in the endeavour
+to arrive at the true size and shape of the earth, and
+the name <i>geodesy</i> is given to the science which deals with
+these operations. As a secondary object, geodesy is concerned
+with the measurement and description of tracts of
+country.</p>
+
+<p class='c007'><span class='sc'>An Arc of Meridian.</span>—The measurement of the size of
+the earth is accomplished by first measuring relatively small
+parts of its surface, and then applying geometrical principles,
+in order to determine the whole circumference. If the earth
+were a true sphere, and we could measure the exact distance
+in miles between two places on the same meridian, a subsequent
+determination of the difference of latitudes of the two
+places would enable us to find the length of a degree,
+measured on the earth’s circumference. As there are 360° in
+a circle, the circumference would be 360 times the length of a
+degree, and the diameter of the earth would be the length of
+the circumference divided by 3·14159, this number expressing
+the constant ratio which exists between the circumference and
+diameter of a circle of any size whatsoever.</p>
+
+<p class='c007'>The determination of the size and shape of the earth thus
+involves two distinct sets of operations; first, measures of
+distances; and second, astronomical observations to determine
+the angular measurements of the arcs on the earth’s surface
+comprised between stations separated by known distances.
+When two such stations lie on the same meridian, the arc
+<span class='pageno' id='Page_131'>131</span>measured in this way is called an <i>arc of meridian</i>. We have
+already seen what means are available for finding the latitudes
+and longitudes of places on the earth, and it now remains for
+us to apply a yard measure, or its equivalent, to the precise
+measurement of the distance between places which are many
+miles apart.</p>
+
+<p class='c007'><span class='sc'>The Base Line.</span>—In the first instance a line of unimpeachable
+straightness is measured with scrupulous accuracy.
+The measuring-rod which has been most successfully employed
+is one consisting of a combination of brass and steel
+bars, which automatically corrects itself for changes of temperature
+in very much the same way that the balance-wheel
+of a chronometer, or of a good watch, corrects itself so as to
+perform its swing in equal periods at all temperatures.
+Several of these compensated rods are used, and they are
+enclosed in wooden boxes which are provided with levels and
+sights. When in use the outer boxes rest on adjustable
+trestles, and instead of putting the rods end to end they are
+placed a certain definite distance apart by the use of microscopes,
+which are themselves mounted on compensating bars.
+The first rod is put in position and levelled, and the others
+are successively placed in line with it by means of the sights.
+As the ground ceases to be perfectly flat it becomes
+necessary to raise the level of succeeding bars, but they are
+kept in the same vertical plane. Six bars are frequently
+employed in laying out a base line, and in order to protect
+them from extremes of temperature they are usually kept
+covered with long tents. In this way a distance of several
+miles can be measured with no greater probable error than a
+couple of inches, and the ends of such a measured base line
+are marked on metal plugs built in columns of masonry.
+The chief base lines measured in connection with British map
+construction were on the sandy shores of Lough Foyle in
+Ireland, 41,614 feet in length, and on Salisbury Plain, 36,578
+feet long.</p>
+
+<p class='c007'><span class='sc'>Triangulation.</span>—When a base line has been accurately
+measured in this way, a distant object which is clearly visible
+<span class='pageno' id='Page_132'>132</span>from both ends is observed with the aid of an instrument
+called the <i>theodolite</i>, and the
+angles between the base line
+and the lines joining its ends
+with the object are very carefully
+determined. Thus if A
+B in Fig. 36 represent the
+base line, and C a conspicuous
+object several miles away, the
+angles C A B and C B A are
+measured, and then it becomes
+easy to determine the distances
+A C and B C by
+trigonometrical calculations.
+A check on the accuracy of
+the observations is obtained by transferring the theodolite
+to C and measuring the angle A C B. The sides of the
+triangle may then be employed as new base lines for the
+measurement of other distances. With the theodolite at C,
+another object, D, is sighted, and the angle D C A is
+measured; similarly, with the theodolite at A, the angle
+C A D is determined, and from these observations the
+distances of D from the points A and C are easily computed.
+These distances again become available for base lines, and so
+the triangulation can be extended indefinitely.</p>
+
+<div id='i_132'  class='figcenter id001'>
+<img src='images/i_132.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 36.</span>—<i>Triangulation.</i></p>
+</div>
+</div>
+
+<p class='c007'>In a mountainous country, the sides of the triangles are
+often as much as 100 miles in length. Signals on the
+Wicklow Mountains in Ireland have been observed from Ben
+Lomond in Scotland and from Scafell in Cumberland. The
+stations are chosen so that none of the angles to be measured
+are very small, and in this way the chances of error are
+greatly reduced. Hence the triangles in the immediate
+neighbourhood of the base line are comparatively small, but
+the sides are gradually extended as the survey proceeds.</p>
+
+<p class='c007'>The process of triangulation forms the basis of the construction
+of accurate <i>maps</i>, and for this purpose the great
+triangles are subdivided by a secondary triangulation, so
+<span class='pageno' id='Page_133'>133</span>that the exact situations of a very great number of places are
+determined. These, again, serve for another set of still
+smaller triangles, with sides perhaps a mile in length; and
+finally the details are filled in by local chain surveys and
+draughtsmanship.</p>
+
+<p class='c007'>There is another point of some importance in connection
+with these triangulations when on a large scale. The larger
+triangles must be corrected for the curvature of the earth’s
+surface. The construction of the theodolite is such that
+two adjacent sides of any triangle, measured from their
+intersection, are referred to the same horizon; but when the
+instrument is transferred to another corner of the triangle,
+the adjacent sides are referred to a new horizon. The sum
+of the three angles of a triangle in these geodetical surveys
+thus exceed two right angles, whereas in plane triangles they
+are always equal to two right angles; the difference is called
+the <i>spherical excess</i>, and in the computations the observed
+angles have to be corrected on this account.</p>
+
+<p class='c007'>Thus, after an extremely laborious survey, it becomes possible
+to determine with great accuracy the distance between
+any two places whatever, and so the number of miles between
+two places at the extremities of an arc of meridian is ascertained.
+An arc of meridian extending nearly 18° has been
+measured in India, and another over 25° long extends from
+Hammerfest in Norway to the mouth of the Danube.</p>
+
+<p class='c007'><span class='sc'>Exact Shape and Size of the Earth.</span>—From the facts
+which have been gleaned by the measurements of arcs of
+meridian in different parts of the world, it is found that the
+length of a degree of latitude as measured on the earth’s
+circumference increases towards the Poles. In latitude 66° N.
+a degree is about 3,000 feet longer than a degree near the
+Equator. This means that the curvature of a meridional arc
+is greatest at the Equator, whence it is concluded that the
+earth is flattened at the Poles. The figure which best accords
+with the observations is the ellipse, and thus it becomes
+possible to calculate the polar diameter, although no arcs have
+been measured in the immediate neighbourhood of the Poles.</p>
+
+<p class='c007'><span class='pageno' id='Page_134'>134</span>Arcs of longitude, extending between two places which
+have the same latitude, have also been measured and applied
+to the determination of the figure of the earth, and, indeed,
+any arcs between two places of known latitude and longitude
+can be utilised.</p>
+
+<p class='c007'>When all the facts are brought together it is found that the
+earth’s polar diameter is about 26 miles shorter than the
+average equatorial diameter, while an equatorial section of
+the earth is also elliptical, the diameter passing through
+longitude 14° E, being two miles longer than the one at right
+angles to it. According to the calculations of Colonel Clarke,
+R.E., we have the following principal dimensions:</p>
+
+<table class='table2'>
+  <tr>
+    <td class='c022'>Earth’s</td>
+    <td class='c022'>mean</td>
+    <td class='c022'>equatorial</td>
+    <td class='c022'>semi-diameter</td>
+    <td class='c029'>= 3,963·296 miles.</td>
+  </tr>
+  <tr>
+    <td class='c022'>„</td>
+    <td class='c019'>„</td>
+    <td class='c022'>polar</td>
+    <td class='c019'>„</td>
+    <td class='c029'>= 3,950·738   „</td>
+  </tr>
+  <tr>
+    <td class='c022'>Polar</td>
+    <td class='c022' colspan='3'>compression</td>
+    <td class='c029'>¹⁄₂₉₃.₄₆</td>
+  </tr>
+</table>
+
+<p class='c007'>A solid which has a shape like that of the earth, with three
+axes of unequal lengths, is called an <i>ellipsoid</i>.</p>
+
+<p class='c007'>A very important consequence of the ellipsoidal form of
+the earth is that lines which are vertical—that is, perpendicular
+to the surface of water—do not pass through the centre of
+the earth, unless they are at the Poles or at certain points on
+the Equator.</p>
+
+<p class='c007'>There is every reason to suppose that at one time the
+earth was in a molten condition, and in response to physical
+laws, such a mass of matter could not retain a spherical form
+when set in rotation, although the sphere would be its
+natural shape if at rest. This has been demonstrated by
+a variety of experiments.</p>
+
+<p class='c007'>Thus, taking it generally, the shape of the earth is very
+intimately associated with its rotation, and it will subsequently
+appear that the same holds good for the sun and
+planets. Those bodies which have the most rapid rotation
+show the greatest flattening in the direction of the polar
+diameter.</p>
+
+<p class='c007'>In addition to direct measurements of the earth, there are
+<span class='pageno' id='Page_135'>135</span>other ways of studying the shape of our planet. One of these
+depends upon observations of the swing of a pendulum at
+different parts of the earth’s surface; as the time of oscillation
+of a pendulum depends upon the force of gravity, which
+itself varies with the distance from the earth’s centre, it is
+evident that this method is a practicable one. It is true that
+the matter is complicated in various ways, but after everything
+has been taken into account, these pendulum observations
+indicate, not only that the earth is flattened at the Poles,
+but they show further that the amount of polar compression
+deduced from geodetical work is in all probably very near
+the truth.</p>
+
+<p class='c007'>Again, the movement of the moon around the earth is
+found to be subject to certain irregularities which would not
+exist if the earth were a perfect sphere. These inequalities
+being deduced from observations of the moon’s position, the
+amount of polar flattening necessary to produce them can be
+calculated, and this is found to agree very closely with the
+value derived from the measurements of arcs of meridian.</p>
+
+<p class='c007'><span class='sc'>Different Kinds of Latitude.</span>—If the earth were a
+smooth spherical body, the latitude of a place would be
+simply equal to the angle made by a line joining it to the
+earth’s centre with the plane of the Equator. Owing to the
+bulging out of the earth in its equatorial part, however, it
+becomes necessary to distinguish between different kinds of
+latitude. If we adopt the definition given above, the name
+of <i>geocentric latitude</i> is given to the angular measurement.
+Taking the earth as a smooth geometrical spheroid, and
+assuming it to have certain dimensions, the angle which
+a line perpendicular to the surface makes with the plane of
+the Equator determines the <i>geographical latitude</i>. As the
+line perpendicular to the surface does not pass quite through
+the centre of the earth, the geographical and geocentric
+latitude differ by as much as 11′ in mid-latitudes, although
+nearly agreeing at the Poles and on the Equator.</p>
+
+<p class='c007'>As there are no direct means of finding the direction of a
+line passing through the earth’s centre, or of one perpendicular
+<span class='pageno' id='Page_136'>136</span>to the imaginary standard spheroid, geocentric
+and geographical latitudes must be calculated from the
+<i>astronomical latitude</i>, which is determined by observations of
+the elevation of the Pole, or its equivalent. The astronomical
+latitude is the angle between the direction of gravity
+and the Equator, and is therefore to a small extent dependent
+upon local irregularities of the earth’s surface.</p>
+
+<p class='c007'>A knowledge of geocentric latitude is chiefly of use in
+making corrections for parallax, in order that the data
+calculated for the earth’s centre may be precisely corrected
+for the place of observation, or <i>vice versâ</i>, as in the case of a
+lunar distance measured for the determination of longitude,
+or in the calculation of a solar eclipse.</p>
+
+<p class='c007'><span class='sc'>Variation of Latitude.</span>—For some years past a widespread
+interest has been taken in the question of a possible
+change in the position of the earth’s axis with regard to its
+surface. The subject is by no means a new one, for as far
+back as two thousand years ago, such variations were suspected.
+Changes amounting to several degrees were then
+believed to have occurred, but it is now certain that the
+supposed variation was due solely to the imperfection of the
+observations. As astronomical science became more and
+more precise, even before the discovery of aberration, it
+became evident that if any changes of latitude were taking
+place at all, they must be very minute.</p>
+
+<p class='c007'>In its geological aspect, the possibility of great changes of
+latitude having occurred in the past history of our globe is
+evidently well worth serious investigation. Granted a
+sufficient change in the position of the earth’s axis, the
+climate of London might become Arctic, or that of Greenland
+tropical. From this point of view the subject has been
+mathematically investigated by Professor G. H. Darwin, and
+it appears that if only the varying distribution of land and
+sea indicated by the geological records be taken into account,
+past changes of more than about three degrees are very
+improbable. Admitting that at any time during the life-history
+of our globe the earth was sufficiently plastic to be
+<span class='pageno' id='Page_137'>137</span>deformed by earthquakes or other disturbances, it is possible
+that changes amounting to 10° or 15° may have occurred.</p>
+
+<p class='c007'>Opinion is perhaps best reserved as to what has happened
+in the past. We are on surer ground when we consider the
+variations of latitude which are now going on.</p>
+
+<p class='c007'>Many competent observers have investigated the present
+movements of the Pole, and it has been conclusively demonstrated
+that changes in the position of the earth’s axis do
+really occur. Dr. Küstner, of Berlin, commenced a series of
+observations for a different purpose in 1884, and found that
+some anomalous results could only be explained by supposing
+that the latitude of Berlin was from 0″·2 to 0″·3 greater
+from August to November, 1884, than from March to May in
+1884 and 1885. Great interest was excited by this striking
+result, and steps were at once taken to test its truth. Old
+observations were re-discussed and compared, and new observations
+were made, with the final result that the movement
+of the earth’s axis of rotation was placed beyond dispute. It
+was not until Dr. Chandler attacked the problem, however,
+in 1891, that the nature of the changes became clear. His
+masterly analysis indicated that the observed variations in
+latitude arise from two periodic fluctuations superposed upon
+each other; one of these has a period of 427 days, and a
+semi-amplitude of 0″·12, while the other is an annual change
+which has ranged between 0″·04 and 0″·20 during the last
+fifty years. The resultant of the two movements produces
+changes which are seemingly very irregular in amount and
+of varying period, but a cycle is completed about every seven
+years. When the two sources of difference are at their
+maximum at the same time, the total range reaches about
+two-thirds of a second of arc. In consequence of the
+inequality of the annual part of the change, the apparent
+average period between 1840 and 1855 approximated to 380
+or 390 days; widely fluctuated from 1855 to 1865; from 1865
+to about 1885 was very nearly 427 days, afterwards increased
+to near 440 days, and very recently fell to somewhat below
+400 days.</p>
+
+<div id='i_138'  class='figcenter id001'>
+<span class='pageno' id='Page_138'>138</span>
+<img src='images/i_138.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 37.</span>—<i>Movements of the Earth’s Pole, 1890–95.</i></p>
+</div>
+</div>
+
+<p class='c007'>At the present time the variation of latitude is being very
+carefully investigated by the International Geodetic Association,
+and the latest results obtained are illustrated diagrammatically
+in Fig. 37. The mean position of the Pole is at
+the centre of the diagram,<a id='r3'></a><a href='#f3' class='c027'><sup>[3]</sup></a> and the horizontal line to the
+right of this point is directed towards Greenwich. The remarkable
+spiral curve shows the wanderings of the Pole about
+its mean position during five recent years. To simplify
+matters, the amount of deviation is represented in feet instead
+of in angular measure, and it will be seen that although
+the variation of latitude may be of considerable interest and
+importance in astronomical matters, it really does not amount
+<span class='pageno' id='Page_139'>139</span>to very much in matters terrestrial, the greatest change in the
+position of the Pole not amounting to more than 20 yards.
+Nevertheless, it is not inconceivable that it may yet have to
+be reckoned with in questions relating to boundary lines
+which depend upon latitude determinations.</p>
+
+<h3 class='c004'>CHAPTER XI.<br> <span class='c014'>THE DISTANCES AND DIMENSIONS OF THE HEAVENLY BODIES.</span></h3>
+
+<p class='c003'><span class='sc'>Parallax.</span>—The problem of determining the distance of a
+heavenly body resolves itself into a measurement of its
+<i>parallax</i>, that is, of the apparent change of its position
+brought about by a change in the situation of an observer.
+If one be seated in a room, about a yard from a window, a
+very simple experiment may be made to illustrate the meaning
+of this term. Closing one eye, the observer will see a
+vertical line, such as the partition between two panes, projected
+upon some particular part of an opposite building;
+when the other eye is used the line will apparently be displaced,
+and the nearer one is to the window the greater will
+be the displacement or parallax. As the heavenly bodies are
+so far away, each of our eyes sees them in the same directions.
+Indeed, the stars are so distant that to <i>all</i> persons situated on
+our planet their apparent positions are identical. With the
+members of the solar system, however, the case is different;
+the earth has an appreciable size as seen from them, so that
+when viewed from different parts of the earth they will not
+appear in exactly the same part of the heavens.</p>
+
+<p class='c007'>The earth’s rotation changes the relation of an observer’s
+position with regard to a heavenly body in pretty much the
+same way as a change in his actual position on the globe.
+<span class='pageno' id='Page_140'>140</span>When an object in the zenith is observed, it will appear in
+precisely the same part of the sky as if it were seen from the
+centre of the earth, but as it approaches the horizon it will be
+displaced. Hence the term <i>diurnal parallax</i>, meaning the
+displacement of a heavenly body depending upon the
+observer’s position as affected by the earth’s rotation. Taking
+it in its general astronomical sense, the parallax of a heavenly
+body is the angle between the two lines which join it to the
+observer and to the centre of the earth respectively. Thus,
+in Fig. 38, let O be an observer, Z his zenith, and C the
+centre of the earth; then the parallax of a body S is the
+angle O S C. As the observer’s
+position is changed to O′ by the
+earth’s rotation, the parallactic
+angle is increased to O′ S C.
+If S be on the horizon, that
+is, when O′ C is perpendicular
+to O′ S, the parallax is a
+maximum, and is then called
+the horizontal parallax. The
+<i>horizontal parallax</i> of a body
+is therefore the greatest angle
+subtended by the earth’s
+radius as seen from the body.
+We have seen, however, that
+the earth’s radius is not of the
+same length in all parts, and it is therefore necessary to
+specify more particularly which radius is in question. The
+standard adopted is the equatorial radius, and, when this is
+employed, our greatest parallactic angle is called the <i>equatorial
+horizontal parallax</i>.</p>
+
+<div id='i_140'  class='figcenter id001'>
+<img src='images/i_140.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 38.</span>—<i>Parallax of a Heavenly Body.</i></p>
+</div>
+</div>
+
+<p class='c007'>In the case of all the heavenly bodies the parallaxes
+are very small; that of the moon averages about 57′, while
+that of the nearest planet does not exceed 40″. The
+parallax of a body evidently diminishes as the distance
+increases.</p>
+
+<p class='c007'><span class='sc'>Distance Deduced from Parallax.</span>—When the
+<span class='pageno' id='Page_141'>141</span>parallax of a heavenly body has been determined, it becomes
+a simple matter to calculate the corresponding distance;
+thus, in Fig. 38, the distance C O′ represents the earth’s
+equatorial radius, O′ S C is the equatorial horizontal parallax,
+C O′ S is a right angle, and the required distance is C S.
+By a simple trigonometrical rule this distance is the earth’s
+radius divided by the sine of the parallax. In the case of a
+small angle, the sine is very nearly equal to the angle itself
+divided by the angle corresponding to an arc of a circle equal
+in length to the radius. As there are 206,265 seconds in an
+arc equal to the radius, the sine of a small angle may be
+taken as the angle itself, expressed in seconds, divided by this
+number. Thus, if <i>p</i> be the equatorial horizontal parallax of
+an object reckoned in seconds of arc,</p>
+
+<table class='table2'>
+  <tr>
+    <td class='c030'>Distance</td>
+    <td class='c031'>= <span class='fraction'><span class='under'>earth’s equatorial radius</span><br>sine <i>p</i></span></td>
+  </tr>
+  <tr>
+    <td class='c030'>&#160;</td>
+    <td class='c031'>= <span class='fraction'><span class='under'>206,265 × earth’s equatorial radius</span><br><i>p</i></span></td>
+  </tr>
+</table>
+
+<p class='c007'>We shall see presently that the average parallax of the sun
+is 8″·80, and its average distance, as derived from the application
+of this formula, is accordingly about 92,790,000 miles.</p>
+
+<p class='c007'><span class='sc'>Diameters.</span>—It is a familiar fact that the further an object
+is removed from us the smaller it appears. The ascent of a
+balloon at once suggests itself as an excellent example. It is
+necessary, therefore, to distinguish very carefully between the
+apparent and the true size of an object. A halfpenny at a
+distance of nine feet from the eye will just cover the moon if
+the line of sight be directed towards that body, but we should
+not say the moon is the size of a halfpenny, because we know
+perfectly well that a disc twice the size would produce just the
+same appearance if removed to double the distance. Apparent
+size must, accordingly, be reckoned in angular measure,
+and we say, for example, that the moon has an apparent
+diameter of a little more than half a degree.</p>
+
+<p class='c007'>When the angular diameter and distance have both been
+<span class='pageno' id='Page_142'>142</span>measured, the real diameter, in miles, can at once be deduced
+by a simple inversion of the process of determining the distance
+of an object from its known parallax. Thus, in Fig. 39
+let A B represent the moon or other heavenly body, and E
+the centre of the earth. The angle M E A is the angular
+semi-diameter, and E M the required distance; then, since
+the angle E A M is a right angle,</p>
+
+<div class='nf-center-c0'>
+  <div class='nf-center'>
+    <div>A M = M E × sine M E A</div>
+  </div>
+</div>
+
+<p class='c007'>That is,</p>
+
+<div class='nf-center-c0'>
+  <div class='nf-center'>
+    <div>Semi-diameter in miles = distance in miles × sine of angular semi-diameter.</div>
+  </div>
+</div>
+
+<p class='c007'>Or,</p>
+
+<div class='nf-center-c0'>
+  <div class='nf-center'>
+    <div>Diameter = twice the distance × sine of angular semi-diameter.</div>
+  </div>
+</div>
+
+<div id='i_142'  class='figcenter id003'>
+<img src='images/i_142.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig 39.</span>—<i>Diameter of a Heavenly Body.</i></p>
+</div>
+</div>
+
+<p class='c007'>Since the apparent diameters are always small, the sine
+may be taken as equal to the circular measure; that is, the
+number of seconds which the angle contains divided by
+206,265.</p>
+
+<p class='c007'><span class='sc'>Distance and Size of the Moon.</span>—If the moon were a
+fixed body outside the earth, its parallax could be easily determined
+by a single observer, who, in that case, would note
+the apparent displacement produced by his rotation. It has,
+however, a very complex movement, and it is therefore difficult
+to separate the real change of position from the parallactic
+<span class='pageno' id='Page_143'>143</span>change. The best method is one in which two observers, far
+removed from each other, can observe the moon’s position at
+nearly the same instant, so that the effect of its movement is
+very small and can be sufficiently allowed for. A necessary
+consequence of this condition is that the two observers should
+be placed as nearly as possible on the same meridian.
+Observations with the object of determining the lunar
+parallax have accordingly been made at Greenwich and the
+Cape of Good Hope. From the known positions of these
+places and the size of the earth, the distance between them is
+very accurately known, and this serves as a base line in a
+triangulation of the moon.</p>
+
+<div id='i_143'  class='figcenter id003'>
+<img src='images/i_143.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 40.</span>—<i>Measurement of the Moon’s Distance.</i></p>
+</div>
+</div>
+
+<p class='c007'>If G and C, in Fig. 40, represent Greenwich and the Cape
+respectively, the celestial equators at the two places will be in
+the directions G E and C E. M being the moon, its declination,
+as measured at G, will be the angle M G E, and as
+measured at C it will be the angle M C E′. Since G E is
+parallel to C E′, the difference of these declinations (when
+both are north declinations, as in the diagram) will be the
+value of the parallactic angle G M C, which is about 1½°.
+From these data it is easy to calculate the distance of the
+moon either from Greenwich, the Cape, or the earth’s centre.
+In this way the distance of the moon is found at some particular
+moment, and the additional knowledge of the shape of
+its orbit enables us to determine the semi-major axis of the
+orbit, which is nothing more than the average or mean distance
+<span class='pageno' id='Page_144'>144</span>of the moon. The mean equatorial horizontal parallax of the
+moon is 3,422″·5, and the corresponding mean distance from
+the earth is 238,855 miles.</p>
+
+<p class='c007'>The average apparent diameter of the moon, as it would
+appear from the centre of the earth, is 31′ 7″, from which it
+results by the method already stated that the true diameter
+is 2,162 miles.</p>
+
+<p class='c007'>The apparent diameter of the moon is affected by the
+observer’s position upon the earth, as well as by the situation
+of the moon in its orbit. An observer to whom the moon is
+directly overhead is nearly 4,000 miles nearer to it than another
+observer who has it on his horizon. Tables have accordingly
+been drawn up to indicate the <i>augmentation</i> of the moon’s
+apparent diameter as it rises above the horizon. The greatest
+possible apparent diameter is about 36″.</p>
+
+<p class='c007'>Everyone must have noticed that when the moon is rising
+or setting, it looks much larger than when it is high up in the
+sky, an appearance which does not seem to accord with the
+fact that its measured angular diameter is least when on the
+horizon. It is evident, however, that the seeming increase of
+size is a subjective phenomenon, due to our incapacity to
+correctly judge distances.</p>
+
+<p class='c007'><span class='sc'>Relative Distances of Planets.</span>—The relative distances
+of the planets from the sun were found long before
+any of the actual distances were known with any reasonable
+degree of accuracy. Kepler discovered the relation which
+exists between these distances, and expressed it in his third
+or harmonic law, which states that “the squares of the periodic
+times of the planets are proportional to the cubes of their
+mean distances from the sun.”</p>
+
+<p class='c007'>In the case of the interior planets, the angles of greatest
+elongation furnish the means of finding their distances from
+the sun as compared with that of the earth. Thus, if V in
+Fig. 41 represents Venus, E the earth, and S the sun, the
+angle E V S is a right angle when Venus is at greatest
+elongation. The observed value of the angle S E V is 46°,
+and this definitely determines the shape, though not the size,
+<span class='pageno' id='Page_145'>145</span>of the triangle S E V. The distance of Venus from the sun,
+S V, is thus found to be 0·72 times the distance of the earth
+from the sun, S E. If Venus be at inferior conjunction, that
+is, at V′, its distance from the sun will be represented by 72,
+if the earth’s distance from the sun be denoted by 100.</p>
+
+<p class='c007'>This method can also be applied in the case of Mercury,
+but as the orbit is so eccentric, it is necessary to take the
+average of a large number of greatest elongation angles.</p>
+
+<p class='c007'>The process of determining the relative distance of an exterior
+planet, such as Jupiter, is a little more complex, but
+involves no considerable difficulties.</p>
+
+<div id='i_145'  class='figcenter id003'>
+<img src='images/i_145.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 41.</span>—<i>Relative Distance of Venus.</i></p>
+</div>
+</div>
+
+<p class='c007'>There is a curious relationship between the relative distances
+of the planets, which is commonly known as <i>Bode’s
+law</i>. A series of figures, 0, 3, 6, 12, 24, 48, 96, 192, 384, each,
+with the exception of the second, being double the preceding
+one, is written down, and the number 4 added to each. Then
+the resulting numbers approximately represent the relative
+distances of the planets from the sun. Thus:—</p>
+
+<table class='table2'>
+  <tr>
+    <td class='c019'>4</td>
+    <td class='c019'>7</td>
+    <td class='c019'>10</td>
+    <td class='c019'>16</td>
+    <td class='c019'>28</td>
+    <td class='c019'>52</td>
+    <td class='c019'>100</td>
+    <td class='c019'>196</td>
+    <td class='c021'>388</td>
+  </tr>
+  <tr>
+    <td class='c019'>Mercury</td>
+    <td class='c019'>Venus</td>
+    <td class='c019'>Earth</td>
+    <td class='c019'>Mars</td>
+    <td class='c019'>Asteroids</td>
+    <td class='c019'>Jupiter</td>
+    <td class='c019'>Saturn</td>
+    <td class='c019'>Uranus</td>
+    <td class='c021'>Neptune</td>
+  </tr>
+</table>
+
+<p class='c007'>It is interesting to note that this law was announced in
+1772, when the asteroids and the planets Uranus and Neptune
+were still unknown, so that there was a break in the
+<span class='pageno' id='Page_146'>146</span>series corresponding to the number 28. The discovery of
+Uranus in 1781, and the fact that its distance agreed roughly
+with Bode’s law, strengthened the conviction that an unknown
+planet revolved round the sun in an orbit between
+those of Mars and Jupiter. An association of astronomers
+was then formed to search systematically for the missing
+planet; but the actual discovery was made in 1801 by
+Piazzi, the Sicilian astronomer, who had not joined the
+association. The new planet was a very small one, and its
+discovery was rapidly followed by the detection of several
+others. At the present time, more than 400 of these asteroids,
+or minor planets, are known, and their average distance fits
+in very well with Bode’s law.</p>
+
+<p class='c007'><span class='sc'>The Sun’s Distance.</span>—One of the grandest problems
+which astronomical science requires us to solve is the determination
+of the sun’s distance. Starting with a knowledge
+of the earth’s dimensions, the subsequent measurement of
+the sun’s distance enables us to get a clear idea of the scale,
+not only of the solar family to which we ourselves belong,
+but of the whole sidereal universe. No wonder then that a
+vast amount of astronomical energy has been expended on
+this investigation.</p>
+
+<p class='c007'>The problem, however, is beset with many practical difficulties,
+and the greatest possible skill is required to cope with
+it. In the first place, the parallax of the sun is so small that
+the method employed for the moon fails, and it can only be
+determined by indirect means.</p>
+
+<p class='c007'>We have already seen that the constant of aberration gives
+us a means of determining the size of the earth’s orbit, and
+consequently the distance of the sun. When proper allowance
+is made for the eccentricity of the orbit, this method is
+a very valuable one.</p>
+
+<p class='c007'>Other methods which have been employed depend upon
+the measurement of the parallax of one of the nearer planets,
+from which the distances of all the planets, including the
+earth, from the sun, can be found from our previous knowledge
+of the relative distances. Mars and some of the asteroids
+<span class='pageno' id='Page_147'>147</span>have been thus utilised at their oppositions, and Venus
+when at inferior conjunction.</p>
+
+<div id='i_147'  class='figcenter id003'>
+<img src='images/i_147.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 42.</span>—<i>The Parallax of Mars.</i></p>
+</div>
+</div>
+
+<p class='c007'>The parallax of Mars can be determined in the same way
+as that of the moon, either by concerted observations at two
+distant places, or by a single observer who utilises the earth’s
+rotation to provide him with a base line. The actual
+measurements do not consist of direct estimations of the right
+ascension and declination of the planet, but of its angular
+distances from stars among which it appears, the measurements
+being made with micrometers or heliometers. In this
+way certain errors due to refraction, etc., are minimised. To
+take an extreme case, let the planet M (Fig. 42) be rising to
+an observer at O; it will then be seen in the direction O M,
+while a neighbouring star will be seen along the line O S.
+After twelve hours the rotation of the earth will have carried our
+observer to O′, and he will now see the planet in the direction
+O′ M, while the star will remain in the same direction, O′ S′.
+In each case he would measure the angle separating the
+planet from the star, and would thus obtain the values of the
+angles S O M and S′ O′ M, which, in the case shown in the
+diagram, would be together equal to the angle O M O′.
+When corrected for the observer’s latitude, and for the planet’s
+change of place in the interval, the equatorial horizontal
+parallax of Mars would be determined. Then the distance of
+Mars from the earth would be known, and at opposition this
+is the difference between the distances of the earth and of
+Mars from the sun; the ratio between the latter is already
+known, and their actual distances at once follow.</p>
+
+<div id='i_148'  class='figcenter id001'>
+<span class='pageno' id='Page_148'>148</span>
+<img src='images/i_148.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 43.</span>—<i>The Transit of Venus.</i></p>
+</div>
+</div>
+
+<p class='c007'><span class='sc'>Transit of Venus.</span>—The planet Venus at inferior
+conjunction is near enough to the earth to have a considerable
+parallax, but the method employed in the case
+of Mars cannot be used, as the planet is not visible when
+between us and the sun, except on the very rare occasions
+when it transits across the sun’s disc. When a transit
+occurs, the distance of the planet from the earth can be
+measured in essentially the same way as that of Mars at
+opposition, when two observers work together. The difference
+is that the apparent place of the planet is referred to the sun’s
+disc instead of to neighbouring stars. Suppose the conditions
+to be as represented in
+Fig. 43, E being the
+earth, V the planet, and
+S the sun. Two observers
+on the earth, at
+<i>a</i> and <i>b</i>, will see the
+planet projected on different
+parts of the sun’s
+disc. If we at first
+regard them as being
+at rest, the observer at
+<i>b</i> would see the planet
+cross the sun along the
+line C D, while to the one at <i>a</i> it would appear to cross the
+line F G. The times of crossing would, under the assumed
+conditions, depend upon the orbital velocity of Venus, and a
+measure of these times at the two stations would determine
+the relative lengths of the chords C D and F G. We already
+know that the distance of Venus from the sun is to its distance
+from the earth at inferior conjunction in the proportion
+72 to 28. (See p. 145.) The rectilinear distance between
+the two places is also known, and the distance <i>x y</i> between
+the chords is ⁷²⁄₂₈ of that from <i>a</i> to <i>b</i>, whatever the actual distance
+of the sun may be. We thus know the ratio of the
+lengths of two parallel chords, and the distance between them
+in miles, from which it is a simple matter to find the diameter
+<span class='pageno' id='Page_149'>149</span>of the sun’s disc in miles. The angular diameter of the sun is
+measured with a transit instrument, and to find the sun’s
+distance we have simply to calculate the distance at which a
+body of known size subtends a known angle.</p>
+
+<p class='c007'>We have supposed the observers at rest, but they are in
+reality carried forward by the earth’s orbital motion, and are
+turned about the earth’s axis. The first of these movements
+will affect both observers in the same degree, and will simply
+lengthen the duration of the transit. The effect of rotation,
+however, depends upon the position of the sun and planet,
+with regard to the observer’s meridian. At sunrise, an
+observer is carried by the rotation of the earth almost directly
+towards the sun, while at sunset he is carried away from it.
+The rate at which the planet traverses the sun’s disc would,
+therefore, be little affected by the earth’s rotation at sunrise
+or sunset. About mid-day, however, the effect of the earth’s
+rotation is to accelerate the apparent motion of the planet,
+and to shorten the time of transit. If the beginning of the
+transit be observed at sunset, and the end soon after sunrise,
+as it may well be in high latitudes, the duration of the transit
+is retarded by the earth’s rotation. Corrections for rotation,
+however, are not difficult to apply.</p>
+
+<p class='c007'>In this method of observing a transit of Venus, which was
+suggested by Halley, when it was impossible that he would
+live to see it carried out, the places of observation must be
+widely separated in latitude, and the beginning and end of
+the transit must both be observed.</p>
+
+<p class='c007'>Another method of utilising a transit of Venus is known as
+Delisle’s method. In this case the two stations are near the
+Equator, and each observer notes the Greenwich time of internal
+contact, when the planet fully enters upon the sun’s
+disc.</p>
+
+<p class='c007'>Owing to various causes, chief among which is the so-called
+“black drop,” the time of ingress and egress cannot be actually
+recorded with the desired degree of accuracy, and the transit
+Venus is no longer looked upon as the best method of
+determining the distance which separates us from the sun.</p>
+
+<p class='c007'><span class='pageno' id='Page_150'>150</span>Some of the results which have been obtained for the solar
+parallax are as follows:—</p>
+
+<table class='table2'>
+  <tr>
+    <td class='c022'>Transit</td>
+    <td class='c019'>of Venus,</td>
+    <td class='c019'>1874,</td>
+    <td class='c022'>contact observations,</td>
+    <td class='c021'>8″·859</td>
+  </tr>
+  <tr>
+    <td class='c022'>„</td>
+    <td class='c019'>„</td>
+    <td class='c019'>„</td>
+    <td class='c022'>photographs,</td>
+    <td class='c021'>8·859</td>
+  </tr>
+  <tr>
+    <td class='c022'>„</td>
+    <td class='c019'>„</td>
+    <td class='c019'>1882,</td>
+    <td class='c022'>contact observations,</td>
+    <td class='c021'>8·824</td>
+  </tr>
+  <tr>
+    <td class='c022'>„</td>
+    <td class='c019'>„</td>
+    <td class='c019'>„</td>
+    <td class='c022'>photographs,</td>
+    <td class='c021'>8·842</td>
+  </tr>
+  <tr>
+    <td class='c022'>Gill’s</td>
+    <td class='c019'>observation</td>
+    <td class='c019'>of</td>
+    <td class='c022'>Mars, 1877,</td>
+    <td class='c021'>8·780</td>
+  </tr>
+  <tr>
+    <td class='c022'>Galle’s</td>
+    <td class='c019'>„</td>
+    <td class='c019'>&#160;</td>
+    <td class='c022'>Flora, 1873,</td>
+    <td class='c021'>8·873</td>
+  </tr>
+  <tr>
+    <td class='c022'>Gill’s</td>
+    <td class='c019'>„</td>
+    <td class='c019'>&#160;</td>
+    <td class='c022'>Juno, 1874,</td>
+    <td class='c021'>8·765</td>
+  </tr>
+  <tr>
+    <td class='c022'>„</td>
+    <td class='c019'>„</td>
+    <td class='c019'>&#160;</td>
+    <td class='c022'>minor planets, 1896,</td>
+    <td class='c021'>8·80</td>
+  </tr>
+</table>
+
+<p class='c007'>From a discussion of all the available data, Professor
+Harkness considers the most probable value of the solar
+parallax to be 8″·80905, with a probable error of 0·00567″.
+Turning this into miles, we find the distance of the sun to be
+92,796,950 miles, and this is in all probability not more than
+60,000 miles in error. This agrees very closely with Dr.
+Gill’s latest value, which has been accepted by the superintendents
+of the British and American nautical almanacs.</p>
+
+<p class='c007'><span class='sc'>The Sun’s Diameter.</span>—The real diameter of the sun is
+found from the parallax, and its mean angular diameter in
+the manner already explained (p. 142). Taking the distance
+as 92,780,000 miles, and the mean apparent semi-diameter as
+962″, we have</p>
+
+<table class='table2'>
+  <tr>
+    <td class='c032'>Sun’s diameter</td>
+    <td class='c031'>= <span class='fraction'><span class='under'>2&#8201;×&#8201;92,780,000&#8201;×&#8201;962</span><br>206,265</span></td>
+  </tr>
+  <tr>
+    <td class='c032'>&#160;</td>
+    <td class='c031'>= 865,400 miles.</td>
+  </tr>
+</table>
+
+<p class='c007'>The sun’s diameter is the same in all directions, so far as
+our measurements give any information on the point, so that
+there is no appreciable polar flattening corresponding to that
+of the earth and some of the other planets. This result is
+what we should expect from the relatively slow rate at which
+the sun turns upon its axis.</p>
+
+<p class='c007'><span class='sc'>Distances and Diameters of Planets.</span>—It has already
+been pointed out that our knowledge of the relative distances
+of the planets from the sun enables us to determine their
+<span class='pageno' id='Page_151'>151</span>absolute distances when the distance of one of them has been
+ascertained. In this way the determination of the earth’s
+distance leads us to those of the other planets.</p>
+
+<p class='c007'>Our additional knowledge of the planetary orbits further
+permits the calculation of the distance of any planet from the
+earth at a stated time. If, then, the angular diameter of a
+planet be measured with a micrometer attached to a telescope,
+the absolute diameter in miles can be determined in the same
+way as that of the sun or moon.</p>
+
+<p class='c007'>To take an actual example, the equatorial angular diameter
+of the globe of Saturn, as measured by Prof. Barnard with
+the great telescope of the Lick Observatory on April 14,
+1895, was 19″·4. It was then computed that if the observation
+had been made from the sun this would have been reduced to
+17″·9. The distance of Saturn from the sun being 9·5388
+times the earth’s distance, it results from this measurement
+that the true equatorial diameter of the ball of Saturn is
+76,500 miles. A number of independent measures made at
+intervals from March to July gave an average value of
+76,470 miles for the diameter.</p>
+
+<h3 class='c004'>CHAPTER XII.<br> <span class='c014'>THE MASSES OF CELESTIAL BODIES.</span></h3>
+
+<p class='c003'><span class='sc'>Mass and Weight.</span>—As a matter of daily experience, we
+know that a certain effort is required to prevent a body from
+falling to the ground, and the larger the bulk of any particular
+kind of matter, the greater is the effort demanded.
+Again, equal bulks of different kinds of matter require unequal
+efforts to sustain them in the hand. From facts such
+as these we get the idea of <i>weight</i>, and we say that one body
+is heavier than another when it has the greater tendency to
+<span class='pageno' id='Page_152'>152</span>fall to the ground. For the purposes of everyday life, the
+weight of a body is used as a measure of the quantity of
+matter which it contains, and the standard of weight in our
+own country is that of a certain piece of platinum kept at the
+Exchequer Office, in London, which is called a <i>pound</i>. The
+weight of the same piece of matter varies at different parts of
+the earth’s surface, and also at different distances from the
+ground, and it is evident, therefore, that weight is not a very
+scientific measure of the quantity of matter which a body
+contains. The standard of comparison must be one which is
+invariable not only in all parts of the earth, but, if we
+wish to investigate the quantity of matter in the celestial
+bodies, it must be unalterable through all parts of the universe.</p>
+
+<p class='c007'>One’s first idea is that the bulk, or space which a body
+occupies, will furnish a means of measuring the quantity of
+matter which it contains, but here again we find that the
+volume of a body can be varied without either adding to or
+subtracting from it, its weight remaining constant. A piece
+of ice, for example, occupies a greater space than an equal
+weight of water.</p>
+
+<p class='c007'>It is evident then that some other property of matter must
+be used as a measure of quantities. Now, there is every
+reason to believe that the same piece of matter, in whatever
+part of space it may be situated, requires the same force to
+set it moving with the same speed in a given time. By the
+continued application of a force, a body will first be set in
+motion, and at the end of a second it will have a certain speed;
+in the next second the velocity will have increased by an
+amount equal to that acquired at the end of the first second,
+and so on for subsequent intervals. For example, if at the
+end of a second the velocity were 3 feet per second, at the
+end of the next second it would be 6 feet per second, and
+after other equal intervals it would be successively 9, 12, 15,
+and so on. In this way the velocity is increased uniformly,
+and is said to be uniformly accelerated, while the gain per
+second is called the <i>acceleration</i>. The greater the force
+<span class='pageno' id='Page_153'>153</span>applied, the greater will be the acceleration it produces, and
+the acceleration can be used as a measure of the force at
+work.</p>
+
+<p class='c007'>If the same force be applied to different quantities of the
+same substance, the acceleration produced will be in inverse
+proportion to the quantities. We thus arrive at the important
+result that two bodies, whatever their nature, contain
+equal quantities of matter, or have equal <i>masses</i>, when equal
+forces give them the same acceleration. The mass of a body
+can thus be ascertained by observing the acceleration due to
+the action of a known force.</p>
+
+<p class='c007'>As a matter of observation, it is found that all bodies, whatever
+their composition or size, fall to the ground from the same
+height in the same time if the observations be made at one
+place. This means that the forces corresponding to weights
+produce equal accelerations in all bodies at the same place,
+and it follows, therefore, that the weights of bodies at the
+earth’s surface, are proportional to their masses. Hence, it is
+that weight can be practically employed in comparing masses,
+or quantities of matter, for the purposes of everyday life. It
+must be clearly understood, however, that a <i>mass</i> of a pound
+is in reality quite distinct from a <i>weight</i> of a pound, the
+former specifying a certain quantity of matter, and the latter
+its tendency to fall towards the earth.</p>
+
+<p class='c007'><span class='sc'>The Law of Gravitation.</span>—The idea that weight is due
+to the attraction of the earth for all bodies in its neighbourhood
+was first suggested by Newton, and an extension of
+this idea led him to formulate the great law which underlies
+the whole science of astronomy. All bodies near the earth’s
+surface are acted upon by forces proportional to their masses,
+and the same acceleration is produced in all of them if they
+are allowed to fall to the ground. Falling freely for a second,
+all bodies whatsoever, when the resistance of the air is
+eliminated, pass through a little over 16 feet, and acquire a
+velocity of just over 32 feet per second. The acceleration due
+to gravity is thus 32⅙ feet per second for bodies near the
+earth’s surface. If the experiment be made at the top of
+<span class='pageno' id='Page_154'>154</span>a high mountain, the distance fallen through and the acceleration
+acquired in a second is found to be less.</p>
+
+<p class='c007'>If we could ascend still higher, the acceleration produced
+in falling bodies would be again reduced, and, in the light of
+what has gone before, it is evident that the force with which
+bodies tend to fall to the earth is diminished as the distance
+from the earth’s surface is increased. It was such considerations
+as these which led Sir Isaac Newton to formulate the
+law that <i>the force with which a body is attracted towards the
+earth diminishes in inverse proportion to the square of the
+distance from the earth’s centre</i>. Terrestrial means of testing
+the truth of this statement are obviously very limited, and
+hence it was that Newton looked to the moon for its verification.
+If the law holds good at the distance of the moon, an
+object so far removed and not acted upon by other forces,
+should fall towards the earth, and as its distance is about
+sixty times that of a body at the surface from the centre of
+the earth, the acceleration produced should be only ¹⁄₃₆₀₀th
+part of that imparted to bodies near the surface. In other
+words, since a body near the surface falls through 16 feet in
+the first second, one at the moon’s distance should only
+fall through about ¹⁄₂₀th of an inch. If, then, the moon be
+subject to the earth’s attraction, this fall towards the earth
+must be exhibited in some form or other, although the fact
+that the moon does not fall down upon the earth shows that
+there is some counteracting tendency.</p>
+
+<p class='c007'>Observations have shown us that the moon moves in a
+curved path. It has been put in motion somehow, and since
+there is no reason why it should turn to one side or the other,
+or come to rest, unless some forces are acting upon it, it
+would tend to go on uniformly in a straight line for ever.
+That its movement is curvilinear is at once an indication of
+the action of a force besides that which originally set it in
+motion. This force is directed towards the earth, and the
+moon is drawn out of its rectilinear path just as far in any
+specified time as it would fall towards the earth if at
+rest.</p>
+
+<p class='c007'><span class='pageno' id='Page_155'>155</span>Let E and M in Fig. 44 represent the earth and moon
+respectively. Then, if the moon were not hindered in any
+way, it would move in the
+direction M <i>b</i>, and would reach
+the point <i>b</i>, let us say, at the
+end of a second. It is, however,
+found to be at the point
+<i>a</i>, and it has therefore fallen towards
+the earth through the
+distance <i>b a</i>. The size of the
+moon’s orbit and the angle
+through which it moves in a second being known, it is easy
+to calculate the distance <i>a b</i>, which is found to be about ¹⁄₂₀th
+of an inch, as demanded by Newton’s law.</p>
+
+<div id='i_155'  class='figcenter id001'>
+<img src='images/i_155.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 44.</span>—<i>The Moon’s Curvilinear Path.</i></p>
+</div>
+</div>
+
+<p class='c007'>In his first attempt to thus verify the law of gravitation,
+Newton failed for the want of a sufficiently accurate knowledge
+of the earth’s diameter, but a few years later a new arc
+of meridian was measured, and he had the untold satisfaction
+of demonstrating its truth.</p>
+
+<p class='c007'>The curved path of the moon is, indeed, similar to that of a
+projectile. A cannon ball thrown out horizontally will reach
+the ground after describing a curved path; but if it could be
+projected from a great elevation, with sufficient velocity, its
+forward movement would prevent its ever reaching the earth’s
+surface at all, and a new satellite of the earth would have been
+manufactured.</p>
+
+<p class='c007'>The same kind of reasoning can be applied to the paths of
+the earth and planets around the sun, and Newton demonstrated
+that the laws of Kepler were a necessary consequence of the
+law of gravitation extended beyond the system of the earth
+and moon. By mathematical reasoning it was proved that if
+one body describes an elliptic orbit around another, and
+the line joining them describes equal areas in equal times, the
+attractive force must be directed to the central body, and,
+moreover, must vary inversely as the square of the distance between
+the two bodies. In this way the movements of the planets
+round the sun are perfectly explained by supposing that an attractive
+<span class='pageno' id='Page_156'>156</span>force, similar to that which causes bodies to fall to the
+earth’s surface, is exerted between all masses of matter, and hence
+the origin of the term <i>Universal Gravitation</i>. In its complete
+form, the law of gravitation states that “any particle of matter attracts
+any other particle with a force which varies directly as
+the product of the masses, and inversely as the square of the
+distance between them.”</p>
+
+<p class='c007'>Confirmation of this grand law, which controls the movements
+of all the vast array of heavenly bodies, is furnished by
+many other phenomena. We see one of its effects in the tides,
+and another in the disturbances of the movements of planets
+brought about by their mutual attractions. Even in the
+depths of stellar space the same law holds good for those
+systems of stars which are sufficiently close together for their
+attractions to produce effects which we can study at our immense
+distance from them.</p>
+
+<p class='c007'>The cause of gravity is still one of the greatest mysteries
+of physical science, although many ingenious attempts have
+been made to furnish an explanation of its mode of action.</p>
+
+<p class='c007'><span class='sc'>Mass of the Sun.</span>—When we know the distance of the
+sun, and the time in which the earth travels completely round
+it, it is easy to calculate the fall of the earth towards the sun
+in the same way that the moon’s fall towards the earth is
+determined.</p>
+
+<p class='c007'>The distance which a body 93,000,000 miles distant falls
+towards the sun in a second is thus found to be 0·116 of an inch.
+A body at the earth’s surface is about 4,000 miles from the
+centre, and it falls 16¹⁄₁₂ feet in a second; if removed to a distance
+of 93,000,000 miles, its fall towards the earth would be
+reduced inversely as the squares of 4,000 and 93,000,000, and
+would amount to ·000,000,349 of an inch. This is only
+1/332,000th part the fall due to the sun’s attraction, and hence
+it is concluded that the mass of the sun is 332,000 times that
+of the earth.</p>
+
+<p class='c007'>Strictly speaking, the accelerations produced by the sun
+and earth should be compared, but the fall during the first
+second is proportional to the acceleration due to gravity, and
+<span class='pageno' id='Page_157'>157</span>the same result is therefore obtained. It may be observed
+also that the fall of the earth towards the sun would not be
+appreciably effected if it were twice the size. All bodies fall
+towards the earth at the same rate, whatever their weights,
+and so in the case of a planet, the distance fallen towards the
+central sun is independent of the planet’s mass; the greater
+the mass the greater the attractive force.</p>
+
+<p class='c007'>The sun occupies about 1,300,000 times the space occupied
+by the earth, and as its mass is only 332,000 times that of the
+earth, it follows that the sun’s density is only about a quarter
+that of the earth.</p>
+
+<p class='c007'><span class='sc'>Masses of Planets.</span>—The process employed for the determination
+of the sun’s mass can be utilised for finding the
+masses of those planets which are accompanied by satellites.
+From the known distance of the planet, the size of the orbit
+of a satellite can be calculated in miles, and knowing the
+period of revolution of the satellite, its fall towards the planet
+can be determined. This fall is then compared with that of
+the planet’s fall towards the sun, and the mass of the planet
+in terms of the sun’s mass is thus arrived at.</p>
+
+<p class='c007'>A convenient way of employing this method is to make
+use of a modification of Kepler’s third law. If <i>m</i> be the mass
+of a planet in terms of the sun’s mass, M, <i>a</i> and T respectively
+denote the semi-axis major of the orbit of the planet
+and its time of revolution round the sun; <i>a′</i> and T similar
+quantities pertaining to the satellites’ revolution round
+the planet: The following formula gives the relation of the
+masses:—</p>
+
+<div class='center'>
+
+<p class='c007'><span class='fraction'><span class='under'><i>m</i></span><br>M</span> =  (<span class='fraction'><span class='under'><i>a′</i></span><br><i>a</i></span>)<sup>3</sup>(<span class='fraction'><span class='under'>T</span><br>T′</span>)<sup>2</sup></p>
+
+</div>
+
+<p class='c007'>This formula can be applied in the case of Mars, Jupiter,
+Saturn, Uranus, and Neptune, but fails in the case of Mercury,
+Venus, and the asteroids, which, so far as we know,
+have no satellites.</p>
+
+<p class='c007'>The mass of Jupiter obtained in this way can be further
+checked by the influence of this giant planet upon other
+<span class='pageno' id='Page_158'>158</span>bodies in its neighbourhood. This planet has such an enormous
+mass that it produces very notable effects on the motions
+of Saturn, the asteroids, and of comets which travel in its
+neighbourhood, and, by measuring the amounts of these <i>perturbations</i>,
+the mass of the planet can be deduced.</p>
+
+<p class='c007'>This method of perturbations is at present the only one by
+which we can obtain a knowledge of the masses of those
+planets which have no satellites. The motion of Mercury is
+disturbed by its nearest neighbours, Venus and the earth;
+that of Venus by the earth and Mercury. The differences
+between the observed positions of the planets and those
+calculated on the supposition that the others did not affect
+them, give the necessary data for the computation of the
+masses. The process, however, is one requiring profound
+mathematical knowledge, and even yet the mass of Mercury
+is not very certainly known.</p>
+
+<p class='c007'>The asteroids, again, present no little difficulty. Their
+feeble light and small size point to small masses, and their
+mutual perturbations are almost insensible, except when two
+of them come into line with the sun. They produce no appreciable
+effects upon the movements of comets, so that it is
+almost impossible to determine their individual masses.
+Each asteroid, however, tends to produce a revolution of the
+major axis of the orbit of the nearest planet, Mars, and all
+tend to give it a motion in the same direction. If the total
+mass of all the asteroids put together were a quarter of the
+earth’s mass, a measurable displacement of the position of
+Mars would be produced. Professor Newcomb has recently
+shown that such a displacement actually occurs, but
+cannot amount to more than 5″·5 per century. From this
+it has been recently calculated that the total mass of
+the asteroids is probably about ¹⁄₁₁₅th that of the earth’s
+mass.</p>
+
+<p class='c007'><span class='sc'>Mass of the Moon.</span>—As the moon has no satellite, we
+must again have recourse to indirect methods if we wish to
+know anything as to its mass. Various processes are open to
+us; but although the moon is so near to us, it is more difficult
+<span class='pageno' id='Page_159'>159</span>to determine its mass than that of the most remote planet in
+our system.</p>
+
+<p class='c007'>It has already been explained (p. 77) that as the earth is
+accompanied by the moon, it is really the centre of gravity of
+the two bodies which obeys the laws of planetary movement.
+As this point lies between the centres of the two bodies, at
+distances which are in inverse proportion to the masses, the
+centre of the earth describes a small monthly orbit, which, as
+we have already seen, produces a small monthly inequality in
+the sun’s apparent movement.</p>
+
+<p class='c007'>By a careful investigation of this monthly oscillation of the
+sun, it has been found that the centre of gravity of the earth
+and moon must lie within the earth at a distance of about
+2,900 miles from the centre. This is about ¹⁄₈₁th of the moon’s
+distance, whence it follows that the mass of the moon is
+about ¹⁄₈₁th that of the earth.</p>
+
+<p class='c007'>Other methods of ascertaining the moon’s mass are also
+available. Among these are the investigation of the parts
+played by the moon in the production of the tides which
+swell our shores, and in the displacement of the earth’s axis
+which causes “nutation.”</p>
+
+<p class='c007'><span class='sc'>Masses of Satellites.</span>—The earth’s satellite is of exceptional
+magnitude in comparison with its primary, and the
+method of finding its mass from the situation of the centre of
+gravity cannot be applied to the satellites attending other
+planets. In the case of the satellites of Jupiter and Saturn,
+the masses have been approximately determined by their
+mutual perturbations, these generally resulting in a revolution
+of the major axes of the orbits. Even this method fails for
+the satellites of Mars, Uranus, and Neptune, so that practically
+nothing is known with regard to their masses.</p>
+
+<p class='c007'><span class='sc'>Mass and Density of the Earth.</span>—So far we have
+been concerned entirely with relative masses, referring the
+masses of the various orders of the heavenly bodies either to
+the earth or sun. Although this is usually all that is required
+for astronomical purposes, it is of great interest to determine
+the absolute mass of the earth, and from this the
+<span class='pageno' id='Page_160'>160</span>absolute masses of the heavenly bodies can at once be
+deduced.</p>
+
+<p class='c007'>We already know the dimensions of the earth, and therefore
+the number of cubic miles or feet which it occupies. We
+know also the weight or mass of a cubic foot of water or lead,
+and if the earth were of uniform specific gravity throughout
+its bulk, and composed of water or lead, we could at once
+calculate its total mass. It is, however, neither water nor
+lead; but if we can compare the mass of the earth with what
+it would be if composed of either of these substances, we can
+deduce either its mass or its specific gravity.</p>
+
+<p class='c007'>A very simple method of “weighing” the earth has been
+employed with much success by Professor Poynting. The
+experiment was carried out at the Mason Science College,
+Birmingham, with a large bullion balance in which the beam
+was 123 centimetres long. Two spheres of lead and antimony,
+each weighing about 21 kilograms, were suspended from the
+arms of the balance. Another sphere of lead and antimony,
+weighing 153 kilograms, was successively brought by means
+of a turn-table under each of the two smaller weights. The
+alteration in the weights of the attracted balls were measured
+by observing the deflection of the beam, this being immensely
+magnified by a simple optical arrangement in which a mirror
+reflecting a pencil of light was made to turn through 150
+times the angle moved through by the beam itself. The
+weight corresponding to a given deflection of the beam was
+determined by observing the disturbance produced by the
+addition of “riders” of known weights. In order to reduce
+the chances of error, the large weight was balanced on the
+turn-table by another mass of half the weight and at twice
+the distance from the centre, this being necessary in order
+that the attracting weight should rotate horizontally. The
+effect of this additional mass was calculated and allowed for,
+and the weighings were also repeated with the weights in
+various positions. The principle of the subsequent calculation
+is briefly as follows:—A mass A of lead and antimony
+of known bulk attracts another mass B with the force
+<span class='pageno' id='Page_161'>161</span>measured; if A were of the same size as the earth, the attraction
+would be increased by as many times as the earth is
+larger than A. If the average specific gravity of the earth
+were the same as that of the mass A, this calculated attraction
+would be equal to the weight of B. The ratio of this
+calculated weight of B to the actual weight accordingly gives
+the proportion between the specific gravity of the experimental
+ball and the average specific gravity of the whole
+earth. From this experiment it was estimated that the mean
+density of the earth is 5·4934 times that of water.</p>
+
+<p class='c007'>The same principle is applied in the case of the famous
+Cavendish experiment, and its subsequent modifications by
+Baily, Cornu, and Boys.</p>
+
+<p class='c007'>Another method of finding the earth’s density, and therefore
+its mass, is chiefly of historical interest. This is known
+as the “mountain method,” and was carried out in 1774 by
+Maskelyne, Hutton and Playfair on the Schiehallion Mountain,
+in Perthshire. A plumb-line suspended at the north
+side of the mountain is drawn towards the mountain, and so
+will not hang quite vertically. If removed to the opposite
+side of the mountain it will be deflected in the reverse direction.
+The amount of this deflection can be measured by
+reference to the stars, the positions of which are in no wise
+influenced by the attraction of the mountain. A survey of
+the mountain was next made in order to determine its bulk,
+and then the average specific gravity of the rocks composing
+it was determined with the greatest possible accuracy.</p>
+
+<p class='c007'>The volume of the earth is 9,933 times that of the mountain,
+and its attraction would be this number of times greater
+if it were composed of the same materials as the mountain
+throughout. It was found to be in reality 17,781 times as
+great as the attraction of the mountain, and as this is 1·79
+times 9,933, it follows that the average specific gravity of the
+matter composing the earth would be 1·79 times that of the
+rocks which build up Schiehallion. The mean specific gravity
+of the rocks being 2·8, the mean density of the earth was thus
+found to be 5·012 times that of water.</p>
+
+<p class='c007'><span class='pageno' id='Page_162'>162</span>As a general result of all the observations which have been
+made, the value of the earth’s density may with much probability
+be considered to be not far from 5·576, or a little
+over 5½ times that of water.</p>
+
+<p class='c007'>Whatever may be the composition of the earth’s interior,
+it is clear that the density must increase as the centre is
+approached.</p>
+
+<p class='c007'>This knowledge of the earth’s density, in conjunction with
+the known number of cubic miles occupied by the earth,
+readily enables us to determine that the total mass of the
+earth is about 6,000,000,000,000,000,000,000 tons.</p>
+
+<h3 class='c004'>CHAPTER XIII.<br> <span class='c014'>GRAVITATIONAL EFFECTS OF SUN AND MOON UPON THE EARTH.</span></h3>
+
+<p class='c003'><span class='sc'>The Tides.</span>—The familiar phenomena of the tides are of
+such importance to commerce in so many parts of the world
+that they have been carefully investigated from very early
+times. The necessities of coast navigation would soon lead
+to the recognition of a periodic character in the tides, as
+well as to their association with the age and position of the
+moon. With the march of science, an explanation of tidal
+phenomena was therefore sought in the motion of the moon.
+A great impetus was given to this inquiry by Newton’s
+generalisation, and the tides were shown to be a necessary
+consequence of the gravitational attraction of the sun and
+moon. Regarding the earth merely as a cosmical particle,
+we have seen that its orbital motion is perfectly explained by
+the gravitational attraction of the sun, and some of its minor
+movements by the attractions of other members of the solar
+system. The law of gravitation, however, compels us, in a
+closer investigation of these mutual attractions, to regard each
+globe as an assemblage of particles, each of which individually
+<span class='pageno' id='Page_163'>163</span>influences and is influenced by other particles. If such a
+collection of particles be spherical and perfectly rigid, it will
+behave precisely as a simple particle in which the whole mass
+is concentrated.</p>
+
+<p class='c007'>When we cease to consider the earth as a mere particle, we
+must regard the waters of the oceans as being free to move
+over the more rigid crust of the globe. Imagine our globe to
+be a spherical mass completely surrounded by a liquid
+envelope. At any moment one half of this is presented towards
+the moon. The solid earth we may conceive to be
+attracted by the moon as a simple particle; but the water on
+the side nearest to the moon is attracted with a greater force
+than the solid globe, because of its greater proximity to the
+attracting body, and it has therefore a tendency to heap itself
+up directly under the moon. Being free to move, the water
+thus remains heaped up under the moon, notwithstanding the
+earth’s rotation, and if there were only one such elevation,
+there would only be one tide a day. Observation shows us
+that there are two high tides a day, and the water must therefore
+be heaped up on the side of the earth which is turned
+away from the moon. This is perfectly true, though seemingly
+at first sight inconsistent with the moon’s attraction.
+The fact is that the solid earth is attracted by the moon with
+greater energy than the water on the side most remote from
+it, so that the heaping up of the water on the side away from the
+moon is to be regarded as due to the earth having left it behind.</p>
+
+<div id='i_163'  class='figcenter id001'>
+<img src='images/i_163.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 45.</span>—<i>The Tides.</i></p>
+</div>
+</div>
+
+<p class='c007'>There is thus a double tidal wave produced by a spheroid of
+water which, in the simple case we have considered, has its axis
+directed towards the
+moon, as in Fig. 45.
+The earth, rotating
+within this liquid shell,
+successively brings different
+parts of the solid
+earth to the points of
+high and low water. If
+the moon were fixed, we should then experience two high and
+<span class='pageno' id='Page_164'>164</span>two low waters every day, but as it revolves in the same
+direction that the earth rotates, the average interval between
+two successive meridian passages is 24 hours 51 minutes.
+This, then, is the period in which alternate high waters or
+alternate low waters are experienced.</p>
+
+<p class='c007'>A similar train of reasoning applies to the attraction of the
+sun upon different parts of our planet, so that there are solar
+as well as lunar tides. Nevertheless, the moon is the dominating
+cause, for although the total attraction of the sun upon
+the earth is about 200 times that of the moon, its differential
+attraction upon the opposite sides of the earth, which is alone
+effective in producing tides, is only about ⅖ths that of the moon.</p>
+
+<p class='c007'>A simple mathematical investigation shows that the tide-raising
+force of a body is proportional to its mass, and
+approximately in inverse proportion to the cube of its distance
+from the affected body. Thus, it appears that if the
+moon were removed to 1·36 times its present distance, solar
+and lunar tides would be equal.</p>
+
+<p class='c007'>At the times of new and full moon, the sun and moon will
+produce two tidal spheroids of water upon our imaginary
+earth, having their axes coincident, and an exceptionally high
+tide will occur. This is a <i>spring tide</i>. When the moon is at
+its quarters the two ellipsoids tend to neutralise each other,
+and an exceptionally low or <i>neap tide</i> results. Two spring
+tides and two neap tides thus occur in each synodic month of
+29½ days.</p>
+
+<p class='c007'>The height of the tide will also be affected by the variations
+in the distance of the moon. If the moon be at perigee the
+tide will be greater because of the smaller distance, and if this
+occur at new or full moon there will be a very high spring
+tide, while a less notable spring tide will occur when the new
+or full moon is at apogee.</p>
+
+<p class='c007'>The combination of the solar and lunar tides gives rise to
+what is called the <i>priming</i> and <i>lagging</i> of the tides. At new
+and full moons the combined tides will produce a spheroid
+of water with its axis directed towards the moon. When the
+moon is a few days old however, the crest will take up a
+<span class='pageno' id='Page_165'>165</span>position intermediate between the direction of the moon and
+that of the sun, and high water will therefore be accelerated.
+The same thing will happen during three or four days
+after full moon. Three days before full or new moon the
+combination of the two tides will displace the crest towards
+the sun, and therefore in advance of the moon, so that high
+water will be retarded. The retardation and acceleration
+correspond to lagging and priming respectively.</p>
+
+<p class='c007'>At the quadratures the combined tides simply reduce the
+height of the crest, since there is no reason why the deviation
+should be to one side any more than to the other. On
+account of priming and lagging, the tides on successive days
+are accelerated or retarded by as much as 13 minutes when
+the effects are greatest.</p>
+
+<p class='c007'>Sufficient has been said to indicate that tidal phenomena
+are very complex even when we suppose the earth to be very
+simply constituted. When we take into account the actual
+configuration of the land and the consequent restrictions in
+the movements of the water, these complications are increased
+tenfold. Yet, by continued observations, the recurrence of
+tides at any port can be predicted with tolerable accuracy.
+It is observed that there is a certain pretty regular interval of
+time between the moon’s meridian passage and the time of
+next high water; this is different at different ports, but is so
+nearly constant at a given place as to be called <i>the establishment
+of the port</i>. Observations being made at a great many
+places, the peculiar movements of the tidal wave can be
+investigated. For this purpose, it is convenient to draw on a
+map what are called <i>co-tidal lines</i> that is, lines passing through
+places at which high water occurs at the same moment. It
+then appears that it is only in the Southern Pacific where the
+water is of sufficient extent to permit the formation of the
+tide crest. The effect of this wave, which commences twice a
+day, is gradually spread over different parts of the world, but
+before it reaches most places other waves have commenced a
+similar journey. The tide at London, for example, coming
+round the north of Scotland and down the North Sea, really
+<span class='pageno' id='Page_166'>166</span>started in the Southern Pacific 66 hours before, and in the
+same way the tide at New York is a little over 40 hours old.</p>
+
+<p class='c007'>The height of a tide is thus regulated by the conditions of
+the sun and moon with regard to the earth when the primary
+tide was formed, and not by their relation when a tide is
+actually observed.</p>
+
+<p class='c007'>In the Pacific Ocean the tides are very feeble, but near the
+coast they vary enormously, and sometimes reach great
+heights. At Bristol the difference between high and low
+water sometimes amounts to fifty feet, and in the Bay of
+Fundy, Nova Scotia, it has been as much as a hundred feet.</p>
+
+<p class='c007'>The peculiarities of the tides at many places are due to
+interference. The primary tidal wave striking the British
+Islands travels partly up the English Channel, and partly
+round to the North Sea by the north of Scotland. At some
+places on the east coast the two waves almost neutralise each
+other, while at others there are even four high tides in a day.</p>
+
+<p class='c007'>The circumstances under which tides occur at a given place
+can only be determined by actual observations, as theory is
+at present utterly inadequate to deal with the manifold complications
+brought about by the configuration of the land, and
+the varying depth of the water.</p>
+
+<p class='c007'><span class='sc'>Tidal Friction.</span>—The regular influx of the tide supplies
+us with a source of mechanical energy, which in the future
+will no doubt become of immense importance to mankind. A
+great mass of water is raised to a higher level, and by suitable
+contrivances it can be made to do useful work during its
+subsequent flow to the ocean from which it came. Ordinarily,
+however, the water simply rushes back without its energy
+being utilised, and the potential power is merely transferred
+to another locality. It is manifest, however, that a certain
+amount of tidal energy is lost by friction as the water
+rolls to and from the rocky shores. This energy is converted
+into heat, and finally radiated into space, or dissipated.
+Now, the principle of the conservation of energy
+tells us that energy can neither be created nor destroyed,
+although its form may change from a useful to a useless one.
+<span class='pageno' id='Page_167'>167</span>It follows, therefore, that the energy lost through the tides
+must be abstracted from one source or another, and it has
+been shown that this energy is really derived from the earth’s
+rotation. As the earth steadily ploughs its way through its
+liquid envelope, the tides act as a break, and its rotational
+velocity is reduced; it is part of this lost energy of rotation
+which is dissipated by the tides.</p>
+
+<p class='c007'>One tendency of tidal friction is accordingly to lengthen the
+period of the earths rotation, and, therefore, to increase the
+length of the day. There are, however, counteracting causes,
+so that there is no certain direct evidence that the day has
+actually lengthened in historical times.</p>
+
+<p class='c007'>All the energy of rotation which is lost by the earth is not,
+however, dissipated by the tides. Some of it is transferred to
+the moon, with the result that the velocity of our satellite, and
+consequently the size of its orbit, must be increasing. From
+this it is inferred that the moon was formerly very much
+closer than at present, and an elaborate investigation of the
+conditions of its retreat has led Professor G. H. Darwin to
+his interesting theory of “tidal evolution.” (See p. 236.)</p>
+
+<p class='c007'>Professor Darwin has shown that if the term “tide” be
+extended to include distortions of the earth and moon at an
+earlier stage of their history, when both were fluid or viscous,
+a similar grinding down of the energies of rotation of both
+bodies must have taken place. The axial rotation of the
+moon, under these circumstances, would be retarded by the
+attraction of the earth on the tides raised in the moon,
+while that of the earth would also be slowed down, but in
+a less degree because of the moon’s smaller mass.</p>
+
+<p class='c007'><span class='sc'>Cause of Precession.</span>—On account of the spheroidal
+form of the earth, we may regard it as a sphere which
+is surrounded by a ring of protuberant matter at the Equator.
+Now the attraction of the sun upon the spherical part
+will be quite independent of the position of its axis of
+rotation, and will, therefore, not affect the position of the
+Equator. It is different, however, with the ring; at the
+solstices the ring is inclined to the line joining its centre with
+<span class='pageno' id='Page_168'>168</span>the sun, and the near side is subject to a greater attraction
+than the side more remote from the sun. On account of this
+difference of pull, there is a tendency for the ring to move
+into the plane of the ecliptic, and this is what would happen
+if the ring were not in rotation. The practical outcome of this
+tendency, combined with the rotation, is to produce the
+twisting of the plane of the ring, and, therefore, of the plane
+of the Equator. At the equinoxes the plane of the ring
+passes through the sun, and although there is still a difference
+of attraction on opposite sides of the ring, the differential
+force is entirely directed to the sun, and therefore cannot
+produce any precessional effect.</p>
+
+<p class='c007'>The ultimate tendency to turn into the plane of the
+ecliptic thus depends upon the <i>difference</i> of the attractions
+on opposite sides of the ring, or rather that part of the difference
+which acts in a direction perpendicular to the Equator.</p>
+
+<p class='c007'>The terrestrial ring cannot change the position of its plane
+without taking the whole earth with it, and the rate of
+precession is thus very slow. The effect of solar precession
+alone would cause the equatorial plane to twist round with
+but little change of inclination; or the earth’s axis would
+travel with a conical movement round a perpendicular to
+the ecliptic passing through the earth’s centre.</p>
+
+<p class='c007'>It will be remarked that as the force-producing precession
+is identical with that which is effective in producing the tides,
+the moon must have a greater precessional effect than the
+sun. This is quite true, and on the average the precession-producing
+force of the moon is 2½ times that of the sun. When
+the moon is on the celestial equator, as it is twice a month,
+the differential force acts in the plane of the ring, and no precessional
+effect results. On the other hand, the greatest
+effect is produced by the moon when the earth’s Equator is
+most inclined to the line joining the earth and moon. The
+amount of this greatest inclination is different in different
+months according to the position of the moon’s nodes. In
+consequence of the revolution of the moon’s nodes, the moon’s
+orbit is inclined to the Equator at all angles from 18° to 28°,
+<span class='pageno' id='Page_169'>169</span>and back again to 18° in a period of 19 years. The precessional
+effect of the moon thus has a principal period of 19
+years, while that of the sun has a period of a year during
+which it has two maxima and two minima. The summation
+of the effects of the sun and moon gives us the <i>luni-solar precession</i>,
+which is very variable in its actual rate, but averages
+about 50″·2 per annum.</p>
+
+<div id='i_169'  class='figcenter id001'>
+<img src='images/i_169.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 46.</span>—<i>Nutation.</i></p>
+</div>
+</div>
+
+<p class='c007'><span class='sc'>Nutation.</span>—If the precession-producing force were of constant
+amount, there would be no change in the inclination of
+the earth’s axis to the ecliptic. When the force is increasing,
+the equatorial ring is slightly tilted towards the ecliptic, and
+when it is decreasing the converse takes place. As the moon
+has the preponderating effect, these changes in the inclination
+will evidently depend mainly upon the changing value of the
+moon’s precessional force; that is, they will have a period of
+19 years. Thus, if <i>P</i>, Fig. 46,
+represents the pole of the
+ecliptic, the north celestial
+pole would travel in a circle
+of 23½° radius about <i>P</i> if precession
+were uniform. Suppose,
+then, the celestial pole to
+be at <i>a</i> when the moon’s node
+is on the Equator—that is,
+when the inclination of the
+moons orbit to the Equator is greatest—from this time the
+integrated effects of the moon’s precessional force will be decreasing,
+and the inclination of the Equator to the ecliptic will
+be increased; the celestial pole will consequently recede a
+little more than the average from the pole of the ecliptic, so
+that after 9½ years it will be at <i>b</i> instead of <i>c</i>. During the next
+9½ years the inclination of the moon’s orbit to the ecliptic will
+be gradually getting smaller, the precessional force will be
+proportionately reduced, and the obliquity of the ecliptic will
+be increased, so that the north celestial pole will have arrived
+at <i>d</i> after the lapse of 19 years. The prolongation of the
+earth’s axis thus describes a wavy curve, each wave extending
+<span class='pageno' id='Page_170'>170</span>over 19 years, so that there are about 1,400 waves during the
+great precessional cycle. This approach and recession of the
+two poles is called <i>nutation</i>, or nodding of the earth’s axis.
+The most recent investigation of its maximum amount, by Dr.
+Chandler, gives it as 9″·202. Besides the principal nutation
+there are others of very much smaller amount, due to the
+monthly changes of the moon’s declination and to the annual
+change of the sun’s declination.</p>
+
+<p class='c007'>The most obvious effect of nutation is that upon the inclination
+of the earth’s axis to the ecliptic—the “nutation in
+obliquity.” There is, however, a displacement of the equinoctial
+point, and corresponding nutations in longitude and
+right ascension.</p>
+
+<p class='c007'>As pointed out by Sir John Herschel, we have in nutation
+a splendid example of a periodical movement in one part of
+a system giving rise to a motion having the same precise
+period in another.</p>
+
+<p class='c007'><span class='sc'>Effects of Precession.</span>—The effects of precession may
+be conveniently summarised here, although some of them
+have necessarily been mentioned elsewhere:</p>
+
+<p class='c007'>(1) The first point of Aries revolves completely round the
+ecliptic, so that it passes through all the constellations of the
+zodiac in a period of 25,800 years. The “signs” of the
+zodiac, accordingly, no longer correspond with the constellations
+after which they are named.</p>
+
+<p class='c007'>(2) The Pole Star is constantly changing, since the
+north celestial pole travels round the pole of the ecliptic at
+a distance of about 23½° in a period of 25,800 years. About
+14,000 years ago the bright star Alpha Lyræ was the Pole
+Star.</p>
+
+<p class='c007'>(3) The position of the north celestial pole is in time
+changed by 47°, and there may accordingly be this change in
+the north polar distances or declinations of all stars whatsoever.
+As the position of the ecliptic is almost constant, the celestial
+latitudes of stars will be but little affected by precession.</p>
+
+<p class='c007'>(4) The right ascensions and longitudes of stars, being
+reckoned from the shifting first point of Aries, are themselves
+<span class='pageno' id='Page_171'>171</span>changeable, passing through all possible values in the
+precessional period.</p>
+
+<p class='c007'>(5) The tropical year is shorter than the sidereal year by
+the time taken for the earth to travel through 50″·2—that is,
+20 minutes 23 seconds.</p>
+
+<p class='c007'>(6) Celestial globes and maps, as well as star catalogues,
+can only represent the right ascensions and declinations of
+stars at a specified epoch.</p>
+
+<h3 class='c004'>CHAPTER XIV.<br> <span class='c014'>INSTRUMENTAL MEASUREMENT OF ANGLES AND TIME.</span></h3>
+
+<p class='c003'><span class='sc'>Graduated Circles.</span>—Astronomy is essentially a science of
+precision, and the progress of our knowledge has to a large
+extent been dependent upon the increasing power of accurately
+measuring angles and time.</p>
+
+<p class='c007'>Let us see, first of all, how to measure angles.</p>
+
+<p class='c007'>A circle is divided into 360 degrees, each degree again
+into 60 minutes, and each minute into 60 seconds of arc; and
+yet, a second of arc is not a small enough quantity for many
+astronomical purposes. Now, unless a very large circle be
+employed, it is mechanically impossible to even mark the
+minutes of arc directly upon it, and if a very large circle were
+constructed, the distortion of its shape produced by its own
+weight would be sufficient to mar its accuracy.</p>
+
+<p class='c007'>What is actually done then is to get a circle of convenient
+size, and to graduate it, as well as the highest mechanical
+skill is capable of, into such parts as may leave distinct and
+equal spaces between the separate divisions. A competent
+instrument maker would, for instance, put 4,320 divisions on
+the <i>limb</i> of a circle 16 inches in diameter, two consecutive
+divisions thus being 5′ apart. For work of the highest precision
+<span class='pageno' id='Page_172'>172</span>it is necessary to strictly investigate the errors of the
+divisions and to correct for them in all observations.</p>
+
+<p class='c007'>For the further subdivision of these graduations, verniers or
+reading microscopes are introduced.</p>
+
+<p class='c007'><span class='sc'>The Vernier.</span>—A graduated circle being attached to an
+instrument, what one has to do is to take a <i>reading</i> with
+reference to some fixed mark. If the fixed mark is seen to
+fall precisely on one of the divisions of the circle when
+observed with a magnifying-glass, the reading can be written
+down exactly. If there be no such coincidence, some means
+are required for accurately reckoning the fraction of a
+division. One method in general use on small instruments,
+and where extreme precision is unnecessary, is to employ a
+subsidiary scale which is called a <i>Vernier</i>, in honour of the
+Frenchman who invented it. This can be applied indifferently
+to a scale of degrees and parts of degrees on a graduated
+circle, or to a straight scale. With the aid of this
+device it becomes possible to measure angles with no greater
+probable error than a few seconds of arc.</p>
+
+<div id='i_172'  class='figcenter id001'>
+<img src='images/i_172.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 47.</span></p>
+</div>
+</div>
+
+<p class='c007'><span class='sc'>The Reading Microscope.</span>—If a greater degree of
+accuracy than 10″ be required, the vernier is superseded by a
+<i>reading microscope</i>. This is a compound microscope (Fig. 48)
+by which the scale can be observed,
+and at the focus of its eye-piece is
+a pair of spider threads which can
+be moved by a fine screw S. Looking
+into such a microscope, one sees
+a magnified picture of a very small
+part of the scale running through
+the field of view, as in Fig. 47.
+Running across the field, in the
+same direction as the marks on
+the scale, are the spider threads <i>a b</i>, which can be given
+a right and left movement by means of the screw.
+At the top of the field is the part called the “comb,”
+having its edge cut with saw-like teeth; like the threads,
+this is at the focus of the eye-piece. The scale is divided
+<span class='pageno' id='Page_173'>173</span>so that the smallest part is 5′, and in that case the teeth of
+the comb are arranged so that five of them equal a scale
+division. The reading microscope is a fixture, and the circle
+is brought into the position in which its reading is required
+by moving the instrument with which it is connected. The
+zero of the microscope is a point at the middle of the comb,
+and one has to determine what part of the scale corresponds
+with it. In order to do this, the threads or “wires” are
+moved until the next division lies between them, and the
+amount which the screw has been turned from the position of
+zero is read off on the graduated head of the screw. The
+dimensions of the parts, and the magnifying power of the
+microscope, are adjusted so that the screw must be turned
+five times to carry the wires
+through a space equal to a division
+on the scale. One division,
+therefore, will move the wires
+through 1′, and as the screw head
+is divided into 60 parts, a movement
+of ¹⁄₆₀th of a revolution
+will shift the wires through a
+second of arc. Even fractions of
+a second can be thus measured.</p>
+
+<div id='i_173'  class='figcenter id001'>
+<img src='images/i_173.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 48.</span>—<i>The Reading Microscope.</i></p>
+</div>
+</div>
+
+<p class='c007'>The introduction of this method
+of measuring minute angles is
+due to Ramsden, who first applied
+it at the end of the last century.
+The microscopes themselves
+are used for measuring
+fractional parts of the graduations
+of the circles, and usually four
+to six of them are applied to different
+parts of the same circle.
+In this way, errors arising from
+flexure of the circle, fluctuations of temperature, want of exact
+circularity, etc., are eliminated, so that finally, after taking
+every conceivable precaution, the astronomer can measure
+<span class='pageno' id='Page_174'>174</span>angles with the accuracy which is absolutely necessary in
+many branches of research.</p>
+
+<p class='c007'><span class='sc'>Astronomical Clocks.</span>—Means for the exact estimation
+of time are of no less importance in an observatory than arrangements
+for the accurate measurement of angles. Astronomical
+clocks are constructed with extreme care, but in principle
+they do not differ from ordinary time-keepers. As sidereal
+time is of the greatest use in an observatory, the hour hand
+only makes one revolution a day, and the face is provided
+with a seconds hand, which is plainly visible. The pendulum
+is of such a length that it performs its swing in a second.
+One of the most important improvements in clocks was the
+introduction of the “compensation” principle, whereby the
+equivalent length of a pendulum remains constant in spite of
+fluctuations of temperature. The mercurial pendulum which
+one very frequently sees in a watchmaker’s establishment has
+a glass or steel cylinder near the bottom partly filled with
+mercury; as the rod lengthens by increased temperature, the
+centre of gravity is raised by a corresponding amount, on
+account of the upward expansion of the mercury, and the rate
+of swing remains constant when the quantity of mercury is
+properly adjusted. The chief defect of this plan is that the
+mercury and the steel rod do not respond equally well to a
+change of temperature.</p>
+
+<p class='c007'>In the most approved clocks the pendulum rod is a compound
+one, consisting of rods, or concentric tubes, of zinc and
+steel. The pendulum bob is hung on a steel rod suspended
+from the top of a zinc tube, which in turn is fixed at the
+bottom end to a larger tube of steel; a rod attached directly
+to the latter is suspended by a flat spring in the usual manner.
+By this arrangement the unequal expansions or contractions
+of the different parts due to changes of temperature neutralise
+each other, so that a constant rate is the result. The tubes
+are pierced with numerous holes so that the inner and outer
+ones acquire the same temperature almost at the same time.</p>
+
+<p class='c007'>The rate of a clock is disturbed slightly by changes in the
+pressure of the atmosphere. When the air is densest there is
+<span class='pageno' id='Page_175'>175</span>a greater resistance to the swinging of the pendulum, and the
+clock will go more slowly. Although this only amounts to a
+small fraction of a second a day, it must necessarily be taken
+into account in such an establishment as that at Greenwich, to
+which all the country looks for the precise control of time-keepers.
+In the standard clock at Greenwich a magnet is
+raised or lowered by the changing height of a barometer, and
+its varying attraction upon a certain piece of iron attached to
+the pendulum compensates for the differences produced by
+change of pressure.</p>
+
+<p class='c007'>Pendulum clocks are obviously unsuitable for use at sea, so
+that <i>chronometers</i> are usually employed on ships. These are
+like large watches, very carefully constructed, with “compensation”
+balance wheels, and can generally be relied upon as
+good time-keepers.</p>
+
+<p class='c007'>After all precautions, however, no astronomer would put his
+faith in any clock for any length of time, as the best of them
+is liable to change its rate rather irregularly. The “error” of
+the clock is therefore very frequently determined by the observation
+of certain standard stars with the transit instrument.
+The stars can be relied upon to come to the meridian at the
+proper time, and any apparent departure from this time
+must be set down to the account of the clock.</p>
+
+<p class='c007'><span class='sc'>The Chronograph.</span>—A good clock, however, is not the
+only requirement of an observatory. It is necessary further
+to be able to record very precisely the moment at which an
+observation is made. If the clock be in the immediate vicinity
+of the observer, the time can be noted by counting the beats
+of the pendulum, and a practised observer will, by this “eye
+and ear” method, record times to the nearest tenth of a
+second. Mere estimation, however, is not very reliable, so
+that a mechanical method, which also permits greater subdivision
+of the second, is very generally adopted. The instrument
+is called a <i>chronograph</i>, and, although constructed in
+various forms, its function is to record on a sheet or strip of
+paper the regular beats of the clock, as well as the signals
+made by the observer. In one form of the instrument the
+<span class='pageno' id='Page_176'>176</span>recording sheet is fixed on a cylindrical drum which is made
+to revolve once a minute by a small clock. Beneath the
+drum is a pair of prickers worked by the armatures of electromagnets.
+One of these magnets is in connection with the clock,
+and a simple arrangement sends an electric current through
+it every second, with the result that the seconds are marked
+by small punctures on the paper. As the cylinder revolves,
+the marker travels slowly lengthwise, so that the clock record
+runs spirally from one end to the other. To facilitate the
+identification of the punctures, one is omitted at the end of
+every minute. When an observation is made, the observer
+presses a button, and a current is sent through the second
+magnet, with the result that a puncture is made alongside
+those made by the clock. In this way the exact moment at
+which an observation is made can be easily registered, and
+read off at any convenient time.</p>
+
+<p class='c007'>At Greenwich a room is set apart for a number of chronographs,
+each in communication with an instrument in the
+various observatories.</p>
+
+<h3 class='c004'>CHAPTER XV.<br> <span class='c014'>TELESCOPES.</span></h3>
+
+<p class='c003'><span class='sc'>The Refracting Telescope.</span>—The function of a telescope
+is two-fold. First, to magnify the heavenly bodies, or, what
+comes to the same thing, to make them look as if they were
+nearer to us, so that we can see them better. Second, to
+collect a much greater number of rays of light than the unassisted
+eye alone can grasp, so that objects too dim to be
+otherwise perceptible are brought within our range of vision.</p>
+
+<p class='c007'>There are two forms of telescope, distinguished as <i>Refractors</i>
+and <i>Reflectors</i>. The simplest form of refracting telescope is
+<span class='pageno' id='Page_177'>177</span>exemplified by the common opera-glass, and large refractors
+are not essentially different. Such instruments depend for
+their action upon the formation of an image by a lens. One
+can easily illustrate this by producing upon the wall of a
+room an inverted image of a candle or gas flame with a
+spectacle lens (one adapted for a long-sighted person), or with
+one of the larger lenses from an opera-glass. Having such
+an image, it may be magnified by means of another lens, just
+as one may magnify a photograph with an ordinary reading
+glass. Technically, the lens which forms the primary image
+is called the <i>object-glass</i> of the telescope, and that which
+is used to magnify this image is called the <i>eye-piece</i>. The
+object-glass is usually a large lens, which is placed at one end
+of a tube, while the eye-piece is a much smaller lens, placed
+at the other end. Means are
+provided for adjusting the distance
+between the two lenses so
+as to admit of distinct vision.</p>
+
+<div id='i_177'  class='figcenter id001'>
+<img src='images/i_177.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 49.</span>—<i>The Achromatic Object-Glass.</i></p>
+</div>
+</div>
+
+<p class='c007'>Matters are, however, not
+quite so simple as has been
+stated. There is a very great
+difficulty introduced by the
+fact that a lens made out of a
+single piece of glass gives an
+image which is surrounded by
+fringes of colour, so that some
+device has to be adopted in
+order to destroy, as far as possible,
+this enemy of good definition.
+In the early history of the telescope, this so-called
+<i>chromatic aberration</i> was considerably reduced by making
+small object-glasses of very great focal length.<a id='r4'></a><a href='#f4' class='c027'><sup>[4]</sup></a></p>
+
+<p class='c007'>Lenses of 100-feet focus, however, are not easy to
+employ as object-glasses, and astronomy was, therefore,
+greatly benefited by Dollond’s invention of the <i>achromatic lens</i>
+<span class='pageno' id='Page_178'>178</span>in 1760. This is a compound lens, usually consisting of a
+double convex crown-glass lens and a concavo-convex, or double
+concave, lens of flint glass. The curvatures of the lenses,
+and the optical properties of the two kinds of glass composing
+them, are such that the colour due to one of them is
+practically neutralised by that due to the other acting in opposition.
+A section of such an object-glass, with the “cell”
+in which it rests, is shown in Fig. 49.</p>
+
+<p class='c007'>In this way the focal length of the lens, and, therefore, the
+length of the telescope tube, can be kept within reasonable
+dimensions, while the definition is improved. There is, however,
+usually a little outstanding colour, due to the imperfect
+matching of the two lenses, and if one looks through a large
+refractor, even of a good quality, a purple fringe will be
+noticed round all very bright objects. This only affects a few
+of the brighter objects, while millions of others which are
+dimmer may be seen free from spurious colour.</p>
+
+<p class='c007'>It may be remarked that the curved surfaces of the lenses
+forming telescopic object-glasses must not be parts of spheres.
+If they are, the images will be rendered indistinct by <i>spherical
+aberration</i>, and the optician has to design his curves to get
+rid of this defect at the same time as chromatic aberration.</p>
+
+<p class='c007'>A new form of telescopic objective, consisting of three
+lenses, which has many important advantages, has recently
+been invented by Mr. Dennis Taylor, of the well-known firm
+of T. Cooke &#38; Sons, York.</p>
+
+<p class='c007'>Such a lens as this illustrates the perfection which the
+optician’s art has now attained. Six surfaces of glass have to
+be so accurately figured that every ray of light falling upon
+the surface of the lens shall pass through the finest pinhole at
+a distance of eighteen times the diameter of the lens.</p>
+
+<p class='c007'><span class='sc'>The Reflector.</span>—In a reflecting telescope, the object-glass
+of the refractor is replaced by a concave mirror. In
+order that such a mirror may reflect all the rays from a star
+to a single point, its concave surface must be part of a
+paraboloid of revolution, that is, a surface produced by the
+revolution of a parabola on its axis. If a spherical surface be
+<span class='pageno' id='Page_179'>179</span>employed, all the rays will not be reflected to a single point,
+and the images which it gives will be ill-defined. Yet it is
+astonishing to find that the difference between a parabolic
+and spherical surface, even in the case of a large mirror, is
+exceedingly small. Sir John Herschel states that in the case
+of a mirror four feet in diameter, and forming an image at a
+distance of forty feet, the parabolic only departs from the
+spherical form at the edges by less than a twenty-one
+thousandth part of an inch.</p>
+
+<div id='i_179'  class='figcenter id003'>
+<img src='images/i_179.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 50.</span>—<i>The Newtonian Reflector.</i></p>
+</div>
+</div>
+
+<p class='c007'>An image being formed by a mirror, it is next to be viewed
+with an eye-piece just as in the case of a refracting telescope.
+Here there is a little difficulty, for if the eye-piece be applied
+in the direct line of the mirror, the interposition of the
+observer’s head will block out the light. Several ways of
+overcoming this have been devised, but the plan most generally
+followed is that which Newton adopted in the first
+reflecting telescope which was ever constructed. With his
+own hands Newton made a small reflector, 6¼ inches long and
+having an aperture of 1⅓ inches, with which he was able to
+study the phases of Venus, and the phenomena of Jupiter’s
+satellites. This precious little instrument is now one of the
+greatest treasures in the collection of the Royal Society of
+London. The general design of this telescope is shown in
+Fig. 50. The concave mirror is at the bottom of the telescope
+tube, and normally it would form an image of a
+star near the end of the tube. A plane mirror, however, of
+small size intercepts the rays and reflects them to the side,
+<span class='pageno' id='Page_180'>180</span>where they converge to a focus. This image is observed and
+magnified by an eye-piece, as in the refractor. It is true that
+in this arrangement the plane mirror, or <i>flat</i>, renders the
+central part of the principal mirror ineffective, but the loss of
+light is very much less than would be the case if the eye-piece
+were placed in position to view the image centrally.</p>
+
+<p class='c007'>In the hands of Sir William Herschel the reflecting
+telescope was greatly developed. The great telescope with
+which he enriched astronomical science had a mirror four feet
+in diameter, and its tube was 40 feet in length. With the
+view of utilising the whole surface of the mirror and dispensing
+with a second reflecting surface, the four foot mirror was
+placed at a small angle to the bottom of the tube, so that its
+principal focal point was no longer at the centre, but at the
+side of the tube.</p>
+
+<p class='c007'>In practice, however, it is found that the Herschellian form
+of reflector does not give the best definition, and it is now
+very seldom seen.</p>
+
+<p class='c007'>Among other forms, the “Cassegrain” is perhaps the most
+important. During the last year or two this form has received
+a great deal of attention, more especially in regard to its
+special adaptability for photographic purposes.</p>
+
+<p class='c007'>In the Cassegrain telescope, the plane mirror of the Newtonian
+form is replaced by a small convex mirror which is
+part of a hyperboloid of revolution, its axis and focal point
+being coincident with those of the primary mirror. The rays
+are in this way reflected back to the mirror at the bottom of
+the tube, and in order that the image may be seen, it is
+necessary to cut out the middle part of the mirror to admit
+the eye-piece.</p>
+
+<p class='c007'>Although the small mirror must theoretically be hyperbolic,
+tolerable definition is obtained even if it be spherical or
+ellipsoidal, and its actual departure from these forms is so
+slight as to be beyond detection by measurement, so that the
+figuring of such mirrors can only be tested in the telescope.
+For photographic purposes this telescope has the very important
+advantage that a short telescope is equivalent to a
+<span class='pageno' id='Page_181'>181</span>very long one of the Newtonian form, or refracting telescope,
+so that the image of sun, moon, or planets formed at the
+focus is very large in comparison with the size of the
+telescope. A modification of this form of telescope, in which
+the small mirror is out of the path of the rays falling upon
+the larger one, and no longer obstructing the central part,
+has been recently revived by Dr. Common, and has become
+generally known as the “Skew Cassegrain.”</p>
+
+<p class='c007'>In reflecting telescopes the mirrors were formerly made of
+<i>speculum</i> metal (an alloy of copper and tin), and the word
+speculum is even now commonly employed to signify a telescopic
+mirror, although it is usual to make the mirror of
+glass, with the concave surface silvered and highly polished.</p>
+
+<div id='i_181'  class='figcenter id003'>
+<img src='images/i_181.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 51.</span>—<i>The Cassegrain Reflector.</i></p>
+</div>
+</div>
+
+<p class='c007'>One is frequently asked for an opinion as to which is the
+better form of telescope, the reflector or refractor, and it is a
+question that one finds some little difficulty in answering.
+On one point, however, all are agreed, namely, that the reflector
+has the advantage in regard to its achromatism; it is
+indeed perfectly achromatic, while the so-called “achromatic”
+refractor is at best only a compromise. For the rest, one
+cannot do better than quote the evidence of Dr. Isaac Roberts
+before the International Astro-photographic Congress:—“The
+reflector requires the exercise of great care and patience, and
+a thorough personal interest on the part of the observer using
+it. In the hands of such a person it yields excellent results,
+but in other hands it might be a bad instrument. The reflector
+<span class='pageno' id='Page_182'>182</span>gives results at least equal, if not superior, to those obtained
+with the refractor, if the observer be careful of the
+centering, and of the polish of the mirror, and keeps the instrument
+in the highest state of efficiency; but when entrusted
+to an ordinary assistant the conditions necessary for its best
+performance cannot be so well fulfilled as the same could be
+in the case of the refractor.” One great practical advantage
+of the reflector is that there are fewer optical surfaces, so that
+a large reflector may be obtained for the price of a much
+smaller refractor.</p>
+
+<p class='c007'><span class='sc'>Eye-Pieces.</span>—So far we have regarded the eye-piece of a
+telescope as a simple lens, but it is evident that the spherical
+and chromatic aberration of such a lens will interfere with its
+performance. For occasional use, however, even a simple lens
+is very serviceable if the object observed is brought to the
+centre of the field of view.</p>
+
+<p class='c007'>Compound eye-pieces are of various forms, each having certain
+advantages, the desiderata being freedom from colour and
+“flatness of field”—that is, stars in different parts of the field
+are to be equally well in focus. Those most commonly employed
+are the Ramsden and Huyghenian eye-pieces. The
+former consists of two plano-convex lenses of equal focal
+lengths, having their curved faces towards each other, and
+being placed at a distance apart equal to two-thirds of the focal
+length of either lens. Such an eye-piece can be used as a
+magnifying-glass, and it is therefore placed outside the focal
+image formed by the telescope with which it is used; on
+this account it is called a <i>positive eye-piece</i>. This kind of eye-piece
+is not quite achromatic, but its flat field of view gives it
+a special value for many purposes.</p>
+
+<p class='c007'>In the Huyghenian eye-piece there are again two lenses,
+made of the same kind of glass. That which comes nearest
+to the eye has a focal length of only one-third that of the
+<i>field</i> lens, and the distance between the two lenses is half the
+sum of the focal lengths. This form of eye-piece cannot be
+used as a magnifying-glass in the ordinary sense, and as the
+field lens must be placed on the object-glass or mirror side of
+<span class='pageno' id='Page_183'>183</span>the focus, it is called a <i>negative eye-piece</i>. The Huyghenian
+eye-piece is more achromatic than the Ramsden, and is more
+widely used when it is only required to view the heavenly
+bodies. In instruments employed for purposes of measurement,
+a positive eye-piece is essential in order that the spider
+threads may be placed at the focus of the telescope. The
+images formed by an astronomical telescope are upside down,
+and neither of the eye-pieces described reinverts them.</p>
+
+<p class='c007'>A special form of eye-piece is therefore used when a telescope
+is employed for terrestrial sight-seeing. The desired
+result is obtained by the introduction of additional lenses, but
+there is a corresponding reduction of brightness.</p>
+
+<p class='c007'>For viewing the sun some device is necessary to reduce the
+quantity of light entering the eye. To look at the sun directly,
+even with a small instrument, is very dangerous. The arrangement
+usually adopted is a <i>solar diagonal</i>, in which the
+light is reflected from a piece of plane glass before entering
+the eye-piece; the piece of glass is wedge-shaped, so that the
+reflection from one surface only is effective; if the glass had
+parallel sides, the solar image would be double.</p>
+
+<p class='c007'><span class='sc'>Magnifying Power.</span>—The magnifying power of a telescope
+depends upon the focal length of the object-glass, or
+speculum, and that of the eye-piece. Optically, it is equal to
+the former divided by the latter, so that the greater the focal
+length of an object-glass, or the smaller the focal length of the
+eye-piece, the greater will be the magnifying power. In a
+given telescope, the object-glass, or speculum, is a constant
+factor, and the magnifying power can only be varied by
+changing the eye-piece. The focal length of the Lick telescope,
+for example, is about 600 inches; with an eye-piece
+which is equivalent to a lens of one-inch focus, the magnifying
+power would be 600; with a lens of half an inch focus, it
+would be 1,200, and so on.</p>
+
+<p class='c007'>The magnifying power which can be effectively employed,
+however, depends upon a great variety of circumstances.
+First, the clearness and steadiness of the air; then there is
+the quality of the object-glass, or speculum, to be considered;
+<span class='pageno' id='Page_184'>184</span>and also the brightness of the object to be observed, for when
+the object is very dim, its light will be spread out into invisibility
+if too high a power be used.</p>
+
+<p class='c007'>In practice, good refractors perform well with powers
+ranging up to 80 or 100 for each inch in the diameter of the
+object-glass. Thus, on sufficiently bright objects, a six-inch
+telescope will work well with a power of about 500, while a
+30-inch may be effectively employed with powers between
+2,000 and 3,000.</p>
+
+<p class='c007'><span class='sc'>Illuminating Power.</span>—It has already been pointed out
+that magnification is not the only function of a telescope.
+As a matter of fact, the most powerful telescopes in the world
+fail to produce the slightest increase in the apparent size of a
+star, for even if these objects be brought to apparently a 3,000th
+part of their real distances, they are still too far away to have
+any visible size. But although a star cannot be magnified, it
+can be rendered more visible by the telescope, for the reason
+that the object-glass collects a greater number of rays than
+the naked eye. The pupil of the eye may be taken to have a
+diameter of one-fifth of an inch; a lens one inch in diameter
+will have 25 times the <i>area</i> of the pupil, and will therefore
+collect 25 times the amount of light from a star; a two-inch
+lens will grasp 100 times, and a 36-inch 32,400 times as much
+light as the pupil alone. Practically all these rays collected
+by the object-glass, or speculum, of a telescope cannot be
+brought into the eye; some are lost through the imperfect
+transparency of the glass, or the imperfect reflecting power of
+the speculum. Still, allowing a considerable percentage for
+loss, there is an enormous concentration of light when a large
+telescope is employed.</p>
+
+<p class='c007'><span class='sc'>The Altazimuth Mounting.</span>—Having got a telescope,
+we have next to see how it can be best supported, for unless
+it be a very small instrument indeed, it will be impossible to
+hold it in the hand like a spy-glass. However a telescope be
+mounted, provision must be made for turning it to any part
+of the sky whatsoever. Very frequently one of the axes on
+which the instrument turns is vertical, while the other is
+<span class='pageno' id='Page_185'>185</span>horizontal. Such a stand for a telescope is called an <i>altazimuth
+mounting</i>, for the reason that it permits the instrument
+to be moved in altitude and in azimuth.</p>
+
+<p class='c007'>As a rule, one finds only small telescopes mounted in this
+manner. The objection to it is that, as one continues to
+observe a heavenly body, two independent movements must
+be given to the telescope in order to follow the body in its
+diurnal movement across the heavens. If we commence
+observing a star newly risen, for example, the telescope must
+trace a stair-like path in order to follow it, as it ascends into
+the heavens.</p>
+
+<p class='c007'><span class='sc'>The Equatorial Telescope.</span>—A much more convenient
+method of setting up a telescope is to mount it as an
+<i>equatorial</i>. The essential feature of this instrument is that
+one of the axes of movement, instead of being vertical, is
+placed parallel to the axis of the earth. This is called the
+<i>polar axis</i>, and, when the telescope is turned around such an
+axis, it traces out curves in the sky which are identical with
+those described by the stars in their diurnal motions. If,
+then, the telescope be directed to a star or other heavenly
+body, it can be made to follow the object and keep it in view
+by a single movement. The axis at right angles to the polar
+axis is called the declination axis, and is necessary in order
+that the telescope may be moved towards and from the Poles
+so that all the heavenly bodies above the horizon may be included
+in its sweep.</p>
+
+<p class='c007'>One very important advantage of the equatorial is that as
+only one motion is required to keep a star in view, so long as
+it is above the horizon, the necessary movement may be
+furnished by clock-work. A good equatorial is accordingly
+provided with a driving clock, which is regulated so that it
+would drive the telescope through a whole revolution once a
+day. Unlike an ordinary clock, the driving clock of a telescope
+is regulated by a governor, in order that the instrument
+may have a continuous and not a jerky movement.</p>
+
+<p class='c007'>The telescope is also provided with clamps and fine adjustments,
+one each in R. A. and declination, in order that it may
+<span class='pageno' id='Page_186'>186</span>be under the control of the observer. It is evident that the
+telescope must be capable of moving independently of the
+driving gear, so that it may first be placed in the desired
+direction; when this is accomplished, the R. A. clamp is used
+to put the telescope in gear with the clock. The declination
+clamp is them made to fix the telescope firmly to the declination
+axis. Fine adjustments in both directions are necessary,
+because it is impossible to sight a large instrument with such
+precision as to bring an object exactly to the centre of the
+field of view.</p>
+
+<p class='c007'>Some of the driving clocks fitted to equatorials are very
+elaborate. As clocks regulated by governors are not such
+reliable time-keepers as those regulated by pendulums,
+arrangements are made by which the accuracy of a pendulum
+can be electrically communicated to a governor clock. One
+of the best forms of electrically-controlled clocks is that
+devised by Sir Howard Grubb.</p>
+
+<p class='c007'>Another important feature of an equatorial is that it can be
+provided with circles which enable the telescope to be pointed
+to any desired object of known right ascension and declination.
+One of these is the declination circle, attached to the
+declination axis and read by a vernier fixed to the sleeve in
+which the axis turns; this is adjusted so as to read 0° when the
+telescope points to any part of the celestial equator, and 90°
+when it is directed to the Pole. The other circle is attached
+to the polar axis, and determines the position of the telescope
+with regard to the meridian; this is called the <i>hour circle</i>, and
+is divided into 24 hours. When the telescope is on the
+meridian, the hour circle reads zero, so that its reading in
+any other position gives the hour angle of the telescope.
+Having given the right ascension and declination of a heavenly
+body which it is desired to observe, the telescope is turned
+until the declination circle reads the proper angle, and the
+hour circle indicates the hour angle which is calculated for the
+particular moment of pointing the telescope. [The hour
+angle is the difference between the right ascension of the
+object and the sidereal time of observation.] In this way it
+<span class='pageno' id='Page_187'>187</span>is easy to find objects of known position which are invisible
+to the naked eye, and one can even pick up the planets and
+brighter stars in full sunshine. Conversely one can determine
+from the circles the right ascension and declination of any
+object under observation, but for various reasons only
+approximate results can be obtained in this way. The chief
+use of the circles on an equatorial is therefore to provide a
+means of pointing the telescope.</p>
+
+<p class='c007'>Telescopes of 4 inches aperture and upwards are usually
+provided with a smaller companion called a <i>finder</i>. This has
+a larger field of view than the main telescope, so that objects
+which are of sufficient brightness can readily be picked up
+and brought to the centre of the finder, the adjustments
+being such that the object is then also at the centre of the
+field of the large telescope.</p>
+
+<p class='c007'>There are, of course, many practical details connected with
+the working of an equatorial with which space does not
+permit us to deal. It may be remarked, however, that the
+adjustment of the polar axis is very simply performed by first
+inclining it at an angle approximately equal to the latitude of
+the place where it is set up, and setting it as nearly as possible
+in the meridian by means of a compass or by observations of
+the sun at noon. The final adjustment is then made by a
+series of observations of stars of known position.</p>
+
+<p class='c007'><span class='sc'>Some of the World’s Great Telescopes.</span>—Thanks to
+the wide public interest taken in astronomical matters, a large
+number of powerful telescopes has been set up in various
+parts of the world. To the British Islands belongs the
+honour of possessing the largest telescope in the world. This
+is the giant reflector erected by Lord Rosse, in 1842, at
+Parsonstown, the mirror being 6 feet in diameter, and the
+focal length 60 feet. Many very valuable observations were
+made with this instrument in its early days, but of late years
+it seems to have fallen into disuse. One reason may be that
+the mounting is not of the most convenient form, and makes
+the telescope unsuitable for photographic work.</p>
+
+<p class='c007'>Coming next in point of size to the Rosse telescope is the
+<span class='pageno' id='Page_188'>188</span>reflector erected at Ealing, by Dr. A. A. Common. The
+glass mirror of this telescope is 5 feet in diameter, 5 inches
+thick, and weighs more than half a ton. Dr. Common aimed
+specially at constructing the largest possible telescope which
+could be equatorially mounted and provided with a driving
+clock, and he was only limited to an aperture of 5 feet by the
+impossibility of obtaining a glass disc of larger size. He has
+attained such great skill in this work that he was able to produce
+a perfect mirror 5 feet in diameter in three months time,
+although no less than 410,000 strokes of the polishing machine
+were required.</p>
+
+<p class='c007'>The telescope is of the Newtonian form, and the mounting is
+quite unique. The polar axis consists of an iron cylinder,
+made up of boiler plates, 7 feet 8 inches in diameter, and
+about 15 feet long. From the top of the cylinder, near its
+outer edge, two horns, each 6 feet long, project outwards, and
+the tube of the telescope swings on trunnions attached to the
+ends of the horns. The main part of the telescope tube is
+square, built up of steel angle iron, and carries the mirror at
+its lower end; the upper part of the tube, which carries the
+“flat” and eye-piece, is round, and of tinned steel strengthened
+by a skeleton framework.</p>
+
+<p class='c007'>It is evident that such an enormous instrument as this cannot
+be made to travel by clock-work with the necessary uniformity
+without some very efficient arrangement for reducing
+friction. Dr. Common’s plan—and it is here that his instrument
+is unlike others—is to make the hollow polar axis
+water-tight, and to fix it in a tank of water. At the bottom
+of the polar axis is a ball and socket joint to keep it in position,
+and at the top is another bearing, which can be adjusted
+so that the polar axis lies truly in the meridian. It was
+found necessary to introduce 9 tons of iron into the bottom of
+the hollow polar axis in order to sink it to the proper angle,
+and to put sufficient weight on the bearings to give stability
+to the instrument. In this way the great mass is brought
+into the region of manageability, and the driving clock, which
+is driven by a weight of 1½ tons, is able to do its work
+<span class='pageno' id='Page_189'>189</span>efficiently. Such, in general outline, is this wonderful telescope,
+which, although not so large as Lord Rosse’s famous
+instrument, is undoubtedly its superior in light-grasping
+power and general utility, and more especially in its
+adaptability for photographing the heavens.</p>
+
+<p class='c007'>Among other large reflecting telescopes now in use are the
+four-foot reflectors at Melbourne and Paris, and the three-foot
+reflectors at South Kensington and the Lick Observatory,
+California.</p>
+
+<p class='c007'>The largest refracting telescope yet constructed is one of
+40 inches aperture for the new Yerkes Observatory of the
+University of Chicago. It is interesting to note here that
+Professor Keeler, in his report as an expert upon the performance
+of the object-glass, considers that there is “evidence
+for the first time that we are approaching the limit of size in
+the construction of great objectives.” Unlike a mirror, a
+lens can be supported only upon its circumference, and it is
+the bending by its own weight that proves detrimental to its
+defining power. If the lens be made thicker with a view of
+overcoming this defect, the absorption of light by the glass
+increases, so that there is in the end no special gain by increasing
+the size.</p>
+
+<p class='c007'>The length of the Yerkes telescope is 62 feet, and it will
+be provided with all accessories pertaining to astrophysical
+research. The Yerkes telescope, however, is not yet in actual
+use, and meanwhile the world-renowned Lick telescope, of
+36 inches aperture, keeps the lead among active big refractors.
+The story of the foundation of this monster instrument
+is not much less wonderful than the telescope itself. Brought
+up in poor circumstances, with few opportunities for intellectual
+development, James Lick, nevertheless, amassed a fortune
+in business, and having few relations, he was anxious
+to dispose of his wealth in such a way as to bring him that
+fame which he had failed to achieve in other directions. Although
+it is very probable that he had never looked through
+a telescope in his life, the idea of a large telescope had taken
+a very firm hold upon his mind, and, thanks to the influence
+<span class='pageno' id='Page_190'>190</span>of his advisers, it was definitely announced in 1873 that Mr.
+Lick’s bid for immortality was to take this form. Several
+sites were examined by experts, and finally Mount Hamilton,
+California, 4,200 feet above sea-level, was selected. An excellent
+road, 26 miles in length, made at the cost of the county
+authorities, connects the observatory with the nearest town,
+San José, 13 miles distant.</p>
+
+<p class='c007'>Owing to various delays, operations were not commenced
+until 1880, and five years were consumed in clearing away
+72,000 tons of rocks and in erecting the buildings.</p>
+
+<p class='c007'>Mr. Lick had stipulated for the erection of “a telescope
+superior to and more powerful than any telescope yet made,”
+and Messrs. Alvan, Clark &#38; Co. contracted to supply a lens
+of 36 inches aperture for the sum of 50,000 dollars. It turned
+out, however, that it was much easier to make such a contract
+than to fulfil it. To produce large discs of optically perfect
+glass, even in the rough, requires the greatest possible skill
+and patience, and this part of the work was undertaken by
+Feil &#38; Co. of Paris. The flint glass disc was safely delivered
+in America in 1882, but the crown disc was cracked in packing.
+The elder Feil having retired from business, the duty
+of providing a new block of crown glass devolved upon his
+sons, who, after two years spent in vain attempts, ended in
+bankruptcy, and it was only through the elder Feil again
+resuming business that the much-required disc was finally
+completed in 1885. After the lapse of another year, the
+rough discs were fashioned, in the workshops of the Clarks,
+into the most marvellous of telescopic lenses.</p>
+
+<p class='c007'>The mounting of the object-glass is worthy of the occasion,
+as will be seen from our illustration (see page <a href='#Page_40'>40</a>). The
+tube is no less than 57 feet long, and 4 feet in diameter in
+the middle part. An iron pier, 38 feet high, beneath which
+lie the remains of Mr. Lick, supports the equatorial head, and
+a winding staircase enables the observer to reach the setting
+circles. Inside the hollow pier is the powerful driving clock
+which turns the telescope to follow the heavenly bodies in
+their apparent movements. Finders of 6, 4, and 3 inches
+<span class='pageno' id='Page_191'>191</span>diameter, rods for the manipulation of the instrument, and
+all necessary accessories, complete what must long remain one
+of the most perfect instruments at the service of astronomical
+science. The 200,000 dollars expended upon it have already
+been amply justified by the work accomplished, while Mr.
+Lick’s dream of immortality has become a reality.</p>
+
+<p class='c007'>The following list indicates some of the large refractors
+now (Feb., 1897) doing active service:—</p>
+
+<table class='table2'>
+  <tr>
+    <th class='c019'><i>Aperture.</i></th>
+    <th class='c021'><i>Observatory.</i></th>
+  </tr>
+  <tr>
+    <td class='c022'>36 inch</td>
+    <td class='c029'>[Lick] California.</td>
+  </tr>
+  <tr>
+    <td class='c022'>30  „</td>
+    <td class='c029'>Pulkowa, Russia.</td>
+  </tr>
+  <tr>
+    <td class='c022'>30  „</td>
+    <td class='c029'>[Bischoffeim] Nice.</td>
+  </tr>
+  <tr>
+    <td class='c022'>28  „</td>
+    <td class='c029'>Greenwich.</td>
+  </tr>
+  <tr>
+    <td class='c022'>27  „</td>
+    <td class='c029'>Vienna.</td>
+  </tr>
+  <tr>
+    <td class='c022'>26  „</td>
+    <td class='c029'>Washington.</td>
+  </tr>
+  <tr>
+    <td class='c022'>25  „</td>
+    <td class='c029'>[Newall] Cambridge.</td>
+  </tr>
+  <tr>
+    <td class='c022'>24  „</td>
+    <td class='c029'>[Lowell] Mexico.</td>
+  </tr>
+  <tr>
+    <td class='c022'>23  „</td>
+    <td class='c029'>Princeton, New Jersey.</td>
+  </tr>
+</table>
+
+<p class='c007'>It is right to add, however, that opinion is still greatly
+divided as to whether these telescopes of large aperture really
+repay the expense and labour involved in their erection and
+use. On the very rare occasion when the “seeing” is practically
+perfect—which occurs perhaps only a few hours in a
+year—it is probable that the superiority of a large telescope
+is very marked, but under average conditions there seems to
+be little advantage over instruments of moderate size for many
+classes of observations.</p>
+
+<p class='c007'>Certain it is that a great deal of valuable work is done with
+comparatively small telescopes, ranging from six to fifteen
+inches aperture, and this in all departments of astronomical
+research. Hence, some of the most active observatories do
+not figure in the above list; among them may be mentioned
+the observatories of Harvard College (U.S.A.), Potsdam, Paris,
+Heidelberg, Cape of Good Hope, Edinburgh, South Kensington,
+Stonyhurst College, and the observatory of Dr.
+Isaac Roberts at Crowborough, Sussex.</p>
+
+<p class='c007'><span class='sc'>Housing of Equatorials.</span>—The building which accommodates
+<span class='pageno' id='Page_192'>192</span>an equatorial telescope must evidently be designed
+to admit of giving a clear opening to any part of the sky.
+Usually this is accomplished by making the roof, or <i>dome</i>,
+with a circular base, provided with wheels, which run on rails.
+It is then only necessary to open a narrow portion of the
+dome, extending from top to base, and to turn the dome until
+this aperture is in the required direction. One of the most
+elaborate domes now in existence is that built by M. Eiffel
+for the great refractor of the Nice Observatory. The lower
+part of the building is in the form of a square (see Frontispiece),
+having a side of about 87 feet, and a height of about
+30 feet. The dome itself is 74 feet in diameter, and the
+moving parts alone weigh 95 tons.</p>
+
+<p class='c007'>As will be seen from the illustration, there are two
+shutters, each a little wider than half the possible opening:
+these run on short rails, and are moved simultaneously by
+means of an endless rope. The whole of the dome is built
+up of steel angle iron, covered with very thin sheet steel. In
+order to facilitate the manipulation of the dome, its great
+weight is buoyed up by means of a float attached to its base
+and immersed in a circular tank of water of a little greater
+size than the base of the dome. If any mishap occurs with
+this gigantic tank, the dome rests on wheels which run on a
+circular rail, so that the work need not be interrupted. The
+whole arrangement is very easily turned with the aid of a
+winch by one man when the dome is floating, but when resting
+on the wheels several men are required at the winch.</p>
+
+<p class='c007'>This brief description will serve to illustrate some of the
+problems which confront the possessor of a very large telescope.
+For smaller instruments, the observatories follow
+pretty nearly the same plan, except that it is unnecessary to
+provide an arrangement for floating the dome.</p>
+
+<p class='c007'>The observatory which shelters a reflecting telescope need
+not differ very greatly from one which contains a refractor.
+If the instrument be a Newtonian, it is generally convenient
+to sink the polar axis below the level of the floor in order
+that the observer may not be at too great a height from the
+<span class='pageno' id='Page_193'>193</span>ground, and in that case, the dome, or its equivalent, is all
+that is necessary. For his five-foot reflector, Dr. Common
+designed an observatory which is not of the ordinary form,
+but gives the necessary opening partly by means of large
+shutters, and partly by a revolution of the whole house. It is
+not everyone who is able to lay out £8,000 on such a dome
+as that erected at Nice by M. Bischoffeim.</p>
+
+<p class='c007'>The varying position of the eye end of a telescope, when it
+is turned to different parts of the sky, makes it necessary to
+provide comfortable and safe seating accommodation for the
+observer, more especially when the telescope is a very large
+one. In the case of the Yerkes telescope, the eye-piece will be
+30 feet higher when observing near the horizon than when observing
+near the zenith, and the observer must necessarily
+follow the telescope. The most convenient arrangement in
+such a case is to raise or lower the floor of the observatory as
+occasion demands. The floor of the Yerkes Observatory is 75
+feet in diameter, and by means of electric motors it can be
+given a vertical motion of 22 feet. A similar arrangement
+was provided for the Lick telescope from the designs of Sir
+Howard Grubb. With smaller instruments, observing ladders
+and adjustable chairs of various forms are employed.</p>
+
+<p class='c007'><span class='sc'>The Equatorial Coudé.</span>—A form of equatorial telescope
+which has possibly a great future before it is one introduced
+at Paris under the name of the <i>equatorial coudé</i>, or elbowed
+telescope. Its practical advantage is that the observer remains
+in a constant and comfortable position, so that revolving
+domes and elevating floors, or other arrangements
+serving similar purposes, are no longer necessary. The telescope
+tube is of two parts of nearly equal length, and what is
+ordinarily the lower half of the tube forms part of the polar
+axis, while the other half is attached to it at right angles. At
+the point of intersection of the two halves of the tube is a
+plane mirror, and there is another mirror in front of the
+object-glass. If the latter mirror were removed, such a telescope
+would only enable the observer to see objects lying
+along the celestial equator, but by its means objects in all
+<span class='pageno' id='Page_194'>194</span>parts of the heavens can be brought within range to an
+observer gazing down the hollow polar axis. The largest
+instrument is that at the Paris Observatory, which has an
+object-glass 23½ inches in diameter for visual observations,
+and another of the same size for photographic purposes.</p>
+
+<p class='c007'><span class='sc'>Fixed Telescopes.</span>—There is still another method of
+using a telescope. The telescope itself may be fixed, and the
+light of the heavenly bodies may be reflected into it by means
+of a mirror which is made to revolve so as to keep pace with
+their movements. Foucault devised an instrument called the
+<i>siderostat</i> for this purpose, and although it is not largely employed
+for telescopic observations, it is very widely utilised
+for spectroscopic work, where the spectroscope is of a kind not
+readily attached to a telescope.</p>
+
+<p class='c007'>Another instrument used for the same purpose has recently
+been brought forward under the name of the <i>coelostat</i>. This
+is simply a mirror which is made to turn on a polar axis in its
+own plane, and since a reflected ray of light moves through
+twice the angle that the reflecting surface turns through, the
+mirror is made to revolve at the rate of one revolution in two
+days. As the name indicates, the whole heavens appear
+stationary in such an instrument, whereas in a siderostat, only
+one star at a time appears at rest, while its neighbours slowly
+revolve round it.</p>
+
+<p class='c007'><span class='sc'>Photographic Telescopes.</span>—The application of photography
+to the study of the heavenly bodies marks one of the
+greatest advances of the present century. The instruments
+which are employed for this purpose range from the ordinary
+tourist camera to the largest telescope. Unlike a person sitting
+for a portrait, the heavenly bodies cannot be made to
+stand still for the purpose, and as instantaneous photographs
+can only be obtained in the case of the sun and moon, it is
+usually necessary to make the camera follow the stars very
+exactly during the time of exposure, in order that the images
+may fall on precisely the same parts of the photographic
+plate.</p>
+
+<p class='c007'>Some guiding arrangement is, therefore, essential, and
+<span class='pageno' id='Page_195'>195</span>generally the photographic camera or telescope is attached to
+an ordinary equatorial which is driven by clock-work, or very
+carefully by hand if the camera be a small one. In the guiding
+telescope are two spider threads at right angles to each
+other, and it is by constantly keeping the image of a star at
+the intersection of these “wires” that the operator ensures
+the images remaining in a constant position upon the sensitive
+plate.</p>
+
+<p class='c007'>An ordinary portrait camera, in the hands of a skilled observer,
+yields very beautiful pictures, but they are naturally
+on a small scale. The field of view of such an instrument is
+so large that a whole constellation may be photographed with
+a single exposure.</p>
+
+<p class='c007'>Portrait lenses of 6 inches aperture in the hands of Dr.
+Max Wolf and Professor Barnard have given magnificent
+delineations of the Milky Way, and of the extremely faint
+nebulosities which are to be found in many parts of the
+heavens.</p>
+
+<p class='c007'>For many purposes, however, telescopes of greater power
+are required, and here it may be remarked that the distance
+between the images of any two adjacent stars will vary in
+direct proportion to the focal length of the telescope. In
+the same way the size of the image of a planet, the moon,
+or a comet, increases as the focal length of the objective is
+increased.</p>
+
+<p class='c007'>Refracting telescopes which are employed for photography
+require object-glasses which are specially “corrected” for
+the photographic rays. White light is compounded of light
+of all colours, but it is the blue and violet constituents which
+are effective in producing photographic action on an ordinary
+sensitive plate. Now, an object-glass which is intended for
+visual purposes is made to focus at the same point as many
+as possible of the rays which are most effective to the human
+eye, that is the green, yellow, and red, and usually there is a
+blue or purple halo round the images of the brighter objects,
+which is, however, too feeble as a rule to interfere with
+visual observations. This blue halo, will evidently result in
+<span class='pageno' id='Page_196'>196</span>defective definition if the lens be employed for photography.
+By putting the plate at the point where the blue rays are
+most nearly focused, a better image is obtained; but for
+really good work a photographic object-glass must be so designed
+that all the blue and violet rays are brought to one
+and the same focus. Such a lens will consequently be a very
+poor one for visual observations. At the present time, 18
+photographic telescopes, each of 13 inches aperture, and
+corrected in this way, are at work in various parts of the
+world for the international star chart.</p>
+
+<div id='i_196'  class='figcenter id001'>
+<img src='images/i_196.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 52.</span>—<i>The Photographic Telescope employed by Dr. Isaac Roberts.</i></p>
+</div>
+</div>
+
+<p class='c007'>The new “photo telescopic” object-glass now manufactured
+by Messrs. Cooke appears to be full of promise. In this lens
+all the colours of the spectrum are brought to almost exactly
+the same focal point, so that it serves equally well for photographic
+or visual purposes.</p>
+
+<p class='c007'>This difficulty in regard to achromatism does not exist in
+the case of the reflecting telescope, since rays of light of every
+<span class='pageno' id='Page_197'>197</span>colour are reflected at precisely the same angles. For this
+reason reflectors, when properly managed, give the best
+photographic results. Dr. Isaac Roberts and Dr. Common
+are especially identified with the application of the reflecting
+telescope for celestial photography. The instrument employed
+by the former consists of a 20-inch reflector, and a
+7-inch guiding telescope of the refracting form. The two
+telescopes are mounted on the extreme ends of the declination
+axis of an equatorial, a photograph of which we owe to
+the kindness of Dr. Roberts.</p>
+
+<p class='c007'>Dr. Common does not employ a guiding telescope at all.
+The photographic plate which he places at the focus of the
+reflector is smaller than the field of view, so that by means of
+an eye-piece fitted with a cross wire at the side of the dark
+slide, he is able to watch a star near the edge of the field.
+Both eye-piece and dark slide are attached to a frame which
+can be controlled by two screws at right angles to each other.
+If the guiding star leaves the cross wire through errors in
+driving, or other causes, the eye-piece and dark slide are bodily
+moved after it by means of the adjusting screws. This method
+not only has the advantage of saving the cost of a guiding
+telescope, but reduces the effects of vibration consequent
+upon the correction of errors by moving the whole telescope.</p>
+
+<p class='c007'>For photographing the sun a special instrument called a
+<i>photoheliograph</i> is usually employed. This differs only from
+an ordinary photographic telescope in being provided with a
+secondary magnifier, by which means the focal image formed
+by the object-glass is amplified before falling upon the
+photographic plate. On a bright clear day, pictures of the
+sun 8 inches in diameter can be taken with an exposure of
+about ¹⁄₅₀₀th of a second, and such a photograph will frequently
+record more facts as to the state of the solar surface than a
+whole day’s observation. Lenses or mirrors of very long
+focus are also occasionally employed in solar photography,
+and in this way a large image is obtained without the use of
+a secondary magnifier.</p>
+
+<p class='c007'>Photographs of the moon and planets may be taken either
+<span class='pageno' id='Page_198'>198</span>with or without a secondary magnifier, but in either case the
+exposures are longer than for the sun.</p>
+
+<p class='c007'>Finally, it may be added that the sensitive plates and
+processes used in astronomical photography do not differ from
+those employed by ordinary photographers.</p>
+
+<h3 class='c004'>CHAPTER XVI.<br> <span class='c014'>INSTRUMENTS OF PRECISION.</span></h3>
+
+<p class='c003'><span class='sc'>The Meridian Circle.</span>—The accurate registration of the
+positions of the heavenly bodies is one of the most important
+functions of an astronomical observatory. When the apparent
+places of an object at a sufficient number of different
+times have been duly recorded, it becomes possible to investigate
+the laws upon which its changes of position depend,
+and to predict its positions at subsequent times for the
+benefit of navigators and others to whom such predictions are
+of practical utility. For this purpose various instruments
+have been devised, but in all cases where it can be employed,
+the <i>transit circle</i>, or <i>meridian circle</i>, as it is indifferently
+called, is generally conceded to give the most trustworthy
+results.</p>
+
+<p class='c007'>With this instrument the observations are made when the
+celestial body under observation is crossing the meridian of
+the place where the instrument is set up, that is, when it
+“transits,” or “souths.” At this time the accuracy of the
+observations is least impaired by the ever-varying effects of
+atmospheric refraction.</p>
+
+<p class='c007'>The meridian circle consists of a refracting telescope—seldom
+exceeding 6 inches in aperture—which is fixed to a
+hollow axis at right angles to itself, and this axis is supported
+<span class='pageno' id='Page_199'>199</span>horizontally in an east and west direction, so that the
+telescope is only free to move in the plane of the meridian.
+A large graduated circle—or frequently two such circles—attached
+perpendicularly to the hollow axis, and read by
+microscopes fixed to the walls or iron pillars which support
+the axis, completes the essential parts of the instrument.</p>
+
+<p class='c007'>As the field of view of the telescope covers a considerable
+area, it becomes necessary to provide some means of marking
+the exact point within it which represents the meridional axis
+of the instrument. This is accomplished by placing at the
+common focus of the object-glass and the positive eye-piece a
+system of “cross wires,” consisting of tightly-stretched spider
+threads, two of which are fixed horizontally and nearly
+in contact, and five or seven vertically at equal distances
+apart. What the observer has actually to do is to incline the
+telescope at such an angle that the star is seen to traverse the
+space between the two horizontal threads, and then to record
+the exact times, by means of a chronograph and sidereal
+clock, at which the star appears to cross each of the equidistant
+vertical threads. By thus making five or seven
+observations and taking the average, greater accuracy is
+attained.</p>
+
+<p class='c007'>The time observations, as we have already seen, determine
+the right ascension of the star under observation, while the
+declination is indicated by the readings of the graduated
+circle, if the latter is so placed as to read 90° when the
+telescope is directed to the Pole.</p>
+
+<p class='c007'>The ideal meridian circle is thus simplicity itself, but the
+mechanical difficulties encountered in making such an
+instrument are insuperable. Perfect right angles and perfect
+circles exist only in our minds, so that after all the undoubted
+skill and care bestowed on its construction, the actual
+meridian circle is only an approximation to the ideal. Still,
+when the instrument is provided with levels and other means
+for estimating its deviation from the meridian plane in which
+it ought to move, the actual observations are capable of
+<span class='pageno' id='Page_200'>200</span>correction by mathematical processes, so that the final statements
+of positions sensibly represent those which would follow
+from the use of a perfect instrument.</p>
+
+<p class='c007'>The greatest possible care is taken to secure rigidity in all
+parts of the meridian circle. The hollow horizontal axis is
+supported on bearings which rest either on heavy piers of
+iron or walls of masonry, and the axis and telescope tube are
+firmly joined together at their intersection. The bearings
+for the axis are turned with extreme precision, and, to reduce
+the friction upon them, the pressure of the instrument is
+counterpoised by an arrangement of balancing weights.</p>
+
+<p class='c007'>Adjustments are provided for every needful purpose. The
+cross wires are fitted in a small frame which can by suitable
+fittings be given a small movement in the field of view until
+the right place for them is found, while the horizontality of
+the axis and its correct direction can be secured by other adjusting
+screws.</p>
+
+<p class='c007'>Since most of the observations have to be made at night,
+the field of view will generally be dark, and the exceedingly
+delicate spider lines will be invisible unless some means of
+illuminating them be provided. Usually a very tiny mirror is
+fixed diagonally at the intersection of the axis and the telescope,
+where it is held in position by a stiff wire. A light
+shining through the hollow axis is thus reflected into the field
+of view, and the threads are rendered visible. The intensity
+of this illumination of the field can be regulated in accordance
+with the brightness of the star under observation.</p>
+
+<p class='c007'>The instrument having been erected, one of the first tests
+applied to it is to see that it is correctly <i>collimated</i>, or, in
+other words, that the optical axis of the telescope is perpendicular
+to the axis of movement. For this purpose the
+telescope is directed to some distant object, such as a building,
+and some mark which falls on the intersection of the
+central spider threads is noted. The axis is then reversed
+end for end by a mechanical arrangement, and the telescope
+again pointed at the same object. If the mark again falls on
+the intersection of the cross wires, the collimation is correct;
+<span class='pageno' id='Page_201'>201</span>if not, the wires are moved with the frame containing them
+until the error is corrected.</p>
+
+<p class='c007'>To test the horizontality of the axis, a spirit-level long
+enough to stretch across the bearings, and called the “striding
+level,” is provided.</p>
+
+<p class='c007'>Various methods are employed for adjusting the instrument
+so that the telescope moves as truly as possible in the plane
+of the meridian. Collimation and level being correct, the
+telescope will move in a vertical plane, whatever may be the
+error in the direction of the horizontal axis, and therefore any
+star passing through the zenith will cross the centre of the
+instrument at the same moment that it crosses the meridian.
+A star away from the zenith, however, will not be seen on the
+cross wires when it crosses the meridian, unless the axis be
+truly east and west. Hence, by taking the difference of time
+between the observed transits of a star near the zenith and
+one a long way from the zenith, and turning the whole instrument
+in azimuth until this difference is equal to the
+difference of right ascensions of the two stars, the instrument
+is readily placed in the meridian.</p>
+
+<p class='c007'>Another useful method of adjustment is to observe the
+upper and lower transits of a circumpolar star. If the instrument
+moves truly in the meridian, the interval between
+the two transits will evidently be twelve sidereal
+hours.</p>
+
+<p class='c007'>Next, the declination circle has to be adjusted so that it
+reads 90° when the telescope is directed to the celestial pole,
+or zero when an equatorial star is under observation. An
+obvious way of doing this is to take the readings when Polaris,
+or other circumpolar star, is at upper and lower transits; the
+celestial pole lying midway between these positions, the
+average of the two readings, when corrected for refraction,
+should be 90°, and the circle would be shifted round in its
+fittings until this was the case.</p>
+
+<p class='c007'>Such, in mere outline, are the processes by which the
+meridian circle is set up. In actual practice, the greatest
+possible refinement is brought to bear on the adjustments,
+<span class='pageno' id='Page_202'>202</span>and every precaution taken to estimate the various errors so
+that due allowance may be made for them in the reduction of
+the observations. It has even been shown that the heat of the
+observer’s body, by affecting the lower side of the telescope
+tube more than the upper, introduces sensible errors in the
+measures of declination. Hence it is important to use metals
+of high conductivity in the construction of meridian instruments,
+so that errors due to the varying temperatures of
+different parts may be reduced to a minimum.</p>
+
+<p class='c007'>As an illustration of a modern meridian circle, we select
+that of the Lick Observatory. (Fig. 53.) This instrument
+has an aperture of six inches, and embodies all the improvements
+which have been introduced by the Berlin firm of
+Repsöld &#38; Co.</p>
+
+<p class='c007'>The observatory containing a meridian circle is usually a
+very simple structure, as it is only necessary to provide an
+opening to the sky along a north and south line. This is
+sufficiently provided for by a series of narrow shutters in a
+building of ordinary construction.</p>
+
+<p class='c007'>To prevent confusion it may be pointed out that the term
+“transit instrument” is frequently restricted to a meridian instrument
+which is not supplied with large circles for the
+accurate measurement of declinations, although it may have a
+small circle to assist in directing the telescope. The use of
+such an instrument is evidently limited to the determination
+of time and right ascension.</p>
+
+<div id='i_203'  class='figcenter id001'>
+<span class='pageno' id='Page_203'>203</span>
+<img src='images/i_203.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 53.</span>—<i>The Meridian Circle of the Paris Observatory.</i></p>
+</div>
+</div>
+
+<p class='c007'><span class='sc'>The Altazimuth.</span>—Although the meridian circle furnishes
+us with the most accurate method of determining
+celestial positions, its use is somewhat restricted by the fact
+that it can only be employed for the observation of objects on
+the meridian. It sometimes happens, however, that bodies
+cannot conveniently be so observed, and other methods become
+necessary. This is especially the case with the moon
+during the first and fourth quarters, when it crosses the
+meridian in daylight, and it is then that an instrument called the
+<i>altazimuth</i> is of special value. This is something like a transit
+circle in which the base supporting the piers is made to
+<span class='pageno' id='Page_205'>205</span>turn on a vertical axis, so that the telescope can be directed
+to any part of the heavens whatsoever. A fixed horizontal
+graduated circle, read by verniers or microscopes attached to
+the revolving part, gives the azimuth of the telescope when an
+observation is made, and the altitude is furnished by the vertical
+circles. The azimuth circle is adjusted to read zero when
+the telescope is pointed due north, and the altitude circle to
+zero when the telescope is horizontal. To secure the first adjustment,
+after correcting level and collimation, a star may be
+observed before it crosses the meridian, and again when it has
+exactly the same altitude after passing to the west; midway
+between the two positions would be due south, and the
+circle should read 180°. In adjusting the vertical circle,
+the telescope is made to point downwards to a trough
+of mercury, and it is known that the telescope is truly
+vertical when the reflected image of the cross wires is
+coincident with the wires themselves; the circle should then
+read 90°.</p>
+
+<p class='c007'>From a knowledge of the sidereal time at which a celestial
+body has an observed altitude and azimuth, the more useful
+co-ordinates of right ascension and declination can be calculated
+by spherical trigonometry.</p>
+
+<p class='c007'>One of the largest instruments of this class has recently
+been erected at Greenwich Observatory. The aperture of the
+telescope is 6 inches, and the rigidity of the various parts may
+be gathered from the fact that the instrument weighs something
+like six tons.</p>
+
+<p class='c007'>A <i>theodolite</i> is a small portable form of altazimuth specially
+adapted for the needs of surveyors, but occasionally employed
+in astronomical work.</p>
+
+<p class='c007'><span class='sc'>The Wire Micrometer.</span>—Notwithstanding that an equatorial
+telescope is usually furnished with circles for estimating
+the positions of objects observed, or to serve as a guide in
+directing the telescope to objects of known position, it is not
+entitled to be called an instrument of precision in the sense we
+are now considering. The provision for driving by clock-work
+and other causes are antagonistic to constancy of adjustment,
+<span class='pageno' id='Page_206'>206</span>and hence determinations of positions by the circles alone
+might be many seconds in error. Most large telescopes, however,
+are provided with some form of micrometer which not
+only serves for the measurement of planets, lunar craters, and
+the like, but may also be used to measure the angular separation
+of adjacent stars. In this way, by making a “triangulation”
+of stars visible in the field of view, and including at
+least two which have had their precise positions determined
+by the meridian circle, the positions of objects can be
+measured with great accuracy.</p>
+
+<p class='c007'>This method is especially valuable in the case of comets,
+which may cross the meridian in daylight, and are often too
+dim to be seen with the altazimuth.</p>
+
+<p class='c007'>Several forms of micrometers are in use, but the so-called
+<i>wire</i> or <i>filar micrometer</i> is most commonly seen in our observatories.
+The essential parts are very similar to those of
+the reading microscope (p. 172). Two parallel spider threads
+are so arranged on sliding frames that they may be brought
+into coincidence, or separated, by means of very finely-cut
+screws. Perpendicular to these are two fixed threads almost
+close together. The system of “wires” is viewed by a
+positive eye-piece, and the whole is attached to a draw tube so
+that it may be placed in position at the eye end of the telescope.
+In order that the wires and telescopic images may be
+sharply defined at the same time, the plane of the wires must
+be at the principal focus of the object-glass. The screws are
+provided with large heads which are graduated so as to show
+the hundredth of a revolution, and counting wheels register
+the numbers of complete turns.</p>
+
+<p class='c007'>Matters are so arranged that when both counting wheels
+indicate zero, the spider threads are coincident. Then, supposing
+one of the screws be turned through a revolution, the
+threads will be separated by a definite amount; an equal
+and opposite movement of the other screw will double
+the separation, and in all cases the distance between the
+threads will be registered in turns, and fractions of turns of
+the screws.</p>
+
+<p class='c007'><span class='pageno' id='Page_207'>207</span>The next proceeding is to ascertain what is called the
+“value,” in angular measure, of the micrometer screw. This
+value will evidently depend upon the pitch of the screw and
+the focal length of the telescope to which the micrometer is
+applied, so that measurements merely stated in terms of revolutions
+of the screw would serve no useful purpose. It can
+easily be calculated that the images of two stars which are
+28′ 39″ apart will be separated by an inch at the focus of a
+telescope of 10 feet focal length; then, if the screws have 100
+threads to the inch, the angular separation of the wires corresponding
+to a single revolution will be one-hundredth part of
+28′ 39″, that is, 17″·15, and the latter would be the value of
+that particular micrometer when used with the telescope in
+question. If the focal length of the telescopic object-glass
+were 20 feet, the linear separation of the images of two such
+stars as we have considered would be 2 inches, and the value
+would therefore be halved, so that measures of twice the accuracy
+would be possible. Since the stellar images and the
+cross wires are equally magnified by the eye-piece, the value of
+the screw is in no way affected by using eye-pieces of different
+powers.</p>
+
+<p class='c007'>In practice it is necessary to determine the value of the
+micrometer screw by actual measurement. For this purpose,
+the wires are separated by a known number of revolutions, say
+twenty, and the micrometer is adjusted so that a star of known
+declination travels exactly between the two fixed wires when
+the telescope remains at rest. With the telescope still fixed,
+the number of seconds required by the image of the star to
+traverse the distance between the separated wires is noted, and
+knowing the angle through which the star must have moved
+in that interval, the angular value of one turn of the screw is
+at once deduced. For work of extreme precision each individual
+turn of the screw must be separately evaluated,
+and allowances must also be made for changes of temperature.</p>
+
+<p class='c007'>When measuring the apparent diameter of a planet, the
+two threads are separated until the image just lies between
+<span class='pageno' id='Page_208'>208</span>them, and the sum of the readings of the two screws multiplied
+by the angular
+value of one turn
+gives the diameter
+in seconds
+of arc. The
+distance having
+been formed by
+other observations,
+the diameter
+of the
+planet in miles
+can be determined
+in the
+manner to which
+reference has
+already been
+made (p. 142).</p>
+
+<div id='i_208'  class='figcenter id001'>
+<img src='images/i_208.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 54.</span>—<i>The Micrometer applied to a Binary Star: a b, Fixed Threads; c d, e f, Movable Threads; s s, Components of Binary Star.</i></p>
+</div>
+</div>
+
+<p class='c007'>One of the
+most important
+applications of the micrometer is in the measurement of double
+and binary stars. In this case the fixed threads are made
+to enclose the two stars, and the movable threads are made
+to bisect the star-images. (Fig. 54.)</p>
+
+<p class='c007'><span class='sc'>The Position Circle.</span>—It is frequently necessary to be
+able to specify a direction, as in the case of a planet’s equator,
+or the line joining the components of a double star. Such
+directions are expressed by “position angle,” which may be
+defined as the angle from the north point, reckoned from 0° to
+360° through east, south, and west. For these observations, a
+<i>position circle</i> is usually attached to the micrometer. This is
+a circle graduated from 0° to 360°, which can remain fixed in
+position as regards the telescope, while the part containing
+the wires and micrometer screws can be rotated by means of
+a rack and pinion. A vernier attached to the movable frame
+indicates the required angles.</p>
+
+<p class='c007'>To adjust the position circle the vernier is set to zero, and
+<span class='pageno' id='Page_209'>209</span>the telescope directed to a star; the circle and micrometer
+are then together turned round until the diurnal movement of
+the star, which is east and west, makes its image to traverse
+the space between the fixed wires. The movable threads will
+then lie in a north and south direction. The circle remains
+in this position during subsequent observations, while the
+micrometer is rotated until the movable threads are in the required
+direction, the position angle then being read off on the
+circle.</p>
+
+<p class='c007'><span class='sc'>The Heliometer.</span>—Another means of measuring small
+angles for astronomical purposes is afforded by the instrument
+called the <i>heliometer</i>, which, as the name will at once suggest,
+was invented for measurements of the sun. This instrument
+is a telescope mounted equatorially, but differs from the
+ordinary telescope, inasmuch as the object-glass is cut across
+the centre, and means are provided for separating the two
+halves by moving one or both parts in the direction of the line
+of bisection, and also for measuring the amount of displacement.
+The cell containing this somewhat peculiar object-glass
+can be rotated so that the line of division of the lens
+may be placed in the same direction as the line representing
+the distance to be measured.</p>
+
+<p class='c007'>The action of the instrument depends upon the fact that
+any small part of a lens is competent to form a complete image
+of a celestial body, so that when an object-glass is bisected,
+and the two halves separated laterally, two distinct images
+will be produced, each differing only from the image formed
+by the complete lens in being less bright.</p>
+
+<p class='c007'>To measure the distance from a star to a planet, let us say,
+as in observations of the parallax of Mars, the lenses are separated
+to such an extent that the image of the star formed by
+one half, coincides with that of the planet formed by the other
+half, and the amount of separation noted. As a check, the
+measurement is repeated with the lenses separated in the
+opposite direction. The angular value corresponding to a
+known separation of the semi-lenses being determined, just as
+in the case of the micrometer screw, the angle between star
+<span class='pageno' id='Page_210'>210</span>and planet at once follows. Angles ranging from a few
+minutes to about two degrees can be measured in this way
+with great accuracy.</p>
+
+<p class='c007'>In the hands of Dr. Gill, of the Cape Observatory, the
+heliometer has yielded very valuable results in connection with
+the distances of the sun and stars.</p>
+
+<p class='c007'><span class='sc'>Other Instruments.</span>—There are other instruments which
+may fairly be classed as instruments of precision, but space
+permits little more than a mention of their names.</p>
+
+<p class='c007'>The <i>zenith telescope</i> is a telescope specially designed for the
+measurement of the angular distances of stars from the zenith,
+for precise determinations of latitude by Talcott’s method.</p>
+
+<p class='c007'>The <i>prime vertical instrument</i> is nothing more than a transit
+instrument, so arranged that the observing telescope swings
+in a vertical plane which is perpendicular to the plane of the
+meridian. From the observed times at which a star passes
+the prime vertical on the eastern and western sides, the latitude
+of the place of observation can be ascertained with great
+accuracy.</p>
+
+<p class='c007'>It is perhaps at sea that the labours of astronomers are of
+most direct value in everyday affairs, and it is precisely here
+that the instruments of high precision cannot be employed, in
+consequence of the absence of firm supports. Nevertheless,
+there is one instrument—<i>the sextant</i>—which yields results
+that satisfy all requirements when carefully constructed and
+placed in good hands. A graduated arc extending over
+about 60° (from which the name is derived) is supported by a
+light framework, and pivoted truly on the centre of the arc is
+the radius bar, or index arm, which carries a vernier for reading
+off the angles to be measured. A plane mirror is fixed to
+the index arm, over the centre of movement, and another, of
+which only half is silvered, is fixed to the frame near its outer
+edge. A small telescope parallel to the surface of the frame
+is directed towards the fixed mirror, so that the continuation
+of its axis is in line with the boundary between the silvered
+and clear part of the glass. Thus, while one object may be
+seen by direct observation through the clear glass, another, in
+<span class='pageno' id='Page_211'>211</span>quite a different direction, may be seen after reflection from
+the surfaces of the two mirrors.</p>
+
+<p class='c007'>The sextant is chiefly used for measuring the altitude of the
+sun, about noon for the determination of latitude, and in the
+morning or evening for the correction of chronometers. In
+such observations, the sextant is held in the right hand, with
+its plane vertical, and the sea horizon is sighted directly with
+the telescope; the index arm is then moved until the reflected
+image of the sun is brought into coincidence with the
+horizon. The reading is then taken, and if the adjustment is
+such that zero is indicated when the reflected and direct
+images of the same object are observed, it will give the altitude.
+The actual angle recorded by the sextant is only half that between
+the objects observed, but by numbering half degrees as
+whole ones, the true angles are read off directly. For observations
+of the sun the instrument is provided with coloured
+glasses of different shades, attached so that they can readily
+be interposed to reduce the intensity of the light.</p>
+
+<h3 class='c004'>CHAPTER XVII.<br> <span class='c014'>ASTROPHYSICAL INSTRUMENTS.</span></h3>
+
+<p class='c003'>So far we have been concerned with instruments which enable
+us to ascertain the positions, dimensions, and appearances of
+the various orders of heavenly bodies; but we can go further
+than this, and learn something of the physical and chemical
+constitutions of the glittering orbs by which we are surrounded.
+We can, for instance, bring instrumental aid to bear upon the
+determination of the brightnesses of the heavenly bodies, and
+by means of that powerful appliance of modern astronomy—the
+spectroscope—we can study the chemistry of all those
+<span class='pageno' id='Page_212'>212</span>bodies which shine by light of their own, and which are not
+so feebly luminous as to be out of our range.</p>
+
+<p class='c007'><span class='sc'>Photometry.</span>—The naked eye was alone employed in observations
+of stellar brightness until quite recently. Each
+step in the advance of astronomical research, as in most other
+branches of science, however, depends upon the greater precision
+of observation which can be introduced, and so we now
+find the eye to be assisted in these inquiries by a <i>photometer</i>
+of some kind or other. The general purpose of photometry
+will be familiar to all in connection with such practical matters
+as the determination of the illuminating power of coal gas.
+The methods here employed, however, are not directly applicable
+to the comparatively feeble light-sources which have
+usually to be dealt with in astronomical photometry.</p>
+
+<p class='c007'>As will be more fully explained in another part of this
+work, the stars visible to the naked eye are divided into six
+grades of magnitude. The brightest of them are classed as
+first magnitude, while those only just visible to the naked eye
+are of the sixth magnitude. Now that telescopes are used,
+this division of stars into magnitudes must be continued in
+some form or other, so as to include telescopic stars. From
+photometric comparisons it has been ascertained that the
+average star of the first magnitude may conveniently be
+reckoned 100 times as bright as a sixth magnitude star.
+Hence, the light-ratio corresponding to a difference of a single
+magnitude is 2·5. Thus, a star which is 2½ times less bright
+than one of the sixth magnitude ranks as seventh magnitude,
+and so on. Fractions of magnitudes are also necessary to
+express the results which can now be obtained.</p>
+
+<p class='c007'><span class='sc'>Limiting Apertures.</span>—For the reason that a large telescope
+enables us to see stars which are too dim to be visible
+in a smaller one, the brightnesses of stars may be compared
+with more or less satisfactory results by reducing the aperture
+of a telescope until the star in question ceases to be visible.
+This is called the method of <i>limiting apertures</i>, and in practice
+a telescope intended for this work would be provided with a
+series of diaphragms, or other arrangement for conveniently
+<span class='pageno' id='Page_213'>213</span>reducing the effective area of its object-glass. A telescope
+which has an object-glass 10 inches in diameter should just
+show stars of the fourteenth magnitude under favourable conditions;
+a star which could just be seen when this aperture
+was reduced to an inch would be of the ninth magnitude, and
+so on.</p>
+
+<p class='c007'>There are numerous reasons why this method fails to give
+satisfactory results, but one of the most important is that the
+image of a star becomes more diffuse with each reduction in
+the aperture of the telescope. At best it must evidently fail
+for a comparison of stars which are visible to the naked eye.</p>
+
+<p class='c007'><span class='sc'>Wedge Photometer.</span>—One of the simplest and best
+methods of estimating star magnitudes is afforded by the
+<i>wedge photometer</i>. This is a strip of neutral-tinted glass about
+six inches in length, and a quarter to half an inch deep, tapering
+from one end to the other, so as to present a gradual reduction
+in depth of tint from the thick to the thin end. A
+similar wedge of clear glass, tapering the opposite way, is
+cemented to this, in order to get rid of prismatic action. Compensated
+in this way, and mounted in a suitable frame, the
+wedge is placed in front of the eye-piece of a telescope, and
+is pushed along until the star under examination is just extinguished.
+A scale is then read off, and from the results of
+a previous evaluation of the wedge in the laboratory, the corresponding
+star magnitude is easily deduced.</p>
+
+<p class='c007'>In order to eliminate the effects of differences in the state
+of the sky, the position of the wedge at which a standard star,
+such as Polaris, ceases to be visible, is determined, and then
+it is the difference of wedge readings upon which the final
+calculation is based.</p>
+
+<p class='c007'>The great value of the wedge in stellar photometry was demonstrated
+by the labours of the late Prof. Pritchard, to whom
+we owe the catalogue of the magnitudes of naked eye stars in
+the northern hemisphere known to the astronomical world as
+the “Uranometria Nova Oxoniensis.”</p>
+
+<p class='c007'><span class='sc'>Other Photometers.</span>—Some photometers depend for
+their action upon comparisons with terrestrial sources of light.
+<span class='pageno' id='Page_214'>214</span>In some cases, an artificial star, consisting of a pinhole illuminated
+by a standard lamp, is brought into the same field
+of view as the star to be compared, and then, by polarising
+apparatus, the brightnesses of the two images are equalised.
+The amount of reduction of either of the stars is determined
+by a scale which measures the rotation of the polariscope, and
+in this way all the stars are compared with an artificial star of
+known brightness.</p>
+
+<p class='c007'>One of the most notable achievements in this field of astronomical
+work is that of Professor Pickering of the Harvard
+College Observatory, who invented and made splendid use of
+the so-called <i>meridian photometer</i>. Here the telescope has two
+object-glasses of equal aperture side by side, and in front
+of each is a silvered flat mirror inclined at an angle of 45° to
+the optic axes. The telescope is supported in an east and
+west direction, so that one mirror reflects the Pole Star into
+its object-glass, while the other can be rotated so as to reflect
+any other star which is on the meridian into the second object-glass.
+Again, by a polariscope at the eye end of the telescope
+the images of the two stars are made of equal brightness,
+and the readings give the data for calculating the required
+magnitude.</p>
+
+<p class='c007'>Photographs of the stars are also largely employed for the
+estimation of magnitudes, stars of different magnitudes being
+represented on the photographs by spurious discs of different
+sizes. If all stars gave out light of the same quality, the
+photographic method would be very trustworthy, but as the
+colours of the stars vary, the photographic and visual magnitudes
+are not invariably in agreement A bright, reddish star,
+such as Betelgeuse, would photographically be only equivalent
+to a white star which was much less bright to the naked eye.</p>
+
+<p class='c007'><span class='sc'>The Prismatic Spectroscope.</span>—Reference has already
+been made in these pages to the wonderful field of astronomical
+research which has been opened up by the discovery of
+the action of a triangular glass prism upon rays of light, and
+the subsequent improvements in the method of utilising this
+effect.</p>
+
+<p class='c007'><span class='pageno' id='Page_215'>215</span>A prismatic spectroscope may be regarded as an arrangement
+which will enable us to get a pure spectrum, and to
+observe it to the best advantage. The light to be analysed
+is admitted through a narrow aperture called the <i>slit</i>, which
+is placed at the focus of a double convex lens. Emerging
+from this <i>collimator</i>, as a parallel beam, the rays pass through
+the prism, and after deviation and dispersion they fall upon
+another double convex lens, which brings them to a focus in
+the form of a spectrum. An eye-piece may then be employed
+to view the spectrum, or a sensitive plate may be placed at
+the focus to photograph it.</p>
+
+<p class='c007'>In a simple form of spectroscope the prism is supported at
+the centre of a graduated circular plate, to which the collimator
+is firmly fixed, while the observing telescope is attached
+to an arm pivoted at the centre of the plate. A vernier
+moving with the telescope indicates the position, on a scale
+of degrees, of any colour brought to the centre of the field of
+view.</p>
+
+<p class='c007'>The best results are obtained when the rays of light emerge
+from the prism at the same angle at which they enter it, in
+which case the prism is said to be at <i>minimum deviation</i>, for
+the reason that the deflection of the rays from their original
+path is then the least possible. As lights of different colours
+are refracted unequally, it is clear that the prism can only be
+at minimum deviation for rays of one particular colour at any
+instant. Frequently, however, there is an automatic arrangement
+by which, as the observing telescope is moved so as to
+bring different colours into the field of view, the prism is
+turned so as to be at minimum deviation for the colour
+actually under observation.</p>
+
+<p class='c007'>The appearances observed in the spectroscope are a series
+of images of the aperture through which the light is admitted.
+If the source of light be yellow, such as that of a spirit lamp
+flame when common salt is introduced, a yellow image of the
+aperture will be seen, and so on for other monochromatic
+radiations. When a white light is observed, images of every
+gradation of colour are formed, and in such a “continuous
+<span class='pageno' id='Page_216'>216</span>spectrum” the separate images cannot be recognised. The
+form of aperture most widely adopted is a narrow straight slit
+with parallel sides. In this case there is the least possible
+confusion, because the several images of the slit appear as so
+many spectrum “lines.”</p>
+
+<p class='c007'>For observations of the sun, where the light is so intense, a
+great number of prisms, each drawing out the spectrum into a
+longer band, may be employed, so that the lines of the
+spectrum may be widely separated, and the peculiarities of
+each more closely investigated. For the fainter bodies, however,
+the instrument must generally be one of comparatively
+small dispersion, so that the light may not be spread out into
+invisibility. It will be evident that the longer the spectrum
+the greater will be the chances of accurate measurements.</p>
+
+<p class='c007'>Another way of obtaining great dispersion is to use prisms
+of the new dense Jena glass, one of which is equal to three or
+four of the flint glass prisms in general use.</p>
+
+<p class='c007'>There are various forms of the prismatic spectroscope. In
+some of them reflecting prisms are introduced to turn the rays
+back through the dispersive train, so as to get increased dispersion
+without increasing the number of prisms. In the so-called
+<i>direct vision spectroscope</i>, prisms of different kinds of
+glass are combined so that the rays of light leave them in
+nearly the same direction that they enter. Here the collimator
+and observing telescope are in the same straight line, and
+this is a great convenience in certain classes of observation.</p>
+
+<p class='c007'><span class='sc'>The Grating Spectroscope.</span>—Sometimes, especially in
+instruments designed for solar observations, the prisms are
+replaced by what is called a diffraction grating. Usually this
+consists of a piece of highly polished speculum metal, upon
+which is ruled a great number of equidistant parallel scratches
+or lines. A portion of the light falling upon the grating is
+simply reflected, while the remainder is spread out into two
+series of beautiful spectra, one on each side of the directly
+reflected beam. The two nearest to the directly reflected
+beam are called spectra of the first order, while following
+these are spectra of the second, third, and fourth orders; the
+<span class='pageno' id='Page_217'>217</span>length of spectrum increasing in each case, and all being
+available for observation if the light dealt with be sufficiently
+bright. The production of these spectra is due to the interference
+of light waves.</p>
+
+<p class='c007'>All gratings produce exactly similar spectra, so that the
+distances between identical lines as seen with one grating are
+always strictly proportional to their distances as seen with
+any other. With prisms, the relative separation of colours is
+by no means constant; a prism made of one kind of glass
+may, for example, separate the green and yellow more than
+another prism made from different material, while the separation
+of yellow and red might be the same in both cases. The
+grating spectrum accordingly affords a constant standard of
+reference, and what is called the “normal solar spectrum” is
+the spectrum of the sun mapped with the various dark lines
+in the relative positions shown by a grating spectroscope.</p>
+
+<p class='c007'>Prof. Rowland, of John Hopkins University, has introduced
+a form of grating spectroscope, in which the grating is
+ruled on a concave spherical surface of speculum metal.
+After passing through the slit the rays of light fall directly
+upon this concave surface, and are brought to a focus after
+reflection, so that no lens except the eye-piece used for visual
+observations is required. Several of these gratings, having
+mostly a radius of curvature of about 21 feet, and a ruled
+surface of about 5½ inches x 2 inches, with 20,000 lines to the
+inch, are in use at the present time. Some idea of the
+difficulties to be faced in making these magnificent aids to
+research maybe gathered from the following remarks of Mr. J.
+S. Ames:—“It takes months to make a perfect screw for the
+ruling engine, but a year may easily be spent in search of a
+suitable diamond point.... When all goes well it takes five
+days and nights to rule a 6 inch grating having 20,000 lines
+to the inch. Comparatively no difficulty is found in ruling
+14,000 lines to the inch.”</p>
+
+<p class='c007'>With the aid of these wonderful gratings, the solar spectrum
+can be photographed with perfect definition, and extending
+<span class='pageno' id='Page_218'>218</span>over a total length of several yards. Thousands of the tell-tale
+Fraunhofer lines are rendered visible in this way.</p>
+
+<p class='c007'><span class='sc'>Measurement of Spectra.</span>—The spectra of many
+substances, including hydrogen and iron, are so characteristic
+as to be recognisable at a glance by an experienced observer,
+but one must as a rule resort to measurement for the identification
+of lines, or for the purpose of locating unknown lines
+for future reference. One of the simplest methods of
+measurement is that of reading the position of the observing
+telescope upon a graduated circle, when the line is seen at the
+centre of the field. If supplemented by a micrometer eye-piece,
+for differential measures with regard to known spectra,
+this method is extremely convenient. As recorded on
+arbitrary scales of this character, the position of the same line
+would be represented by a number which would be different
+for every instrument, and it is therefore necessary to reduce
+all measurements to a common scale; that now universally
+adopted is the natural one of wave-lengths. The position of
+a line in the spectrum depends upon the length of the waves
+constituting the rays of light which produce it, so that a
+measure of wave-length completely specifies the situation of
+a line whatever spectroscope maybe employed. Light waves
+are excessively minute, but by the use of the diffraction grating
+they can be measured with great accuracy. So small are
+they, that the most convenient unit of wave-length is the
+ten-millionth part of a millimetre<a id='r5'></a><a href='#f5' class='c027'><sup>[5]</sup></a>—or tenth metre, as it is
+technically named. Expressed in this way, the wave-length
+of the glorious red line seen in the spectrum of hydrogen is
+6563·07, while that of the blue line characteristic of the same
+gas is 4861·51.</p>
+
+<p class='c007'>When the positions of a certain number of lines of known
+wave-length have been read off on the scale of any spectroscope,
+the required wave-lengths of other lines are ascertained
+by a graphical interpolation, or by calculation. Elaborate
+tables of the wave-lengths of the lines in the spectra of the
+sun and chemical elements have been prepared by various
+<span class='pageno' id='Page_219'>219</span>investigators, and these are in constant demand by all workers
+in the field of astrophysics.</p>
+
+<p class='c007'><span class='sc'>The Telespectroscope.</span>—For the examination of the
+spectra of the heavenly bodies, a spectroscope is attached to
+the eye end of a telescope from which the eye-piece has been
+removed, such a combination forming a <i>telespectroscope</i>. The
+slit is placed at the principal focus of the object-glass of the
+main telescope, and an image of the object to be observed is
+thus produced upon it. If the sun be under observation, any
+special part of it, such as a sun-spot or the chromosphere, may
+be separately investigated by bringing the corresponding part
+of the image upon the slit.</p>
+
+<p class='c007'>In the case of the sun, moon, comets, planets, or nebulæ,
+the image is one of sensible size and the spectrum lines have
+a perceptible length. With a star, however, the image is only
+an illuminated dot upon the slit, and the spectrum would have
+no appreciable breadth, so that all but the strongest lines
+would in general fail to show themselves. Accordingly, when
+observing star spectra, a cylindrical lens is placed in front of
+the slit, so that the stellar image is drawn out into a bright
+line, and the necessary breadth of spectrum and length of the
+spectrum lines are secured.</p>
+
+<p class='c007'>For photographing the spectra of the heavenly bodies it is
+simply necessary to replace the eye-piece by a small camera,
+and to expose a sensitive plate for a length of time dependent
+on the brightness of the spectrum. The spectrum of a terrestrial
+substance, such as hydrogen or iron, photographed in
+juxtaposition, is always a great convenience, and is essential
+for the investigation of stellar movements by the displacement
+of spectrum lines.</p>
+
+<p class='c007'><span class='sc'>The Lick Star Spectroscope.</span>—Among the most complete
+and perfect spectroscopes adapted for use with the telescope
+is that designed by Prof. Keeler for the great refractor
+of the Lick Observatory. It is illustrated in Fig. 55, and it
+will be at once evident that there are ample means for keeping
+the instrument under control. Towards the upper part of the
+diagram, on the left, is the eye end of the telescope, without
+<span class='pageno' id='Page_220'>220</span>the eye-piece. Two stout brass rods 3 inches in diameter and
+6 feet long are attached by clamps to a revolving jacket which
+surrounds the end of the telescope tube, and on these the
+spectroscope is supported by clamps which allow of it being
+moved inwards or outwards from the focus of the telescope.
+The collimator of the spectroscope lies midway between the
+rods, and in order to facilitate the focussing of the image upon
+the slit, it has a small longitudinal movement independently
+of that of the whole spectroscope. The observing telescope is
+seen on the left of the diagram, while the grating rests on
+the circular graduated plate over which the observing telescope
+can be moved. The grating has 14,438 lines to the
+inch.</p>
+
+<p class='c007'>Three prisms can also be used with the spectroscope, two
+of them being single prisms of 30° and 60° refracting angles
+respectively, and the third a compound prism giving a very
+high dispersion. Two observing telescopes are provided, one
+being of extra power for use with the grating in solar
+spectroscopy</p>
+
+<p class='c007'>The instrument is generously supplied with the small refinements
+which contribute so largely to easy and successful
+manipulation. Among these are a diagonal eye-piece for viewing
+the image of the object on the slit plate, electrical illumination
+of the graduated scale and wires of the micrometer
+eye-piece, and an automatic arrangement for keeping the
+prisms at minimum deviation.</p>
+
+<p class='c007'>There is a small totally-reflecting prism covering half of
+the slit, by which the light from an electric spark, or other
+source of luminosity, can be made to pass through the spectroscope
+so as to produce a series of known reference lines
+which serve as so many mile-posts for the measurement of the
+spectrum of the celestial body under observation. The induction
+coil, seen to the right of the diagram, is for the purpose
+of producing these electrical sparks.</p>
+
+<p class='c007'>In mounting the spectroscope, which weighs no less than
+200 pounds, the eye end of the great telescope tube is first
+supported by a prop, and the long rods are inserted. The
+<span class='pageno' id='Page_223'>223</span>spectroscope is then placed on the rods, and balancing weights
+equivalent to the weight of the spectroscope are removed from
+the lower part of the telescope tube.</p>
+
+<div id='i_221'  class='figcenter id003'>
+<img src='images/i_221.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 55.</span>—<i>The Spectroscope adapted to the Eye End of the Lick Telescope.</i></p>
+</div>
+</div>
+
+<p class='c007'><span class='sc'>The Objective Prism.</span>—It is a very remarkable fact that
+many of the recent advances in our knowledge of the spectra
+of stars have followed from the revival of a method first employed
+by Fraunhofer in 1814, in which the slit and collimating
+lens, forming part of an ordinary spectroscope, are
+dispensed with. The rays coming from a star being already
+parallel, and the star itself being a virtual slit without length,
+a large prism placed in front of the object-glass of a telescope
+makes a complete stellar spectroscope. A prism employed in
+this way is known as an <i>objective prism</i>.</p>
+
+<p class='c007'>In place of the image of a star, which would be seen in the
+absence of the prism, a spectrum without appreciable width
+appears at the focus of the telescope, and the spectrum lines
+will be represented by mere dots. To turn these dots into
+lines so that they may be better visible, a cylindrical lens
+must be employed in conjunction with the eye-piece.</p>
+
+<p class='c007'>It is to the application of photography, however, that we
+owe so much, and in this case the cylindrical lens is removed,
+while a small camera replaces the eye-piece of the telescope.
+In this form the instrument is often called a <i>prismatic
+camera</i>.</p>
+
+<p class='c007'>The prism is so arranged that the spectrum lies along the
+meridian passing through the star, and it is then only necessary
+to allow the driving clock to be slightly in error in order
+that the spectrum may trail a short distance perpendicular to
+its own length, and in this way broaden the photographed
+spectrum. On the proper regulation of the clock rate, and
+consequent “trail” of the spectrum across the plate parallel to
+itself, depends very largely the success of the photograph obtained.
+The spectrum of a bright star must obviously be
+made to travel more quickly than that of a fainter one, and a
+short exposure suffices. For the same clock rate, and in the
+same time, a star near the Pole will give a shorter trail than
+one nearer the Equator, and declination must therefore be
+<span class='pageno' id='Page_224'>224</span>taken into account in adjusting the clock error for this method
+of photography.</p>
+
+<p class='c007'>One great advantage of the objective prism in the photography
+of stellar spectra depends upon the fact that all the
+light passing through the object-glass is utilised in the production
+of the spectrum, whereas in an ordinary telespectroscope
+a large percentage of the light is lost in the jaws of the
+slit. The large focal length of the telescope also enables a
+long spectrum to be obtained even with a single prism of
+small angle.</p>
+
+<p class='c007'>When the dispersion is only small, the spectra of stars as
+faint as the tenth or eleventh magnitude can be photographed
+by this method, so that sometimes as many as 200 spectra are
+registered with a single exposure. Here, again, the objective
+prism has an immense advantage over the telespectroscope.</p>
+
+<p class='c007'>Professor Pickering, of Harvard College, was among the first
+to recognise the value of the objective prism for the photography
+of stellar spectra, and the munificent endowment of
+this research, by Mrs. Draper, as a memorial to Dr. Henry
+Draper, has enabled him to produce the Draper catalogue of
+stellar spectra, giving the chief characteristics of the spectra
+of over 10,000 stars.</p>
+
+<p class='c007'>Professor Norman Lockyer, at South Kensington, has also
+been conspicuously successful in this department of astrophysical
+research. The chief instrument he employs is a
+photographic telescope of only six inches aperture, with an
+objective prism of 45° refracting angle. The spectra thus obtained
+show hundreds of lines in such stars as Arcturus, with
+very fine definition, so that they bear almost unlimited
+enlargement.</p>
+
+<p class='c007'>An objective prism of twenty-four inches aperture will form
+one of the accessories of the fine telescope which is now being
+erected at the expense of Dr. Frank McClean, for the Cape
+Observatory, and there can be no doubt that the use of this
+gigantic prism will add greatly to our knowledge of the
+chemistry of the fainter stars.</p>
+
+<p class='c007'>As yet there is no very practicable method of employing
+<span class='pageno' id='Page_225'>225</span>the objective prism for determining the velocities of stars in the
+line of sight from the displacement of spectrum lines, and
+herein lies its one great disadvantage as compared with the
+telespectroscope. The difficulty is to ensure that the spectrum
+always falls absolutely in the same position with respect to the
+terrestrial spectrum, which must be photographed alongside
+for purposes of measurements. It is true that the spectrum
+of an approaching star is somewhat shorter, and of a receding
+star slightly longer than that of one at rest relatively to the
+observer, but these changes are so small as to little more than
+indicate the direction of movement even when a large instrument
+is employed.</p>
+
+<p class='c007'>Under the direction of Professor Norman Lockyer, the objective
+prism was very successfully used for photographing the
+spectra of the solar surroundings during the total eclipses of
+1893 and 1896. In place of the picture of the solar corona,
+which would appear in the absence of the prism, the prismatic
+camera shows a spectrum consisting of bright rings. If, for
+instance, the corona were wholly composed of hydrogen,
+there would be a picture of it in red, blue-green, blue, and
+violet, corresponding to the lines ordinarily seen in the
+spectrum of that gas. These rings thus indicate the chemical
+nature of the corona, and at the same time show, by their
+differing forms, the distribution of different gases throughout
+its extent. The spectra of the solar prominences and
+chromosphere are also depicted during the brief time of their
+visibility, during an eclipse, with such distinctness that a
+series of “snap shots” is all that is required to give a lasting
+record.</p>
+
+<p class='c007'><span class='sc'>The Spectroheliograph.</span>—A special form of spectroscope—called
+the <i>spectroheliograph</i>—has been devised by
+Prof. Hale, of Chicago, for photographing the sun in
+monochromatic light. It consists of a spectroscope, arranged
+for photography, in which the slit can be made to travel by
+clock-work across the sun’s image, which is projected upon it
+by the telescope to which the instrument is attached. In
+front of the photographic plate there is a secondary slit, so
+<span class='pageno' id='Page_226'>226</span>that only a very restricted part of the spectrum reaches the
+sensitive film. The secondary slit is connected by mechanism
+with the primary one, so that as the latter traverses the sun’s
+image, the former exposes different parts of the photographic
+plate to the light which passes through it, and in this way
+builds up an image of the sun in monochromatic light, matters
+being so arranged that light of the same wave-length always
+falls upon the secondary slit. By utilising the brightest lines
+which appear in the spectrum of the solar prominences,
+monochromatic images of those interesting appendages to
+our luminary have been successfully photographed without
+waiting for a total solar eclipse.</p>
+
+<p class='c007'><span class='sc'>The Bolometer.</span>—Besides the luminous effects of the
+spectrum, there are heating effects which can be measured by
+the <i>bolometer</i>, an instrument invented by Prof. Langley. A
+very thin strip of metal is connected with a delicate galvanometer,
+and is arranged so that it can be passed a long the
+whole spectrum. The electrical resistance of the strip varies
+according to its temperature, and the galvanometer at once
+signals any fluctuations which may occur. If, for instance,
+the strip comes to the place occupied by a dark line, there
+will be a notable fall of temperature. In this way, the
+bolometer is used to map the solar spectrum in the “infra-red”
+region—a part of the spectrum invisible to the eye, and
+of which we should otherwise have remained in ignorance.</p>
+
+<div><span class='pageno' id='Page_227'>227</span></div>
+<div class='chapter ph1'>
+
+<div class='nf-center-c0'>
+<div class='nf-center c001'>
+    <div>ASTRONOMY</div>
+  </div>
+</div>
+
+</div>
+
+<div id='i_228'  class='figcenter id003'>
+<span class='pageno' id='Page_228'>228</span>
+<img src='images/i_228.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p>DONATI’S COMET, OCTOBER 9, 1858. (<span class='sc'>From Langley’s “New Astronomy”.</span>)</p>
+</div>
+</div>
+
+<div class='chapter'>
+  <span class='pageno' id='Page_229'>229</span>
+  <h2 class='c006'>SECTION III.—THE SOLAR SYSTEM.</h2>
+</div>
+
+<div class='nf-center-c0'>
+<div class='nf-center c002'>
+    <div><span class='large'><span class='sc'>By Agnes M. Clerke.</span></span></div>
+  </div>
+</div>
+
+<h3 class='c013'>CHAPTER I.<br> <span class='c014'>THE SOLAR SYSTEM AS A WHOLE.</span></h3>
+
+<p class='c003'>The solar system consists of one supereminent body, with a
+train of miscellaneous attendants. By its immense gravitative
+power, their movements are so governed that they not only
+revolve round it as a common centre, but accompany its
+march through space; they are, in various degrees, warmed
+and enlightened by its copious emissions of heat and light;
+they are linked with it by origin and destiny. Some, indeed,
+much more closely than others. Planets, satellites, and
+asteroids belong to the immediate family of the sun;
+periodical comets and revolving meteoric rings have been
+adopted into it. The planets are eight in number; the six
+nearest the sun—Mercury, Venus, the Earth, Mars, Jupiter,
+and Saturn—have been known immemorially; Uranus and
+Neptune were discovered respectively in 1781 and 1846.
+Mercury, Venus, and Mars form, with the Earth, a group of
+“terrestrial planets,” so-called because they differ not very
+greatly in scale from our globe, and are constructed on nearly
+the same lines. The outer quartette of planets are giants by
+comparison, and show obvious symptoms of being in a very
+different physical condition. And it is noteworthy that the
+zone of asteroids, lying between Mars and Jupiter, divides the
+planetary classes.</p>
+
+<p class='c007'><span class='pageno' id='Page_230'>230</span>The asteroids are sometimes designated minor planets;
+but the former term is preferable, as accentuating their
+distinctive character. For they are not simply diminutive
+planets. A planet revolves in solitary state within its own
+broad domain. The asteroids traverse intercrossing and
+entangled paths, indefinitely numerous, ranging widely in
+celestial latitude, and covering with their network nearly the
+entire chasm of space between Mars and Jupiter. The small
+bodies moving in them have doubtless been formed in a
+manner totally different from that by which the single
+body they seem to replace would have taken shape.</p>
+
+<p class='c007'>Satellites bear in many respects the same relation to planets
+that planets bear to the sun. They are united with them
+into secondary systems, one of which is particularly well
+known to us, since it is constituted by the earth and the
+moon. The existence of twenty-one satellites has been
+ascertained, and many more possibly remain to be detected.
+Their apportionment is singularly unequal. Only three of
+the twenty one belong to the four small interior planets, while
+eighteen are attached to the four exterior giants. Moreover,
+both Mercury and Venus are solitary; so that the solar
+neighbourhood appears to be a region unpropitious to the
+development of subordinate systems.</p>
+
+<p class='c007'>Seventeen comets certainly, and many more probably, are
+domiciled in the solar kingdom. And even these preserve
+traces of an alien origin. They revolve round the sun in
+closed orbits, and are hence periodical in their apparitions;
+but their periodicity has to be qualified by a saving clause.
+They come up to time <i>barring accidents</i>. For their orbits, not
+being adjusted to stability, are liable to violent changes
+through the influence of the powerful masses, the tracks of
+which they intersect. In running up to, or back from perihelion,
+comets have to cross many railroads, so to speak, and
+do not always escape disturbing or destructive encounters
+with passing trains. Thus, many are entered in our astronomical
+visitor’s book as lost or strayed. Halley’s is the
+only well-secured cometary prisoner of the sun of imposing
+<span class='pageno' id='Page_231'>231</span>magnitude; the rest are of little spectacular, although of
+very high theoretic, interest. Comets are the only self-luminous
+members of the solar system.</p>
+
+<p class='c007'>Meteorites, besides being intrinsically obscure, reflect, owing
+to their minuteness, so little sunlight that they remain invisible
+until ignited in our atmosphere. They travel round the sun
+in annular systems, each mote-like component of which
+pursues its way, independently of the others, under the strict
+regimen of gravitational law. The number of these meteoric
+rings must be prodigious. Some hundreds have been brought
+to our acquaintance, which can only include such as cut the
+earth’s orbit; and these must be an insignificant fraction of
+the whole. The innumerable closely-related orbits grouped
+into each ring are ill-regulated for the safety of the bodies
+moving in them, since they conform in no way to the rules of
+planetary circulation. Hence the numerous encounters with
+the earth announced by the luminous trails of shooting stars.</p>
+
+<p class='c007'>Our system, as at present known, is 5,585 millions of miles
+in diameter. It is limited by the orbit of Neptune. But no
+less than three trans-Neptunian planets have been, on some
+show of evidence, alleged to exist. One of them, held by
+Professor Todd of Amherst College, U.S., to be responsible
+for some outstanding perturbations of Uranus, was placed by
+him in 1877 at a distance from the sun fifty-two times that of
+the earth (the radius of Neptune’s orbit being measured by
+thirty of the same units); the two others, called into existence
+by Professor Forbes of Edinburgh in 1880, to account for the
+formation of two groups of comets with aphelia respectively
+at one hundred, and three hundred astronomical units, were
+believed to occupy those enormously remote positions. Although
+none of the three, in spite of telescopic and photographic
+search, has yet been found, the possibility is not excluded
+that the appearance on a long-exposed sensitive plate
+of a line in lieu of a dot as the representative of a seeming
+star, may in the future announce the annexation by the sun
+of a further immense slice of territory out in the depths of
+<span class='pageno' id='Page_232'>232</span>space. The boundaries of our system are thus only provisionally
+fixed.</p>
+
+<p class='c007'>Intra-Mercurian planets have proved equally recalcitrant to
+prediction; and it may safely be said that no globe of the
+superficial dimensions of an English county lies concealed in
+the comparatively narrow space available for its circulation.
+The necessity for the presence of “Vulcan” was deduced by
+Leverrier from an unexplained displacement of Mercury’s
+perihelion, and a transit of the required body, supposed to have
+been observed March 26, 1859, was thereupon, in all good
+faith, brought forward by Dr. Lescarbault of Orgères. Another
+pseudo-discovery—this time of a pair of Vulcans—was
+made during the total eclipse of July 29, 1878; but neither
+on nor off the sun has the body needed to satisfy the French
+mathematician’s theory been genuinely seen, and few believe
+that it will ever be forthcoming.</p>
+
+<p class='c007'>Professor Titius of Wittenberg pointed out in 1772 that the
+relative distances of the planets from the sun could be expressed
+by adding 4 to the series 0, 3, 6, 12, 24, 48, etc. Thus,
+if the distance of Mercury were called 4, those of Venus, the
+Earth, Mars, and so on, would severally be 7, 10, 16. The
+validity of this relation—known as “Bode’s Law”—was
+strengthened by the conformity to it of Uranus and Ceres,
+neither of which had been discovered when it was enunciated;
+Neptune, however, proved to be much nearer to
+the sun than he should have been, and the formula hence
+ranks as an empirical one, not grounded in the nature of
+things.</p>
+
+<p class='c007'>Yet the grand outlines of the solar system are traced on a
+visibly symmetrical plan. The larger bodies composing it
+move nearly in the same plane, in orbits nearly circular, and
+at regulated intervals, augmenting rapidly outward. All revolve
+from west to east, or “counter clockwise,” and this
+fundamental current of motion carries with it, besides the asteroids,
+all the periodical comets, save Halley’s. Among
+secondary systems only the Uranian and Neptunian escape
+from its sway; there being a visible tendency towards deviations
+<span class='pageno' id='Page_233'>233</span>from rule towards the confines of the solar domain.
+These deviations, however, are not of a subversive character.</p>
+
+<p class='c007'>The planetary machine may continue working forever without
+a hitch. Such irregularities as would be likely to throw
+it out of gear are found only in parts of almost evanescent
+mass and negligeable influence. Two modes of action which
+should, in the long run, bring about a collapse, are non-existent
+or insensible. These destructive agencies are a resisting
+medium, and the progressive transmission of gravity. The
+presence of either should prove fatal in the same ultimate
+fashion. Along slowly narrowing tracks, the planets would
+descend, one after the other, into the ample lap of the sun.
+Their circulation is, however, to the best of our present knowledge,
+unimpeded and undeflected; the disturbances affecting
+it are self-compensatory.</p>
+
+<p class='c007'>But while the mechanical stability of the system is assured,
+its physical state is continually changing. And the change
+is always in the same direction. A degradation of energy
+steadily progresses. The sun is, in fact, spending his capital,
+and even with a millionaire of his stamp this cannot last. The
+time must come, if science is to be believed, when his radiative
+powers will have become exhausted. Five millions of years
+hence they will, in all probability, be much less efficacious than
+they are now. Within twice or thrice that interval they may
+have become almost extinct.</p>
+
+<p class='c007'>Planetary globes, too, grow old through the wasting of their
+internal heat. The moon seems in a measure to prefigure the
+future condition of all, should their decay not be arrested.
+Possibly the lunar stage is not the last. Death may, in the long
+ages to come, be succeeded by disintegration, when a ring of
+rubbish will be substituted for our “wan-faced” companion.
+To what purpose, then, our readers will ask, the mechanical
+perfections of a system destined eventually to be involved in
+darkness and destruction? To what purpose its exquisite
+balance, the nicely-adjusted relations of its members, its self-righting
+faculty, its compensatory springs? We can reply only
+by recalling that the extreme conclusions of science are invariably
+<span class='pageno' id='Page_234'>234</span>pessimistic, because they are reached without taking any
+account of the intelligent control perpetually, though insensibly,
+overruling the workings of blind forces. If, in one
+sense, heaven and earth pass away, we still know that, in good
+time, “a new heaven and a new earth” shall inscrutably arise.
+Not “faintly,” then, but boldly and ardently, we “trust the
+larger hope” that renovation will succeed, or anticipate
+subversion.</p>
+
+<p class='c007'>Whatever <i>can</i> have an end <i>must</i> have had a beginning, and
+the origins of things have an especial fascination for our
+minds. As regards the history of the planetary world, we are
+not altogether in the dark. The problem of the maintenance
+of the sun’s heat was satisfactorily solved by Helmholtz in
+1854. Its radiative supplies, as he showed all but conclusively,
+are derived from gravitative power. As they are
+diffused into space, the cooled particles from which they proceed,
+clash together, and their arrested motion is converted
+into a fresh thermal stock. This implies a steady diminution,
+although to a surprisingly slight extent, in the bulk of the
+solar globe. It has been computed that a shortening of the
+sun’s diameter by 380 feet yearly would suffice to keep this
+grand heat-producing machine in full working order; and at
+least ten thousand years should elapse before the contraction
+became measurable by any instrumental means at our command.
+Its progress should, nevertheless, eventually reduce
+our glowing luminary to an obscure, inert mass.</p>
+
+<p class='c007'>Now, evidently, its shining in the past was sustained in the
+same way as at present. The globe that blazes in our
+summer skies is, accordingly, but the shrunken remnant of
+what it once was. It is shrunken in proportion to the vast
+quantity of its former emissions. Hence, the farther we
+look back into the ages, the more voluminous its dimensions.
+And, sounding the utmost profundities of time, we arrive at
+an epoch when all the planets were swallowed up in a sphere
+girdled by the present orbit of Neptune.</p>
+
+<p class='c007'>The tenuity of this distended body was unimaginable. At
+ninety miles of altitude, our air is one hundred million times
+<span class='pageno' id='Page_235'>235</span>rarer than it is at sea-level; yet the primitive solar “nebula”
+was considerably more attenuated still. This aerial mass had,
+doubtless, been in some way impressed with a slow movement
+of rotation, which, by mechanical necessity, quickened as condensation
+progressed. The planets represent a few fragments
+detached during the process; nearly the whole of its substance
+being compacted into the sun. How the fragments came to
+be detached is the crux of cosmogonists. According to
+Laplace’s famous hypothesis, equatorial rings of matter
+separated successively from the parent nebula at certain
+critical epochs when gravity was overcome by the gaining
+centrifugal tendency due to accelerating rotation. These
+rings drew together into planets, from which satellites
+were generated by a repetition of their own birth-process.
+Many incongruities are, however, involved in this <i>modus
+operandi</i>. Only two need here be mentioned. Reason and
+experience teach us that globes of small interior consistence
+easily break up into rings, while cosmic rings show not the
+slightest tendency to collect into globes. Again, Laplace
+supposed that the production of each planet relieved a long
+antecedent strain. But nebulous stuff is almost absolutely
+incoherent. Hence it <i>cannot be stretched or strained</i>. As the
+nebula condensed and whirled, it would, accordingly, have left
+behind innumerable disaggregated particles, but no massive
+rings.</p>
+
+<p class='c007'>M. Faye of the French Academy has attempted to remedy
+these defects. The planets, he considers, were not abandoned,
+but formed at centres of condensation within the nebular
+matrix. The order of their formation would thus have been
+quite different from that assigned by Laplace, in whose theory
+the exterior globes were necessarily the earliest to take shape.
+M. Faye, on the contrary, argues Uranus and Neptune, from
+their retrograde rotation, to be the <i>youngest</i> instead of the
+<i>oldest</i> members of the solar system, while the terrestrial group
+belong to the first era of planetary development.</p>
+
+<p class='c007'>Astronomers are now virtually agreed that “The world was
+once a fluid haze of light,” but by what precise means, in what
+<span class='pageno' id='Page_236'>236</span>succession, under what compulsion, its constituent bodies
+were set wheeling in the void, they are less ready to pronounce
+than were their predecessors, who, dazzled with the
+analytical triumphs of the eighteenth century, accepted unquestioningly
+the plan of creation it complacently transmitted
+to them. The complexities of world-making have, besides,
+been instructively illustrated by Professor G. H. Darwin’s discovery
+that tidal friction was essentially concerned in the process.
+By an able mathematical investigation, he showed, in
+1879, that it was particularly effective in modelling the
+earth-moon system, owing to the fact that our satellite,
+comparatively to its primary, is by far the largest in the
+solar system.</p>
+
+<p class='c007'>Tidal friction may be regarded under a two-fold aspect.
+Its effect in grinding down the speed of rotation has been explained
+in Section II. (page <a href='#Page_166'>166</a>). The energy, however, thus
+apparently destroyed is only transformed. The rotational momentum
+subtracted from the earth is added to the orbital
+momentum of the moon, which thus travels (setting aside other
+causes of change) along continually widening spires. This
+retreat from the earth is even now going on, although with
+elusive slowness, amid the rise and fall of secular change. Its
+effects in past ages, nevertheless, coupled with those due to
+the slackening of rotation by the friction of the tidal wave—the
+two forming, as it were, the obverse and reverse of
+one medal—must have been of overruling importance. Laying
+hold of the clue they offer, Professor Darwin succeeded in
+tracing back the history of the moon through a “corridor of
+time” nearly a hundred million years long. It was then
+spinning at a vertiginous rate, round, and nearly in contact
+with the earth, which must have been fluid or plastic, while of
+about its present size. The <i>month</i> of that epoch was three or
+four hours in duration; the <i>day</i> was shorter still. The actual
+existence of the moon convinces us of this latter fact. Otherwise,
+the huge tidal wave raised by the moon upon the earth
+should have lagged, however slightly. Its attraction would
+have pulled the moon backwards at the decisive moment of
+<span class='pageno' id='Page_237'>237</span>its emergence into separate being, and led infallibly to its
+re-engulfment.</p>
+
+<p class='c007'>The origin of the moon has been, by Professor Darwin’s
+analysis, made clearer than that of any other heavenly body.
+Certainty regarding such remote events is unattainable; but
+it is highly probable that our globe, at a late stage of its development,
+gave birth, amid the throes of disruption, to its
+solitary offspring. But the case is unique. The terrestrial
+system presents conditions not repeated elsewhere. Generalisations
+founded upon them are sure to be misleading. We
+have indeed gained, from all recent inquiries into cosmogony,
+the profound conviction that no single scheme will account
+for everything; that the utmost variety prevailed in the circumstances
+under which the heavenly bodies attained their
+present status; and that a rigidly constructed hypothesis can
+only misrepresent the boundless diversity of nature.</p>
+
+<h3 class='c004'>CHAPTER II.<br> <span class='c014'>THE SUN.</span></h3>
+
+<p class='c003'>The sun is an immense reservoir of radiant energy. For our
+daily uses we have no other store worth mentioning to draw
+upon, our fuel being the embalmed sun-heat of former ages;
+and all the physical and vital operations carried on over the
+whole globe derive their motive power from the same copious
+source. Yet only 1/2,128,000,000th part of the sum total of solar
+radiations strike its comparatively diminutive surface; while
+all the planets combined intercept no more than 1/234,000,000th
+of that inconceivable effluence.</p>
+
+<p class='c007'>The sun gives as much light as 600,000 full moons, or two
+and a half billions of the most powerful electric lights, or as
+1,575 billions of billions of standard candles. And since his
+<span class='pageno' id='Page_238'>238</span>disc is the projection of a hemisphere, and is thus equivalent
+only to one-fourth the globular surface, these vast numbers
+must be quadrupled to represent the whole luminous emissions
+of this surpassing body. Their amazing profusion is the combined
+result of immensity of shining area, and vivid intrinsic
+brilliancy. Each square inch of the sun’s surface has been
+estimated to integrate the lustre of twenty-five electric arcs,<a id='r6'></a><a href='#f6' class='c027'><sup>[6]</sup></a>
+and Professor Langley, by direct experiment, proved it to be
+5,300 times brighter, and 87 times hotter, area for area, than
+the white-hot “pour” from a Bessemer converter; notwithstanding
+that the circumstances of the comparison were exceedingly
+“unfair to the sun.”<a id='r7'></a><a href='#f7' class='c027'><sup>[7]</sup></a></p>
+
+<p class='c007'>Radiant heat and light do not indeed differ in themselves,
+but only in their effects. The sun sends out into space ethereal
+waves of various lengths, but all of the same kind, subject to
+the same laws, and travelling with the same velocity of
+186,000 miles a second. They appear, however, under diverse
+forms of energy according to the qualities of the substances
+upon which they impinge. Thus a small section of this long
+range of undulations affects our eyes as light, the human
+retina being so fashioned as to be able to <i>see</i> with their help.
+There is nothing in the nature of the rays themselves to make
+them visible, and it is in fact more than probable that other
+living creatures perceive vibrations to which we are blind.
+Our eyes are sensitive over nearly two octaves; from waves
+measuring about 760 millionths of a millimetre, to those of
+less than 400 millionths. In the solar spectrum the limits are
+roughly marked at one end by a great dark band in the deep
+red—Fraunhofer’s “A,”—and at the other by “H,” in the extreme
+violet. Beyond H extend undulations so short as to
+be visually imperceptible, while photographically active. This
+means that certain salts of silver are capable of taking up the
+energy they bring from the sun, and of using it to break their
+chemical bonds; while on differently prepared plates similar
+effects can be produced by rays in all parts of the spectrum,
+<span class='pageno' id='Page_239'>239</span>even in the ultra-red, where the undulations, too long to be
+sensible as light, are mainly felt as heat. Here, as Professor
+Langley has shown by “bolometric”<a id='r8'></a><a href='#f8' class='c027'><sup>[8]</sup></a> explorations, reside three-fourths
+of the energy distributed throughout the solar spectrum;
+nor is it impossible that this great stretch of heat waves
+may merge, without interruption, into electrical <i>rollers</i>,
+measured, not by millionths of a millimetre, but by metres,
+or even by kilometres. The important point to be borne in
+mind, however, is that the solar energy is diffused abroad by
+means of ethereal vibrations of a single type, but immensely
+varied size and frequency, and hence susceptible of dispersion
+into a spectrum.</p>
+
+<p class='c007'>The “solar constant” expresses the quantity of heat
+received by the earth from the sun. Its value, according to
+the most trustworthy determinations, is three calories per
+square centimetre per minute. This means that a vertical
+sun pours down upon each square centimetre of the globe
+heat enough (supposing the atmosphere out of the way) to
+raise the temperature of three grams of water by one degree
+centigrade in a minute. Putting it otherwise, the energy
+imparted would suffice to keep an engine of three-horse power
+continually at work on every square yard of the terrestrial
+surface. Or, if the heat were distributed uniformly in all
+latitudes, it would annually melt a complete ice-jacket one
+hundred and seventy feet thick.</p>
+
+<p class='c007'>The temperature of the body lavishing heat at this
+tremendous rate must obviously be very high; but enquiries
+on the point are necessarily limited to the actual emitting
+shell, or “photosphere.” Their success is testified to by a
+noteworthy reduction of late in the range of uncertainty.
+The difficulty attending them consists mainly in our ignorance
+of any systematic relation between temperature and
+radiation. Excessively hot bodies lose heat much more
+rapidly, under the same conditions, than moderately hot ones;
+and empirical “laws of radiation” have been, over and over
+<span class='pageno' id='Page_240'>240</span>again, arrived at as the upshot of long series of laboratory
+experiments. But such laws are only too apt to turn traitors
+if trusted without control; and since the thermal power of
+the sun vastly exceeds that of any terrestrial source, they are
+precarious guides in this particular research. Nevertheless,
+as the outcome of various improvements and refinements, it
+has, within the last few years, been prosecuted with excellent
+results. That obtained in 1894 by Messrs. Wilson and Gray
+deserves particular confidence. The <i>effective</i> temperature of
+the sun was by them fixed at 8,000°, or allowing for absorption
+in the solar atmosphere (measured by Wilson and Rambaud),
+at 8,800° centigrade. This estimate, which makes the sun’s
+surface more than twice as hot as the carbons of the electric
+arc, is unlikely to be widely erroneous. The word “effective”
+signifies the condition that the photosphere is equivalent in
+radiative power to a stratum of lampblack; if it fall short of
+this standard, as appears probable, then the temperature must
+be raised by a corresponding amount.</p>
+
+<p class='c007'>The solar atmosphere, of which the absorptive effects have
+just been alluded to, is a shallow envelope, stopping predominantly
+the shorter wave-lengths of the light transmitted
+through it. Hence, if it were removed, the sun would
+appear, not only much brighter, but also much <i>bluer</i> than it
+does at present. The general darkening of the limb due to
+its action is apparent to visual, and conspicuous in photographic,
+observations. By its aid, “faculæ”—brilliant and
+elevated portions of the photosphere—were early detected.
+Invisible on or near the middle of the disc, they stand out in
+relief against its dusky edges as they are brought round, and
+carried off again by the sun’s rotation.</p>
+
+<p class='c007'>The magnitude of this astonishing luminary fairly baffles
+our conceptions. Its mass is 745 times that of all the planets
+taken together. Its volume is such, that if Jupiter were
+located centrally within it, two of his Galilean moons, besides
+the lately discovered inner satellite, would have “ample room
+and verge enough” to revolve round him, keeping well inside
+the photosphere. The entire Uranian system could be easily
+<span class='pageno' id='Page_241'>241</span>accommodated in the same way; while Neptune and his
+satellite, and the earth and moon, could very nearly perform
+their evolutions side by side in the sun’s excavated
+interior.</p>
+
+<p class='c007'>The sun is 865,000 miles in diameter, and in figure is
+sensibly spherical. Its surface is 12,000 times, its volume
+1,300,000 times that of the earth. In mass it is equal to
+332,000 earths. Its mean density, then, is only one-quarter
+that of the earth, or 1·4 times that of water. In other words,
+the terrestrial globe, if equally bulky, would contain four
+times the quantity of matter contained in the solar globe.
+Yet we know that it is largely made up of iron and still
+heavier metals; while gravity at its surface is 27·6 more
+powerful than it is here. Thus, the sun’s materials are
+weighed down by an inconceivable pressure, and would be
+of a density utterly transcending our experience but for the
+counteracting agency of heat. The comparative insubstantiality
+of such a globe gives us some faint notion of the
+violent molecular agitation affecting every particle of its mass.
+Contrasted with the fires raging within, the surface temperature
+of 8,000° or 9,000° might perhaps be deemed
+moderate or cool. There is much evidence that it is throughout
+gaseous, although of a consistence approaching more
+nearly that of pitch or treacle than can easily be reconciled
+with established ideas as to the qualities proper to an aerial
+substance. Yet the laws governing the gaseous state are
+plainly those obeyed in the sun.</p>
+
+<p class='c007'>Its function, as a great thermal engine, is to produce and
+diffuse heat For these purposes it is essential that the
+interior stores should be brought rapidly to the surface; and
+this is accomplished, not, as in solids, by conduction, but by
+actual transport, or “convection.” Only the enormous elasticity
+of highly compressed gases could render this process
+swift enough to sustain the incessant outpourings of heat
+from the photosphere. It may be accompanied by an actual
+rise in temperature. If the sun be truly gaseous throughout,
+it <i>must</i> be so accompanied. The reason of this seeming
+<span class='pageno' id='Page_242'>242</span>anomaly is that a sphere of radiating and contracting gas
+develops by shrinkage more heat than it can dispose of by
+radiation. Whether or no the sun comes within the scope of
+this principle, known as “Lane’s Law,” cannot at present be
+decided. It is, in other words, an open question whether the
+sun is growing hotter or colder. Help towards answering it
+might have been expected from the study of geological
+climates; but their variations have evidently been due to a
+complexity of causes. At any rate, the sun’s decline, if the
+inevitable turning-point has already been reached, is going
+on with extreme slowness.</p>
+
+<p class='c007'>The visible structure of the photosphere, or lustrous
+envelope of the solar globe, is, in itself, suggestive of the
+vertical circulation by which the indispensable communications
+between its interior and exterior are kept up. It is
+composed of brilliant granules and dusky interstices, the
+former representing, it is supposed, the vividly incandescent
+summits of uprushing currents, the latter the cooled, descending
+return-flows. It may be safely described as the limiting
+surface of thermal interchange, and is often spoken of as a
+cloud-sphere, or level of condensation, where the ascending
+vapours, like mounting volumes of water-gas in our atmosphere,
+are chilled into liquid droplets. To the brilliant
+luminosity of these incandescent droplets, the blaze of the
+solar emissions is ascribed. Or the droplets might equally
+well be solid particles on the model of the ice-spicules
+collected to form the delicate fields of cirrus in our upper
+air. The cloud theory of the photosphere is, however,
+hampered by the difficulty of finding a substance capable of
+liquefying or solidifying at a temperature of 8,000° C. Carbon
+has generally been selected as the material of the solar
+“granules,” but carbon evaporates at about 4,000°, and although
+its boiling point might be raised by enormous pressure, there
+are no signs that the requisite conditions exist in the sun.
+Hence, some speculators turn towards electricity as the exciting
+agent of the photospheric radiance; but it would be
+waste of time to attempt, at present, to discuss the vague
+<span class='pageno' id='Page_243'>243</span>possibilities connected with an hypothesis which offers no
+holding ground for distinct reasoning.</p>
+
+<div id='i_243'  class='figcenter id003'>
+<img src='images/i_243.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 1.</span>—<i>Photograph of a Sun-spot.</i> (From <cite>Knowledge</cite>, February, 1890.)</p>
+</div>
+</div>
+
+<p class='c007'>The photospheric texture is often rent and perforated. This
+ragged condition (well exemplified in Fig. 1 from a photograph
+taken by Dr. Janssen at Meudon) is accompanied or
+caused by a violent disturbance of the sun’s bodily circulation.
+A typical sun-spot consists of a dark opening, or “umbra,”
+within which a still darker “nucleus” can often be discerned.
+The umbra is garnished all round with a semi-luminous
+“penumbra,” composed of elongated shining bodies placed
+side by side, and all, when undisturbed, pointing radially inwards
+towards the centre of the spot. The effect has been
+compared to that of “straw-thatching,” although the solar
+“straws” are, at times, thrown somewhat wildly about.
+Where they hang over the <i>eaves</i> of the spot they are always
+brightest, because set most closely together. The penumbra
+<span class='pageno' id='Page_244'>244</span>may be called a modified extension of the ordinary mottled
+surface of the photosphere, the lustrous grains being drawn
+out into filaments, the “pores” into obscure interspaces.</p>
+
+<p class='c007'>Spots commonly occur in groups (as in our Figure) belonging
+to a single area of disturbance marked by the brightening,
+and probably by an elevation of the photosphere. The
+members of such families show curious and unexplained
+mutual relations. The size of these extraordinary formations
+is on the gigantic scale of all solar phenomena. They are
+often visible, individually or collectively, to the naked eye,
+and attracted notice accordingly in pre-telescopic times. In
+1858, a spot opened to the extent of 144,000 miles, so that
+sixteen earths, side by side, might have been engulfed in it.
+A still more remarkable outbreak took place in February,
+1892. Three thousand three hundred and sixty million square
+miles of the photosphere were riddled as if by some tremendous
+bombardment, the extreme dimensions of the affected
+district being 150,000 by 75,000 miles. This spot, the
+largest ever photographed at Greenwich, attained its acme on
+February 13th, when a magnetic storm and widely diffused
+auroral display attested the sympathy of the earth with commotions
+in the sun. Five times brought back to view by the
+sun’s rotation, its history was followed from November until
+March; but this duration is not an extreme case, a spot having
+been known to survive throughout eighteen rotations. Although
+the group of February, 1892, covered ¹⁄₇₀₀th of the
+sun’s entire surface, its proportions were outdone by those
+of a spot and its immediate attendants, without counting outliers,
+measured by Sir John Herschel at the Cape, March 29th,
+1837.</p>
+
+<p class='c007'>Spots are always associated with faculæ. The two are
+correlated phenomena. There is no certainty as to their
+order of precedence, if any fixed order there be, but faculæ
+both survive spots and develop apart from them. Not infrequently
+the faculæ garlanding a spot throw a “bridge”
+right across it (see Fig. 1). In stereoscopic views these
+<span class='pageno' id='Page_245'>245</span>brilliant projections show as veritable <i>suspension bridges</i>.
+They float almost palpably at a high altitude above the black
+gulf they span.</p>
+
+<p class='c007'>The distribution of spots is easily perceived to depend
+immediately upon the sun’s rotation. Two zones of its surface,
+parallel to the solar equator, are alone infested by them.
+These may be defined as lying between 6° and 35° of north
+and south latitude; but the prohibition of spot-development is
+much more absolute in the polar than in the equatorial
+direction. One solitary macula has been observed in 50°
+north latitude.</p>
+
+<p class='c007'>The periodicity of sun-spots was first recognised by Schwabe
+at Dessau in 1851. Since abundantly confirmed, it constitutes
+one of the fundamental data of solar physics. Once in
+about eleven years a “maximum” is attained; for months
+together the photosphere is never calm and unbroken; its
+agitated condition betrays the turmoil of the interior. The
+superabundance of spots is succeeded, after some years,
+by a scarcity, or “minimum,” when the perturbing agencies
+appear to have sunk into repose, preparatory to another outburst
+of activity. In this highly irregular, although well-marked,
+cycle, the ascent is almost always much more rapid
+than the descent; the upspringing of the disturbance occupies,
+as a rule, not much more than half the time allotted to its quieting
+down. Nor is its intensity by any means uniform. High and
+low maxima alternate with, or succeed each other, with no
+obvious regularity. Sometimes we have a divided or double
+maximum, as in 1882–4, followed by an unusually swift ebb of
+agitation. The minimum of 1889 was premature and brief;
+for spots were again numerous in 1891, and developed prodigiously
+throughout the years 1892 and 1893. Only in
+January, 1894, a slight falling off became apparent, and the
+tranquillity which set in with 1895 may very probably reign
+with only temporary interruption for some time. The
+cause of these vicissitudes is completely unknown; but they
+so closely resemble, in character, the changes of variable stars,
+that it seems impossible to exclude the sun from that category,
+<span class='pageno' id='Page_246'>246</span>spot-maxima corresponding with stellar light-maxima
+and <i>vice versâ</i>.</p>
+
+<div id='i_246'  class='figcenter id003'>
+<img src='images/i_246.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 2.</span>—<i>Sun-spots and Magnetic Variations.</i> (From Langley’s “New Astronomy.”)</p>
+</div>
+</div>
+
+<p class='c007'>Solar disturbances, however originating, are a sort of universal
+pulse-beat, with
+which the earth, and
+doubtless every other
+member of the solar
+cortège, throb in unison.
+The accompanying diagram
+(Fig. 2) shows how
+closely the magnetic
+needle sympathises with
+the variations in the
+state of the sun. The
+amplitude of its daily
+oscillations is represented
+by the dotted
+curve, while the smooth
+curve is constructed from
+the relative numbers of
+spots. The striking
+conformity in point of
+time-development, between
+two effects so disparate
+in their nature,
+extends to minute details.
+Violent commotions
+on the sun seldom
+fail to be reflected in
+magnetic storms and
+auroral manifestations
+on the earth; and exact
+correspondences have
+sometimes been observed;
+yet it does not
+seem possible to trace these simultaneous effects to the
+immediate magnetic action of the sun.</p>
+
+<p class='c007'><span class='pageno' id='Page_247'>247</span>No meteorological cycle corresponding with the spot-cycle
+has yet been satisfactorily made out. The direct diminution
+of heat and light through the obscuration of a small part of
+the sun’s photosphere amounts, at the utmost, to ¹⁄₁₀₀₀th of the
+whole. The spots are far from being totally dark or cool.
+Their blackest nuclei are really no less brilliant than limelight;
+while about half as much heat is derived from them as
+from the surrounding disc when they are centrally situated,
+and 80 per cent. when they are near the limb.<a id='r9'></a><a href='#f9' class='c027'><sup>[9]</sup></a> Their dimming
+and cooling effects then are insignificant; they are probably
+more than compensated by the quickening of the sun’s circulatory
+processes, and consequent increase of emission,
+through the disturbance of internal equilibrium of which outbreaks
+of spots are among the consequences.</p>
+
+<p class='c007'>The spot-zones do not always occupy the same positions.
+They shift with the progress of the eleven-year cycle. This
+curious circumstance, discovered by R. C. Carrington in 1856,
+illustrates, in his words, “the regular irregularity, and irregular
+regularity,” distinguishing solar periodicity. At maxima, the
+mean latitude of the zones in question is about 16°; but they
+close down towards the equator as each wave of agitation
+dies out, its few latest products appearing in quite low latitudes.
+Then, when minimum is passed, a fresh start is made
+with the opening of a few small spots in 30° or 35° north or
+south latitude; and this newly-organised disturbance begins
+to descend as before, gaining strength as it proceeds. Thus,
+each impulse acts independently of the succeeding one.</p>
+
+<p class='c007'>The most cursory observation of sun-spots suffices to show
+that the shining body marked by them rotates on an axis
+from west to east, in the same direction as the planetary
+revolutions. True, they emerge to sight on its eastern, and
+vanish at its western limb; but this is because we are located
+at its <i>backside</i>, and see their courses inverted. Attempts,
+however, to fix the sun’s period of rotation were long baffled;
+for the spots, instead of being carried round as if attached to
+a rigid surface, gave signs of possessing “proper motions” of
+<span class='pageno' id='Page_248'>248</span>uncertain and inconstant amount. The subject was first
+thoroughly investigated by Carrington; and he reached the
+unexpected conclusion that the sun has no uniform period,
+but gyrates in a composite fashion, quickest at the equator,
+and gradually slower towards the poles. From less than
+twenty-five days, he found the time of circuit to lengthen
+steadily to twenty-seven and a half in 50° of latitude. The
+axis round which this remarkably conditioned movement is
+performed makes an angle of 7° 15′ with the pole of the
+ecliptic; it inclines towards the earth’s northern hemisphere
+from June to December, when the spots describe, in crossing
+the disc, paths curved downwards (to the eye of a northern
+observer); but the conditions being reversed between
+December and June, their paths are then curved upwards;
+while on June 3rd and December 5th, they pursue straight
+tracks, the earth being on those two days in the line of intersection
+between the sun’s equatorial plane and that of the
+ecliptic.</p>
+
+<p class='c007'>Only a rough approximation, however, to the laws of solar
+rotation can be derived from spots. For they do not simply
+drift with the photospheric currents, but are subject to accelerations
+and retardations connected with their internal economy,
+as well as to mutual attractions and repulsions depending, it
+is supposed, upon their electrical condition. Fortunately,
+however, a method has been perfected by which these complications
+are abolished. Something has already been said
+as to spectroscopic determinations of motion in the line of
+sight. They are evidently applicable to the sun’s axial
+movement. For, through its effect, his eastern limb is always
+advancing uniformly towards us, while the western limb is
+retreating at the same rate. Thus, the whole Fraunhofer
+spectrum is shifted slightly upward, or towards the blue, at
+the left-hand edge of the solar disc, and as much towards the
+red at the right-hand edge. The same lines of solar absorption,
+in fact, taken from opposite sides of the solar equator, and
+placed end to end, appear evidently notched, and can be
+distinguished at a glance from terrestrial absorption lines,
+<span class='pageno' id='Page_249'>249</span>which, having nothing to do with the sun’s rotation, show
+no break at the junction of their sections. They in this
+way “virtually map” themselves, as Professor Langley proved
+experimentally in 1877.</p>
+
+<p class='c007'>In 1887–9, M. Dunér, of Upsala, succeeded in extending
+these delicate measurements to within fifteen degrees of the
+sun’s poles, where the movement is so slow that it can only,
+by incredible refinements, be dealt with successfully. The
+upshot was to emphasise the law of slackening <i>angular</i> speed
+detected by Carrington and confirmed by Spoerer. From
+25½ days at the Equator, the sun’s period of rotation was
+found to become protracted to 38½ days at the seventy-fifth
+parallel of latitude. Its investigation from photographs of
+faculæ has been lately carried out by M. Stratonoff at Taschkent
+in Russia. The results of the three methods are collected
+in the following little table.<a id='r10'></a><a href='#f10' class='c027'><sup>[10]</sup></a></p>
+
+<table class='table1'>
+  <tr><th class='c008' colspan='4'>THE SUN’S ROTATION.</th></tr>
+  <tr>
+ <th class='btt bbt blt c024'>Mean Solar Latitude.</th>
+ <th class='btt bbt blt c024'>Period from Faculæ.<br>(Stratonoff.)</th>
+ <th class='btt bbt blt c024'>Period from Spots.<br>(Spoerer.)</th>
+ <th class='btt bbt blt brt c024'>Period from Spectroscopic Measures.<br>(Dunér.)</th>
+  </tr>
+  <tr>
+ <td class='blt c024'>0°</td>
+ <td class='blt c024'>24<sup>d</sup>·66</td>
+ <td class='blt c024'>25<sup>d</sup>·09</td>
+ <td class='blt brt c024'>25<sup>d</sup>·46</td>
+  </tr>
+  <tr>
+ <td class='blt c024'>15°</td>
+ <td class='blt c024'>25 ·26</td>
+ <td class='blt c024'>25 ·44</td>
+ <td class='blt brt c024'>26 ·35</td>
+  </tr>
+  <tr>
+ <td class='bbt blt c024'>30°</td>
+ <td class='bbt blt c024'>25 ·48</td>
+ <td class='bbt blt c024'>26 ·53</td>
+ <td class='bbt blt brt c024'>27 ·57</td>
+  </tr>
+</table>
+
+<p class='c007'>These facts, although so various, are not necessarily discordant.
+They apply to different parts of the great solar
+machine, each one of which may rotate with a certain independence.
+The spots drift, more or less passively, <i>with</i> the
+photosphere. The faculæ are elevated above it, and appear to
+be everywhere accelerated relatively to its systematic currents.
+The strata originating the Fraunhofer lines, to which alone
+the spectroscope is applied, display, on the contrary, effects of
+retardation. “This peculiar law of the sun’s rotation,” Professor
+Holden remarks, “shows conclusively that it is not a
+<span class='pageno' id='Page_250'>250</span>rigid body, in which case, every one of its layers in every
+latitude must necessarily rotate in the same time. It is more
+like a vast whirlpool where the velocities of rotation depend
+on the situation of the rotating masses, not only as to latitude,
+but also as to depth beneath the exterior surface.”</p>
+
+<p class='c007'>Solar chemistry progresses by successive interpretations;
+and the characters to be read are so multitudinous and so
+similar as to require very delicate discrimination. The work,
+carried on simultaneously in the sun and laboratory, becomes
+more arduous as it advances, and is still far from complete.
+Indeed, the difficulties attending detailed comparisons between
+the Fraunhofer lines and the innumerable components
+of terrestrial spectra, would be insuperable but for the aid
+of photography, here, as elsewhere, the versatile handmaiden
+of physical astronomy.</p>
+
+<p class='c007'>Here is a list of 36 solar elements published by Professor
+Rowland of Baltimore in 1891, and arranged according to the
+number of their representative lines in the solar spectrum.</p>
+
+<div class='lg-container-b'>
+  <div class='linegroup'>
+    <div class='group'>
+      <div class='line'>Iron (2000 +)</div>
+      <div class='line'>Nickel</div>
+      <div class='line'>Titanium</div>
+      <div class='line'>Manganese</div>
+      <div class='line'>Chromium</div>
+      <div class='line'>Cobalt</div>
+      <div class='line'>Carbon (200 +)</div>
+      <div class='line'>Vanadium</div>
+      <div class='line'>Zirconium</div>
+      <div class='line'>Cerium</div>
+      <div class='line'>Calcium (75 +)</div>
+      <div class='line'>Scandium</div>
+      <div class='line'>Neodymium</div>
+      <div class='line'>Lanthanum</div>
+      <div class='line'>Yttrium</div>
+      <div class='line'>Niobium</div>
+      <div class='line'>Molybdenum</div>
+      <div class='line'>Palladium</div>
+      <div class='line'>Magnesium (20 + )</div>
+      <div class='line'>Sodium (11 + )</div>
+      <div class='line'>Silicon</div>
+      <div class='line'>Hydrogen</div>
+      <div class='line'>Strontium</div>
+      <div class='line'>Barium</div>
+      <div class='line'>Aluminium (4)</div>
+      <div class='line'>Cadmium</div>
+      <div class='line'>Rhodium</div>
+      <div class='line'>Erbium</div>
+      <div class='line'>Zinc</div>
+      <div class='line'>Copper (2)</div>
+      <div class='line'>Silver (2)</div>
+      <div class='line'>Glucinium (2)</div>
+      <div class='line'>Germanium</div>
+      <div class='line'>Tin</div>
+      <div class='line'>Lead (1)</div>
+      <div class='line'>Potassium (1)</div>
+    </div>
+  </div>
+</div>
+
+<p class='c007'>Only two of these substances, carbon and silicon, are non-metallic,
+hydrogen ranking as a gaseous metal. Neither
+oxygen, nitrogen, nor argon, have yet spoken their “Adsum,”
+but it is not impossible that they may do so in the future.
+Negative evidence, at any rate, is, in spectroscopic inquiries,
+absolutely inconclusive.</p>
+
+<p class='c007'>The spectra of sun-spots are, as might have been expected,
+<span class='pageno' id='Page_251'>251</span>characterised by a great increase of absorption. There is a
+general darkening which extends far up in the ultra-violet, and
+is modified, in the green and blue, into remarkable dusky
+gratings made up of closely-set fine rays; and some of the
+ordinary Fraunhofer lines are besides thickened and blackened.
+The formation in spots of oxides is thought by Dr. Scheiner
+to be possibly indicated by these symptoms; “if so,” he adds,
+“the presence of oxygen in the sun would thus be indirectly
+suggested.”<a id='r11'></a><a href='#f11' class='c027'><sup>[11]</sup></a> Bright lines, too, flash out in the immediate
+neighbourhood of sun-spots, especially the “great twin
+brethren,” “H” and “K,” due to calcium, which stand in imposing
+breadth and strength at the violet end of the Fraunhofer
+spectrum, and are of corresponding importance as indexes to
+solar phenomena. With this pair, brilliant hydrogen rays are
+often associated, besides other “reversals,” by which, upon
+the customary dark lines, flaming rays of identical wave-lengths
+are superposed. But these signs of incandescence
+evidently belong to the facular stratum high up above the
+spot-umbra.</p>
+
+<p class='c007'>So long ago as 1769, the observations of Dr. Wilson of
+Glasgow were believed to have established, once for all, that
+spots are funnel-shaped depressions in the photosphere. But
+the perspective effects from which he argued are certainly not
+always, perhaps not very often, present. Mr. Howlett, after
+thirty-five years—1859 to 1895—devoted to testing the truth
+of the traditional conviction, has at last succeeded in shaking,
+if not in overthrowing, it. Most solar observers now admit
+that spots are of extremely various and extremely variable
+construction, so that the obscure umbra, at times a sort of pit
+or crater, in which vapours, cooled by expansion, well up from
+below, may, at another stage in the life-history even of the
+same spot, represent an actual accumulation of absorbent
+material above the brilliant solar cloud envelope. In any
+case, a spotted area appears to be an area of elevation. This
+might be due to a wide-spreading relief of pressure, or an
+accession of internal heat. The fact emerged clearly from a
+<span class='pageno' id='Page_252'>252</span>series of measurements of the sun’s diameter executed by M.
+Sykora at Charkow, Russia, in 1895.<a id='r12'></a><a href='#f12' class='c027'><sup>[12]</sup></a></p>
+
+<p class='c007'>The intensity of the agitations connected with sun-spots
+can be most fully appreciated from spectroscopic observations.
+Lines torn, displaced, and <i>branching</i>, testify to velocities in
+the line of sight of the matter surrounding or overlaying them
+up to three or four hundred miles a second! These tumultuous
+uprushes and downrushes are not of a systematic nature;
+they afford no insight, consequently, into the formative laws
+of spots. Of these we are indeed far more ignorant than Sir
+William Herschel supposed himself to be. Recent work on
+the sun has provided a grand store of facts ascertained with
+surprising skill and ingenuity. But they want <i>colligating</i>.
+No framework has yet been constructed that will hold them,
+each in its proper place. It has been truly said: “Considering
+the rapid progress which has been made in the observational
+or practical side of solar physics, it must be confessed
+that the theoretical side has been very imperfectly developed.
+Almost every student of solar physics has his own theory, and
+usually he himself is the only one who believes in it.”</p>
+
+<p class='c007'>Since Sir John Herschel propounded his “cyclonic theory”
+of sun-spots in 1847, there has been a marked tendency to assimilate
+solar to terrestrial phenomena. But the circumstances
+of the two bodies are so utterly unlike that such attempts can
+only prove misleading. The earth is a solid globe warmed
+from without, hence, with hot tropical and frigid polar regions.
+This disparity is the prime motor in the circulation of its
+atmosphere and oceans; a circulation, essentially in latitude,
+directed towards the equalisation of temperature. The sun,
+on the contrary, is heated from within; there is no appreciable
+difference of temperature between its poles and equator; and
+its circulation is of the bodily kind belonging to fluid masses,
+and is carried on by vertical currents effecting exchanges of
+heat between the surface and the profundities beneath.
+Were these to stop, or even notably to slacken, the sun would
+promptly cease to shine, and lapse into the condition of a
+<span class='pageno' id='Page_253'>253</span>“dark star.” It is not then surprising that the drifting
+movements of the photosphere are <i>along</i>, not <i>across</i>, parallels of
+latitude. Solar meteorology, in a word, has almost nothing
+in common with terrestrial meteorology; and explanatory
+schemes, based upon an analogy which does not exist, must
+sooner or later be consigned to the limbo of vanities.</p>
+
+<h3 class='c004'>CHAPTER III.<br> <span class='c014'>THE SUN’S SURROUNDINGS.</span></h3>
+
+<p class='c003'>“What we ordinarily call the sun,” wrote the late Mr.
+Ranyard, “is only the bright spherical nucleus of a nebulous
+body.”<a id='r13'></a><a href='#f13' class='c027'><sup>[13]</sup></a> But it is only when the interposing moon cuts off
+the dazzling rays of the nucleus that we see directly anything
+of its nebular surroundings. Partial or annular eclipses are
+of little or no use for this purpose; the revelation belongs
+exclusively to the sombre, yet splendid moments of totality.
+No sooner has the last glint of sunshine vanished than the
+corona starts into view, encompassing the black lunar globe
+with a sort of “glory” of silvery streamers. Its radiated
+shape suggests vacillation of form and a flickering radiance;
+yet its immobility is absolute. The awe and wonder of the
+sight tend, for the moment, to supersede scientific curiosity,
+and they are enhanced by the perception, at the base of the
+corona, of the serrated scarlet “chromosphere” fringing the
+moon’s circumference, while the towering “prominences” that
+are usually seen to spring from it produce the startling effect
+of a conflagration.</p>
+
+<p class='c007'>These marvellous appendages received no adequate notice
+until their disclosure during the total eclipse of July 8, 1842.
+Even the uninstructed crowds in the streets of Milan and
+<span class='pageno' id='Page_254'>254</span>Pavia shouted with amazement at what they saw; while by
+solar students the recurrence of similar opportunities has ever
+since been eagerly anticipated and diligently turned to
+account. The question that first pressed for solution related
+to the local habitation of prominences; for some unwisely
+persisted in attaching them to the moon. A decisive answer
+was given by photography at its first <i>effective</i> application to
+eclipses on July 18, 1860. From a comparison of negatives
+exposed at the beginning and end of totality, it became
+clearly apparent that the moon had, in the interval, moved
+<i>over</i> the prominences, uncovering, to a small extent, those on
+the west side and concealing those on the east.</p>
+
+<p class='c007'>Their solar connexion having thus been established by
+the camera, the spectroscope was called upon to determine
+their physical and chemical nature. An admirable opportunity
+for taking this further step was presented by the Indian
+eclipse of August 18, 1868. The result was decisive. The
+light of a huge spire of flame, 89,000 miles high, had no
+sooner passed through a prism than its gaseous origin declared
+itself. The spectrum consisted of several hydrogen lines, and
+one unknown line in the yellow, slightly more refrangible
+than the sodium-pair D<sub>1</sub>, and D<sub>2</sub>, and hence called D<sub>3</sub>.
+“Je verrai ces lignes-là en dehors des éclipses!” M. Janssen
+exclaimed, as they caught his eye; and on the following
+morning, at Guntoor in the Neilgherries, he actually started
+daylight spectroscopic work at the edge of the sun. He
+owed his success to a perfectly simple principle. The
+ordinary invisibility of prominences is due to the drowning of
+their light in reflected sunshine. But sunshine, because it is
+continuous—that is, made up of beams of all refrangibilities—can
+be weakened to almost any extent by dispersion,
+while the detached prominence-rays lose nothing by being
+separated. Hence, the result of passing the mixed light from
+near the solar limb through a train of prisms is that the
+tell-tale bright lines stand out distinctly from an <i>emaciated</i>
+prismatic background. The method was independently
+discovered by Mr. Norman Lockyer in England, and his
+<span class='pageno' id='Page_255'>255</span>and Janssen’s communications on the subject were laid before
+the French Academy of Sciences on the same day of October,
+1868. It has proved of inestimable value, and was further
+improved in 1869 by Dr. Huggins’s device for viewing these
+objects in their proper shapes through an open slit, instead of
+building them up in narrow sections by successive observations
+through a narrow one. This was made possible by the
+intensity of their light. They can be observed in variously
+coloured images corresponding to the different rays they
+emit; but the least refrangible of the hydrogen series—the
+blood-red C (alias Hκ)—is generally chosen as being the
+most brilliant and best defined.</p>
+
+<p class='c007'>The unrecognised substance giving the yellow prominence-line
+was named by Dr. Frankland “helium.” It evidently
+existed near the sun in enormous quantities, and in close
+companionship with hydrogen. Yet no dark line corresponding
+to its absorption was to be found in the Fraunhofer spectrum,
+although it now and then emerged in spot-spectra.
+Conjectures were rife as to its nature and relations. It
+was generally believed to be specifically lighter than hydrogen,
+and some held it a product of its dissociation, and so
+of a different elemental standing. Everything about it,
+however, remained doubtful until, in March, 1895, Professor
+Ramsay produced a sample for inspection close at hand,
+extracted by heat from the rare mineral “clevite.” The recognition-mark
+was its emission, when electrically excited, of
+the solar D<sub>3</sub>, with which were associated several other chromospheric
+rays previously registered as of unknown origin,
+but now linked together as vibrations of the same molecules.
+A sudden and entirely unlooked-for advance was thus made
+in the chemistry of the sun’s surroundings.</p>
+
+<p class='c007'>Helium is a colourless gas of about twice the density of
+hydrogen. Its peculiar qualities are shared only by argon,
+the new constituent of the earth’s atmosphere. Both have unusual
+thermal relations; both are chemically inert. They
+refuse to combine with any other element, and thus stand
+apart from the round of multiform change involving the whole
+<span class='pageno' id='Page_256'>256</span>material world. Helium is nevertheless distributed freely
+throughout the universe. Hydrogen itself is scarcely more
+ubiquitous.</p>
+
+<p class='c007'>A considerable mass of information regarding the solar
+prominences was rapidly collected by means of the Janssen-Lockyer
+invention. They were at once divided into two
+classes. The “quiescent” kind occur in all solar latitudes;
+they change their shapes very gradually; they have no immediate
+relationship with spots. In form they resemble
+<i>pillared clouds</i> resting in banks like heavy cumuli, or floating,
+like expanses of thin cirrus, high above the chromosphere
+with which they are ordinarily connected by slender supports
+or conduit-pipes. But these are at times invisible or non-existent.
+Father Secchi occasionally watched isolated cloudlets
+form and grow spontaneously as if by condensation from
+saturated air; and on October 13, 1880, Professor Young
+made a confirmatory observation. About 11 <span class='fss'>A.M.</span> he noticed
+a detached fiery mass at an elevation of 67,500 miles above
+the limb. “It grew rapidly, without any sensible rising or
+falling, and in an hour developed into a large stratiform
+cloud, irregular on the upper surface, but nearly flat beneath.
+From this lower surface pendent filaments grew out,
+and by the middle of the afternoon the object had become
+one of the ordinary stemmed prominences.”<a id='r14'></a><a href='#f14' class='c027'><sup>[14]</sup></a> The size of
+these formations is enormous. They vary in height from
+about 10,000 to 100,000 miles; and ranges of them 450,000
+miles in extent have been photographed during total
+eclipses.</p>
+
+<div id='i_257'  class='figcenter id001'>
+<img src='images/i_257.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 3.</span>—<i>Curves showing the development of Sun-spots and Prominences during the period 1880 to 1891.</i> (Sidgreaves.)</p>
+</div>
+</div>
+
+<p class='c007'>The second class of prominences, known as “eruptive,” are
+obviously manifestations of intense energy. In some of their
+forms they suggest geyser-like spoutings of incandescent
+vapours. They represent swords and scimetars, palms with
+twisted trunks composed of mounting flames, igneous vegetation
+of sundry types. Their chemistry is much more complex
+than that of the quiescent sort. Not only hydrogen and
+helium, but iron, magnesium, sodium, and a number of other
+<span class='pageno' id='Page_257'>257</span>metals enter into their composition. Belonging to the same
+order of disturbance with spots, they are closely conjoined
+with them, both in time and space. They conform to the
+sun-spot cycle, as well as to the “law of zones,” showing that
+photospheric and chromospheric disturbances spring from a
+common cause. Fig. 3 (from the <cite>Observatory</cite> for March,
+1893) embodies a comparison between the “spotted area”
+as determined at Greenwich 1880–1891, and the “profile area”
+of prominences (without distinction of kind) observed spectroscopically
+at Stonyhurst during the years 1880–1892. The
+agreement between the two curves is very striking; but the
+<span class='pageno' id='Page_258'>258</span>minimum of solar activity in 1889 is decidedly better represented
+by the prominence-tracing. Father Sidgreaves, director
+of the Stonyhurst Observatory, adds the important remark that
+wide-spreading elevations of the chromosphere attend spot-maxima,
+while depressions of equal extent occur at minima.</p>
+
+<p class='c007'>The chromosphere is a solar envelope, but not a solar atmosphere.
+It completely surrounds the sun to the depth of
+about 4,000 miles with a close tissue of scarlet flames, their
+filamentous or tufted summits swaying and intercrossing as if
+under the gusty sweep of fiery winds. Any of these summits
+which attain an unwonted height become “prominences,” but
+it is a mere matter of convention when the change of nomenclature
+should take place. The chemical composition of the
+chromosphere does not differ essentially from that of prominences.
+Its permanent constituents were found by Professor
+Young to be hydrogen, helium, “coronium,” and calcium, the
+last represented <i>only</i> by “H” and “K.” But disturbances never
+failed to be indicated by the blaze of metallic lines, of which
+273 in all have been determined by the same authority. Their
+appearance signified, without doubt, the injection from below
+of the corresponding vapours, chiefly those of iron, titanium,
+sodium, magnesium, strontium, barium, and manganese. At
+moments the reinforcement of the spectrum with bright rays
+was so extensive that it seemed as if the entire “reversing
+layer” had been uplifted bodily into the chromosphere.</p>
+
+<p class='c007'>The reversing layer lies quite close to the photosphere. It
+is scarcely more than 300 miles deep, and is hence invisible
+except during about a second at the beginning and end of
+total eclipses. Young was the first to be favoured with a sight
+of it, on December 22, 1870. No sooner was the direct solar
+spectrum intercepted by the moon, than “all at once, as
+suddenly as a bursting rocket shoots out its stars, the whole
+field of view was filled with bright lines, more numerous than
+one could count. The phenomenon was so sudden, so unexpected,
+and so wonderfully beautiful, as to force an involuntary
+exclamation.”<a id='r15'></a><a href='#f15' class='c027'><sup>[15]</sup></a> It was afterwards frequently observed, and
+<span class='pageno' id='Page_259'>259</span>at last satisfactorily photographed by Mr. Shackleton, a member
+of Sir George Baden-Powell’s expedition to Novaya
+Zemlya, for the purpose of observing the total solar eclipse
+of August 9, 1896. The permanent record then secured was
+of peculiar importance as affording the means of confronting
+in detail the components of the vario-tinted flash at the
+eclipsed sun’s limb with the dusky legion of the Fraunhofer
+lines. The correspondence is striking, and leaves no doubt
+that Young’s stratum is the actual locality where the characteristic
+solar spectrum is produced. It may be described
+as an universal solar ocean of glowing metallic vapours, the
+rays emanating from which, although vivid when seen <i>off</i> the
+sun, are thrown out in dark relief by projection upon the white-hot
+photosphere. The existence of just such a heterogeneous
+absorbing layer had been predicted, on theoretical grounds,
+some years before it came into view.</p>
+
+<p class='c007'>The movements taking place in eruptive prominences are
+often of portentous speed. They are betrayed, so far as they
+coincide with the visual ray, by spectroscopic line-displacements;
+so far as they are directed <i>across</i> the visual ray, by
+immediate observation of the spectroscopic images. Thus,
+the up-and-downrushes of flaming hydrogen above spots on
+the disc reach velocities of 320 miles a second; and solar
+tornadoes (detected by Mr. Lockyer more than a quarter of a
+century ago) are often observed to whirl at rates which would
+be incredible were they less well authenticated. Vertical
+explosions at the limb, on the other hand, of still more unruly
+violence are rendered manifest by displacements, not of the
+emitted lines, but of the radiating substances themselves.</p>
+
+<p class='c007'>On September 19th and 20th, 1893, Father Fényi, director
+of the Kalocsa Observatory in Hungary, witnessed the
+development and dissolution of a pair of objects perhaps the
+most extraordinary in the astonishing record of solar phenomena.<a id='r16'></a><a href='#f16' class='c027'><sup>[16]</sup></a>
+They broke out within nineteen hours of each other,
+showed a close similarity of shape and structure, underwent
+analogous changes, and, strangest of all, were situated at
+<span class='pageno' id='Page_260'>260</span>almost diametrically opposite points of the solar limb. The
+first was already, when first viewed at 2 <span class='fss'>P.M.</span>, 168,000 miles
+high; within half an hour, it had sprung up to 224,000 miles
+(8′ 18″), and again subsided into a commonplace flame of the
+modest dimension of 13,650 miles (30″). The rate of ascent,
+directly measured (always necessarily through the medium of
+the spectroscope), was 132 miles a second. This vast, though
+transient construction, seemed to be formed of a multitude of
+distinct fiery tongues, each leaping and flaring independently.
+As a whole, it was also tongue-shaped, and “stood erect
+nearly in the direction of the sun’s radius,” travelling, meanwhile,
+towards the earth at an average rate of 186 miles a
+second.</p>
+
+<p class='c007'>The companion-prominence began to show at nine next
+morning, and, rising with a velocity of 300 miles per second,
+attained in twelve minutes to a height of 220,000 miles. This
+tremendous apparition was of the same “ragged” texture as
+its predecessor, and shone, even in its loftiest fragments, with
+the same intense glow. As might have been expected from
+its opposite position, its radial movement was <i>from</i> the earth.
+A prominence measured by the same observer, July 15, 1895,
+was diminishing its distance from the earth with the extraordinary
+velocity of 533 miles a second; and on September
+30 of the same year, a colossal object resembling the bent
+and riven trunk of a great tree, was in the course of half an
+hour flung upwards to a minimum altitude of 313,000 miles,
+and had again faded out of sight. “The appearance,” Father
+Fényi wrote, “of all the numerous great eruptions which I
+have observed has been such as would be produced by a kind
+of explosion over a spotted region, which, seizing upon a
+prominence already developed, hurls it upward from the
+surface, tears it to pieces, and brings it to a speedy end.”
+The matter thus acted upon is of enormous volume, but
+negligeable mass.</p>
+
+<p class='c007'>Photographs of prominence-spectra, obtained by Dr.
+Schuster during the eclipse of May 17, 1882, brought
+out the remarkable predominance in their light of the
+<span class='pageno' id='Page_261'>261</span>“H” and “K” emissions of calcium. It was re-discovered
+by means of spectrographs of those objects, taken in 1891
+without an eclipse, by Professor Hale at Chicago, and by
+M. Deslandres in Paris. Both investigators promptly seized
+upon the advantage it offered for their chemical delineation
+in full daylight. The lines in question are dark and abnormally
+wide in the sun itself, bright and sharp in prominences.
+Thus, at these particular parts of the spectrum, the obliterating
+effects of scattered sunlight are non-existent. Just here, too,
+photographic sensitiveness is at its maximum. Hence, by
+working with either of these lines (K is preferable) nothing
+could be easier than to get impressions of the brilliant
+forms of prominences relieved against the background of
+solar absorption. (See Figures 4 and 5.) The thin, bright
+line is <i>sheltered</i> from daylight glare by the dusky, broad
+one. By the use of a “double slit,” the method was completed.
+This, again, was simultaneously invented by Hale
+and Deslandres, although they had, without suspecting it,
+been anticipated by Janssen in 1869. The second slit is
+adjusted so as to exclude all but a single ray of the spectrum
+formed by dispersing the light admitted through the first. An
+unlimited power of selection is in this way afforded as to the
+quality of light to be employed; but for general purposes, K
+is not likely to be superseded.</p>
+
+<p class='c007'>In the Chicago spectroheliograph, two moveable slits,
+together with a powerful diffraction spectroscope, are attached
+to a twelve-inch refractor. With this instrument, monochromatic
+impressions of the sun with its spots, faculæ, and flame-garland
+are obtained without difficulty. To begin with, the
+solar disc is covered with a metal diaphragm, then the first slit
+is caused to traverse the artificially eclipsed image, the second
+following at such a rate that the K line alone always falls upon
+the sensitive plate. The result is a complete photographic record
+of the chromosphere and prominences. The diaphragm
+having been then removed, the return journey of the slits is
+very quickly made, so as to guard against the formidable
+actinic strength of even that small element of direct sunlight
+<span class='pageno' id='Page_262'>262</span>contained in the K line. The object of the second transit is
+to <i>insert</i> an autographic print of the sun itself into the space
+previously left blank to receive it. The entire operation occupies
+less than one minute. Portrayed thus in calcium light, the
+solar disc has a strange effect. It is entirely overspread with a
+reticulation of irregular bright markings, greatly emphasized
+over the spot-zones, and corresponding in general with the
+positions of faculæ. According to Professor Hale, these masses
+and wreathings of calcium vapour <i>are</i> faculæ. M. Deslandres
+regards them rather as gaseous formations connected with
+faculæ. Their extension and intensity are at times so great
+that M. Deslandres has actually succeeded, through the
+prevalence of their light, in photographing the sun as a
+“bright-line star.” The double-slit method also affords the
+means of studying the distribution of each element of the
+reversing layer in the leisure of ordinary daylight, as M.
+Deslandres has shown by some preliminary experiments.<a id='r17'></a><a href='#f17' class='c027'><sup>[17]</sup></a></p>
+
+<p class='c007'>To this extent astronomers have made themselves independent
+of eclipses. These momentous occurrences are, fortunately,
+not needed for researches concerned with distinct
+coloured rays separable by dispersion from diffuse sunshine.
+But with the corona it is different. For here we have a white
+glory to deal with. Coronal light is derived from three
+sources: from the original incandescence of solid or liquid
+particles, from sunshine reflected by them, and from gaseous
+emissions. The most conspicuous of these is a green ray of
+unknown chemical meaning. It proceeds from every part of
+the corona, even from the dark rifts separating its brilliant
+streamers, and the inconceivably tenuous substance to which
+it owes its origin has, accordingly, received the name of
+“coronium.” The coronal spectrum includes many other
+bright lines, especially in the ultra-violet, photographed during
+eclipses; but the hydrogen, helium, and calcium lines which
+accompany them probably represent scattered chromospheric
+light.</p>
+
+<div id='i_264'  class='figcenter id001'>
+<img src='images/i_264.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 4.</span>—<i>Eruptive Prominence photographed by Professor Hale at the Kenwood Observatory, March 24, 1895, at 22h. 40m. Chicago mean time.</i> (<i>The photosphere is covered with a metallic disc.</i>)</p>
+</div>
+</div>
+
+<div id='i_265'  class='figcenter id001'>
+<img src='images/i_265.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 5.</span>—<i>The same, 18m. later.</i><br>  <br>  (From the <cite>Astrophysical Journal</cite>, May, 1896.)</p>
+</div>
+</div>
+
+<p class='c007'>The green coronal ray is much too faint to be isolated with
+<span class='pageno' id='Page_267'>267</span>the spectroscope; but the continuous coronal spectrum has
+maxima of intensity compared with ordinary daylight, which
+suggested to Dr. Huggins, in 1882, a differential method of
+photographing the entire structure apart from eclipses. It
+has however, as yet come to nothing, and Hale and
+Deslandres have been equally unsuccessful with their “double
+slit” apparatus. Hence, it is only by favour of the moon
+that this wonderful appendage can be investigated, and the
+available moments have not been allowed to pass in vain.</p>
+
+<div id='i_267'  class='figcenter id001'>
+<img src='images/i_267.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 6.</span>—<i>The Eclipsed Sun, photographed at Sohag in Egypt, May 17, 1882. A Comet is almost involved in the Corona.</i> (From “Philosophical Transactions,” vol. clxxv.)</p>
+</div>
+</div>
+
+<p class='c007'>One result fully ascertained is that it changes in form
+<span class='pageno' id='Page_268'>268</span>concurrently with the progress of the sun-spot period. The
+maximum coronal type is entirely different from the minimum
+type, and reappears in unmistakable connexion with
+vehement solar disturbance. This cyclical relation was
+first pointed out by Mr. Ranyard. On July 29, 1878, a
+totality of 165 seconds was observed, under splendid
+conditions of weather, in the Western States of North
+America. No prominences worthy of note were visible, but
+the corona wore a most surprising aspect. A pair of enormous
+equatorial streamers stretched east and west of the sun
+to a distance of at least ten millions of miles. Indeed, they
+came to no definite end. They were best seen with the
+naked eye, and made no show on sensitive plates, but the application
+of low telescopic powers disclosed, near the base of
+the effusions, a mass of delicate and complex detail. The
+solar poles were as distinctively, although not so strikingly,
+garnished as the solar equator. Each was the centre from
+which diverged a dense brush of straight, electrical-looking
+rays. The sun was at the time in a state of profound tranquillity;
+and it was recalled that, at the previous minimum, in
+1867, Grosch had delineated, at Santiago, just the same
+equatorial extensions, and just the same polar brushes. The
+connexion was emphasised during the maximum of 1882–4,
+by the substitution, when the moon covered the sun on May
+17, 1882, and May 6, 1883, of a dazzling stellate formation for
+the winged corona of 1878. In Fig. 6 is reproduced a photograph
+by Dr. Schuster of the Sohag, or Egyptian corona,
+with the added embellishment of a comet hurrying up to
+perihelion, conspicuous to the eye at the time, but never seen
+again.</p>
+
+<p class='c007'>In 1889 the minimum type of corona reasserted itself. A
+drawing made by Miss M. L. Todd during the eclipse of
+January 1, gave the characteristic equatorial “fish-tails,”
+reaching out on the west to four solar diameters.<a id='r18'></a><a href='#f18' class='c027'><sup>[18]</sup></a> And although
+the camera, owing to special difficulties, has not yet
+been able to pursue them so far, Professor Barnard’s exquisite
+<span class='pageno' id='Page_269'>269</span>picture (Fig. 7), taken at Bartlett’s Springs, California,
+with an exposure of 4½ seconds, portrays the type to perfection,
+with its suggested indefinite expansions, “the soft
+feathery details of the inner corona, and the delicate fan-structures
+at the poles.” Two minute notches mark the
+points where a couple of prominences have, by the intensity
+of their actinic power, <i>eaten into</i> the black circumference of
+the lunar image.</p>
+
+<div id='i_269'  class='figcenter id001'>
+<img src='images/i_269.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 7.</span>—<i>The Corona of January 1, 1889, photographed by Professor E. E. Barnard.</i></p>
+</div>
+</div>
+
+<p class='c007'>Nine negatives were secured by the artist, but at a considerable
+personal sacrifice. “So impressive,” he wrote, “was
+the magnificent spectacle upon the crowd that had gathered
+just outside our enclosure, that not a murmur was heard.
+<span class='pageno' id='Page_270'>270</span>The frightened, half-whining bark of a dog, and the click-click
+of the driving clock, alone were audible. When the sun
+suddenly burst forth, an almost instantaneous and highly-surprised
+cackling of the chickens, that had hastily sought
+their roosts at the beginning of totality, would have been
+amusing could one have shaken off the dazed feeling at
+the unexpectedly rapid termination of the semi-darkness.
+My own feelings were those of excessive disappointment
+and depression. So intent was I in watching the cameras
+and making the exposures, that I did not look up to the
+sun during totality, and therefore saw nothing of the
+corona.”</p>
+
+<p class='c007'>On April 16, 1893, at the height of the last sun-spot maximum,
+a shadow-track crossed South America and Central
+Africa. Once more the coronal type had changed. Not a
+trace remained of the equatorial “wings”; not a trace of the
+polar “fans.” Instead, the “compass-card” aureole of 1882
+and 1883, shaped regardless of heliographic latitude, reemerged
+from beneath the veil of daylight. That the sun’s
+filmy “crown” follows, after its own inexplicable fashion,
+the general round of solar vicissitudes, no longer admitted of
+a doubt. The fact is thus stated by M. Deslandres, who observed
+the eclipse at Fundium, in the Senegal district.</p>
+
+<p class='c007'>“The form of the corona,” he says, “undergoes periodical
+variations, which follow the simultaneous periodical variations
+already ascertained for spots, faculæ, prominences, auroræ,
+and terrestrial magnetism. This important relation, indicated
+by preceding eclipses, is strongly confirmed by the eclipse of
+1893.”<a id='r19'></a><a href='#f19' class='c027'><sup>[19]</sup></a></p>
+
+<p class='c007'>Professor Schaeberle’s photographs, taken on the same occasion
+at Mina Bronces in Chili, marked a decided advance
+in coronal portraiture. The sun’s disc measured four inches
+on his plates, exposed with a photoheliograph forty feet in
+length; and the details of inner coronal construction came
+out accordingly with unprecedented perfection. The corona
+of August 9, 1896, reproduced the most striking features of
+<span class='pageno' id='Page_271'>271</span>the corona observed August 29, 1886; and both corresponded
+to an intermediate epoch of the spot-cycle. The polar
+brushes were present without the equatorial extensions, while
+in both a protruding ray made an angle of some thirty or
+forty degrees with the solar axis. This distinctive trait imprinted
+itself with surprising emphasis on some of Sir George
+Baden-Powell’s Novaya Zemlya photographs.</p>
+
+<p class='c007'>Researches, prosecuted under cover of eighteen eclipses,
+have greatly strengthened the visible analogy between coronal
+streamers, auroral coruscations, and comets’ tails. The persuasion
+that electrical discharges in high vacua are concerned
+in all these phenomena is not easily resisted. Repulsive
+forces such as are at work in Crookes’ tubes perhaps come
+into play, on the vast solar scale, to produce the strange and
+beautiful luminous forms revealed during eclipses. Their
+tenuity is certainly extreme. They probably contain very
+much less matter, volume for volume, than the incredibly
+exhausted tubes of modern physicists. The unresisted
+passage of comets through the corona demands this supposition,
+which is in complete accord with the fineness of the
+Fraunhofer lines. The corona shows no increase of density
+downwards, and the chromosphere very little. Hence neither
+can be a true solar atmosphere, weighing freely upon the
+sun’s surface. For, under the immense power of solar gravity,
+the accumulated pressure of the superincumbent layers, even
+if there were only one hundred miles’ thickness of them, could
+not be intelligibly conveyed in figures; how much less when
+the piling-up of the aerial strata is reckoned by thousands of
+miles!</p>
+
+<p class='c007'>To recapitulate. Starting from the photosphere, we meet
+first an envelope producing the <i>general</i> absorption, by which
+sunlight is enfeebled and reddened as if by the interposition
+of a slightly rufous shade. Next comes the reversing layer
+composed of mixed incandescent vapours, giving rise, by
+their <i>selective</i> absorption, to the Fraunhofer lines. No alterations
+in correspondence with the spot-cycle have yet been
+determined in either of these couches, which, close as they lie
+<span class='pageno' id='Page_272'>272</span>to the photosphere, remain, nevertheless, apparently indifferent
+to its agitations. They are overspread by the chromosphere
+and prominences; while above and beyond shines the
+mysterious corona; both chromosphere and corona strictly
+conforming, by manifest changes, to the sun’s periodicity.
+One other solar appendage remains to be noticed.</p>
+
+<p class='c007'>After sunset in spring, and before sunrise in autumn, a
+mass of soft luminosity, often brighter than the Milky Way,
+may be seen tapering upward from the horizon along an
+axis approximating to the line of the ecliptic. Its more
+conspicuous visibility at those times just reverses the case of
+the harvest moon. As a rule, the apex of the cone barely
+reaches the Pleiades; but it does not really end here.
+Thrice during the present century, by Brorsen, Backhouse,
+and Barnard, the zodiacal “counterglow” has been independently
+discovered and studied. This is a hazy, luminous
+patch, ten to fifteen degrees across, and exactly 180°
+from the sun. It represents the <i>opposition aspect</i> of the
+Zodiacal Light, hence proved to be a formation in planetary
+space, extending considerably beyond the earth’s orbit. Two
+plausible hypotheses as to its nature have been proposed.
+Professor Searle<a id='r20'></a><a href='#f20' class='c027'><sup>[20]</sup></a> holds it to represent the reflection of
+sunlight from “an infinite number of small asteroids.” Professor
+Bigelow<a id='r21'></a><a href='#f21' class='c027'><sup>[21]</sup></a> considers it as an amassment in the plane of
+the sun’s equator—“a place of zero potential”—of the
+particles electrically expelled from the poles. The Light is
+then, if this view be correct, an extension of the corona—a
+sort of “pocket or receptacle, wherein the coronal matter is
+accumulated and retained as a solar accompaniment.” A
+continuous spectrum is derived from it; no element of
+original emission can be detected; so that the spectroscope
+“holds the balance even” between the two theories. If,
+however, the latter were true, the Zodiacal Light should spread
+out from the sun’s equator; if the former, then its medial plane
+<span class='pageno' id='Page_273'>273</span>should deviate very slightly from that of the ecliptic, to
+which the fundamental, or “invariable” plane of the solar
+system is inclined only one and a half degrees. M.
+Marchand’s observations from the Pic du Midi<a id='r22'></a><a href='#f22' class='c027'><sup>[22]</sup></a> appear to
+be decisive on the point. During three years, he mapped
+down the limits assigned by his observations night after
+night, to an emanation which, in that pure air, was seen to
+compass the entire sphere. The eventual comparison of his
+collected data showed its axis to be a great circle sensibly
+coincident with the sun’s equator. All reasonable doubt as
+to the nature of the Zodiacal Light has thus been removed.
+It is a reservoir for the sun’s waste matter—the sink, into
+which are daily flung the particles rejected through the
+agency of the aigrettes and streamers composing the wonderful
+eclipse-vision of the corona.</p>
+
+<h3 class='c004'>CHAPTER IV.<br> <span class='c014'>THE INTERIOR PLANETS.</span></h3>
+
+<p class='c003'>The Interior Planets are those which revolve within the
+earth’s orbit. They are two in number—Mercury and Venus.
+Mercury, with a diameter of three thousand miles, is the
+smallest of the eight principal planets. It pursues a track,
+too, more eccentric and more highly inclined to the ecliptic
+than any other planetary orbit. The zodiac had of old to be
+made 16° wide in order to afford room for its excursions. These
+irregularities are, however, quite innocuous as regards the stability
+of the system, for the reason that they belong to a body of
+insignificant mass. The successive approaches to it of Encke’s
+<span class='pageno' id='Page_274'>274</span>comet have afforded a means of ascertaining its gravitative
+power; and, according to the latest report from this filmy
+messenger, it is even less than had been supposed. Mercury,
+it appears, weighs little more than one-ten-millionth of the
+sun, or one-thirtieth of the earth. And since its volume is
+about one-nineteenth the terrestrial, the matter of which it is
+composed must be less dense in the proportion of 30 to 19.
+So that the planet would turn the balance against one equal
+globe of granite, or three and a half of water. We can hence
+easily calculate that gravity, at Mercury’s surface, possesses less
+than one-fourth its power at the earth’s surface. A man of
+sixteen stone transported thither, would find himself relieved
+of fully three-quarters of his habitual burthen.</p>
+
+<p class='c007'>The plane of Mercury’s orbit makes an angle of 7° with the
+ecliptic, and he traverses it with a speed varying from 23 to
+35 miles a second. The corresponding distances from the sun
+are 43½ and 28½ million miles, while the mean distance, or
+semi-major axis of the ellipse, measures just 36 millions. Independently
+then of what we call seasons, Mercury is subject,
+in the course of its year of 88 days, to considerable vicissitudes
+of temperature. At perihelion it receives nine times, at
+aphelion only four times, more heat than is imparted by the
+sun to an equal area of the earth.</p>
+
+<p class='c007'>The crucial point as regards the physical condition of a
+planet is the presence or absence of an atmosphere. And
+there is decisive evidence that Mercury is in this respect
+poorly provided. Certain luminous phenomena, often observed
+during its transits across the sun, appear to be of purely
+optical production, since they are less conspicuous with good
+than with indifferent telescopes; while, on the other hand,
+genuine refractive effects are absent. A corresponding indication
+is afforded by the low “albedo,” that is, the slight reflective
+power of this planet. Of the light flooding its surface only
+17 per cent.<a id='r23'></a><a href='#f23' class='c027'><sup>[23]</sup></a> is returned; 83 per cent. is absorbed. Now the
+albedo of clouds is about 72; a cloud-wrapt globe is little less
+<span class='pageno' id='Page_275'>275</span>brilliant than if it were covered with fresh-fallen snow.
+Hence a high albedo accompanies a dense, vapour-laden atmosphere;
+a low albedo indicates a transparent one. And
+since Mercury, which sends back only about as much light as
+if it were made of grey granite, has the lowest albedo of any
+of the principal planets, it may be safely concluded to possess
+the thinnest aerial covering. Yet it is not, apparently, a
+totally airless globe. Spots upon its surface have been seen to
+become effaced as if by atmospheric veilings; and the spectroscope
+hints (although doubtfully) at aqueous absorption.</p>
+
+<p class='c007'>Mercury is “new” when nearest to the earth, and “full”
+when most remote from it. At both these periods, moreover,
+its position with regard to the sun renders it ordinarily invisible;
+so that it is usually seen as either gibbous or
+crescent shaped. The study of its phases has brought out
+a noteworthy circumstance. It is easy to understand that
+geometrical light changes will not proceed by the same
+gradations upon a smooth and upon a rugged globe, where
+they are complicated by irregular shadows and illuminations.
+The laws of variation are quite different in each
+case, and their respective prevalence can be distinguished
+by steady observation. There seems no reason to doubt that
+the latter are obeyed by Mercury. After several years’ watching
+of its phases, Professor G. Müller<a id='r24'></a><a href='#f24' class='c027'><sup>[24]</sup></a> of Potsdam concludes
+them to be such as characterise a broken and uneven surface.</p>
+
+<div id='i_276'  class='figcenter id001'>
+<span class='pageno' id='Page_276'>276</span>
+<img src='images/i_276.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 8.</span>—<i>Map of Mercury, by Schiaparelli.</i> (From <cite>Astronomische Nachrichten</cite>, No. 2944.)</p>
+</div>
+</div>
+
+<p class='c007'>Little or nothing was known about the rotation of Mercury
+when Schiaparelli of Milan undertook its determination in
+1882. His observations were made in full daylight, in order
+to reduce atmospheric disturbances to a minimum; and he
+executed, in the course of a few months, a series of 150
+Mercurian delineations upon which is founded the planisphere
+exhibited in Fig. 8. The surface of the planet,
+coloured light rose with a coppery tinge, was seen to be
+diversified by brownish-red markings which became effaced
+towards the limb as if through atmospheric absorption.
+Although evidently of a permanent nature, their outlines
+escaped precise definition. The most remarkable circumstance
+about them was that they showed no effects of rotation.
+During several consecutive hours of watching, they
+remained sensibly fixed in their places. The conclusion was
+finally arrived at that Mercury rotates on a nearly upright
+axis in the same time that it revolves round the sun. Its
+day, no less than its year, is equal to 88 of our days. Consequently
+it turns at all times substantially the same face
+towards the sun; and the “terminator,” that is, the dividing-line
+between darkness and light, only “librates,” without
+travelling right round the globe. The librations of Mercury
+are, however, extensive in proportion to the eccentricity of
+its orbit; hence, five-eighths of its surface come in for some
+<span class='pageno' id='Page_277'>277</span>share of illumination during the Mercurian year. Over the
+remaining three-eighths darkness reigns supreme.</p>
+
+<div class='lg-container-b c015'>
+  <div class='linegroup'>
+    <div class='group'>
+      <div class='line'>“There is no light in earth or heaven,</div>
+      <div class='line in2'>But the cold light of stars.”</div>
+    </div>
+  </div>
+</div>
+
+<p class='c007'>Satisfactory confirmation of this curious result was obtained
+by Mr. Percival Lowell at the Flagstaff Observatory in
+Arizona during the autumn of 1896.<a id='r25'></a><a href='#f25' class='c027'><sup>[25]</sup></a> In Schiaparelli’s map,
+the axis of rotation lies in the plane of the paper, and the centre
+of the projected sphere thus represents the point on Mercury’s
+surface where the sun is vertical at perihelion and aphelion;
+A and B, 23° 41′ to the east and west of it respectively, marking
+the places where the sun is vertical at the libration-limits.
+That formidable luminary oscillates from the zenith of A to the
+zenith of B and back in 88 days, occupying, in consequence of
+the planet’s unequal motion, 51 in describing the arc from east
+to west (left to right), but only 37 in retracing it from west to
+east.<a id='r26'></a><a href='#f26' class='c027'><sup>[26]</sup></a></p>
+
+<p class='c007'>The effects of these arrangements upon climate must be
+exceedingly peculiar. They cannot readily be traced in
+detail; but, thin as the Mercurian atmosphere is, it must be
+to some extent operative in modifying the contrast in temperature
+between the two hemispheres. Except in a few
+favoured localities, the existence of liquid water must be
+impossible in either. Mercurian oceans, could they ever have
+been formed, should long ago have been boiled off from the
+hot side, and condensed in “thick-ribbed ice” on the cold side.</p>
+
+<p class='c007'>Mercury is then, according to our ideas, totally unfitted to
+be the abode of organic life. Nor can it at any time have
+been more favourably circumstanced than at present. We
+need not hesitate to assert that its rotation was reduced to its
+actual minimum rate by the power of tidal friction. The
+brake was, moreover, applied by the sun. The attainment
+of rapid gyration was prevented by the resistance of solar
+tides raised on a plastic mass. Disruption was accordingly
+<span class='pageno' id='Page_278'>278</span>rendered impossible. The planet was, by anticipation,
+deprived of satellites, and remained undivided and solitary.</p>
+
+<p class='c007'>Venus, the earths nearest planetary neighbour, might be
+called its twin. Its diameter being 7,700 miles, it is of nearly
+the same size; it is not greatly inferior in mean density;
+gravity at its surface is of more than four-fifths its terrestrial
+strength, and it is supplied with an extensive atmosphere.
+Its movements are placid and well-regulated. In a period of
+225 days it revolves at the rate of 22 miles per second in an
+almost circular track, deviating but slightly from the plane of
+the ecliptic. Its distance from the sun is 67,200,000 miles;
+hence it receives just twice as much heat and light as the
+earth. Moreover, it reflects at least 65 per cent. of the light
+incident upon it. Viewed in the same telescopic field with
+Mercury during a close conjunction in 1878, it shone, James
+Nasmyth reported, like burnished silver, while Mercury
+appeared as dull as zinc or lead. Yet Mercury is illuminated,
+on an average, three and a half times more intensely than its
+neighbour.</p>
+
+<p class='c007'>Atmospheric effects are conspicuous on Venus. At the
+beginning and end of transits, the part of the little black disc
+off the sun, has constantly been seen silver-edged through
+refraction; and when the planet, at inferior conjunction, passes
+above or below the sun, its whole circumference is not unfrequently
+bordered with a halo of solar rays, bent inwards as
+if by the action of a lens. Just in the same way, the
+<i>geometrical</i> rising of the heavenly bodies is <i>visually</i> anticipated,
+and their setting delayed on the earth, by the curvature
+of the beams refracted in passing through its atmosphere—or
+rather, through half of it; while we, as spectators of Venus
+from the outside, perceive the entire effect. Made on equal
+terms, the comparison is greatly to the disadvantage of the
+earth. Refraction, as directly measured on Venus, considerably
+exceeds its terrestrial amount; and the measurable
+refraction is only that produced in the higher part of the air
+surmounting the shell of clouds which constitutes the planet’s
+visible surface. Thus, at the cloud-level a barometer would,
+<span class='pageno' id='Page_279'>279</span>by the lowest estimate, stand at 35 inches, while at the
+same altitude of, say, two miles, the column of mercury would,
+on the earth, drop to 21 inches. It is, indeed, very likely
+that the aerial envelope of Venus weighs twice as much as
+our own.</p>
+
+<p class='c007'>The occasional visibility of the dark side of Venus is still
+unexplained. The appearance is indistinguishable except in
+scale from that of the “old moon in the new moon’s arms”;
+but illumination by earthshine, which is fully competent to
+produce the lunar effect, practically vanishes at the distance
+of Venus. The “ashen light,” as it is called, ordinarily
+shows only when the planet figures as a narrow crescent;
+but M. Brenner of the Manora Observatory, who has a
+knack of being unprecedented, saw it in June, 1895,<a id='r27'></a><a href='#f27' class='c027'><sup>[27]</sup></a> during
+the gibbous phase. The appearances of this pale gleam
+follow no traceable law. They occur unsought; and are
+recalcitrant to vigilant expectation. Their closest analogy
+is with our auroræ. The “phosphorescence” of the dark side
+of Venus may quite reasonably be set down as of an electrical
+nature. But it does not seem, like terrestrial auroræ, to
+follow the lines of a magnetic system.</p>
+
+<p class='c007'>Distinct spectroscopic indications of aqueous absorption in
+the atmosphere of Venus were perceived, during the transits
+of 1874 and 1882, by Tacchini, Riccò, and Young. They
+accord well with the “snow-caps,” which are one of the many
+puzzling Cytherean features. Since these can be resolved
+into groups of brilliant points, they represent, in the opinion
+of the late M. Trouvelot, mountainous formations penetrating
+the reflective stratum, and shining, lustrous with snow, in the
+clear upper air. They might almost equally well be cloud-like
+condensations of a permanent kind, called into existence
+by topographical peculiarities, and hence, after a fashion,
+<i>rooted in the soil</i>. On the other hand, Mr. Lowell questions
+their reality in any form; and his drawings represent extraordinarily
+sure seeing.</p>
+
+<div id='i_280'  class='figcenter id001'>
+<span class='pageno' id='Page_280'>280</span>
+<img src='images/i_280.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 9.</span>—<i>Venus, from a drawing by Mascari.</i> (<cite>Nature</cite>, February 20, 1896.)</p>
+</div>
+</div>
+
+<p class='c007'>The only point regarding the planet’s rotation upon
+which astronomers are agreed is that its axis is nearly
+perpendicular to the place of its orbit. As to its period,
+the divergence is enormous. It reaches all the way from 24
+hours to 225 days. Bad as is the telescopic holding-ground
+on Mercury, that afforded by Venus is worse still. The disc
+falls off rapidly in brightness from the limb towards the
+terminator, and is sometimes diversified by filmy and indefinite
+markings, obviously of atmospheric origin (in Fig.
+9 the shadings are much too pronounced).
+Attempts to use them as
+fiducial points are foredoomed to
+failure. The period, accordingly, of
+23<sup>h</sup> 21<sup>m</sup> arrived at by forcing into
+artificial agreement the observations
+of Cassini at Bologna, of Bianchini
+and De Vico in Rome, obtained
+small credit. The subject lay, as it
+were, dormant until Schiaparelli made,
+in 1890, the provisional announcement
+that Venus rotates on the same plan
+as Mercury. A clamour of contradiction
+was immediately raised, and a large amount of evidence on
+both sides of the question has since been collected. It is curious
+to notice that, setting aside the opposite conclusions of
+Terby and Brenner, the Alps mark a dividing-line between
+the pros and the cons. Schiaparelli’s period of 224·7 days
+(ratified by himself in 1895) is supported by Perrotin’s observations
+both at Nice and Mont Mounier; by Tacchini’s
+at Rome, Cerulli’s at Teramo, and Mascari’s at the complementary
+establishments of Catania and Mount Etna; while
+Niesten, Trouvelot, Villiger, Stanley Williams, and Flammarion,
+all under some disadvantage as regards climate,
+aver that the debated gyration is performed in “about”
+24 hours. Now, in the first place, a period of 24 hours is
+in itself open to suspicion, since all delicate observations are
+liable to be affected by diurnal atmospheric variations; in
+the second, it is mainly, if not entirely, based upon supposed
+<span class='pageno' id='Page_281'>281</span>changes in almost evanescent shadings, while the long period
+of 224·7 days has been derived fundamentally, from the immobility
+relative to the terminator, of definite and permanent
+topographical features. The perfect roundness of the disc of
+Venus affords independent proof of extremely slow rotation.</p>
+
+<p class='c007'>Spectroscopic evidence may before long become available.
+The quantity to be measured by the exquisite method of line-displacements
+is, indeed, at the most extremely small. The
+equatorial velocity of Venus would, with the 24-hour period,
+but slightly exceed a quarter of a mile a second; but this
+effect being doubled by reflexion from the planet, and doubled
+again by juxtaposition of light from its east and west limbs,
+could probably be made distinctly perceptible. In the negative
+case, the value of the support lent to the long-period hypothesis
+can only be appraised by the degree of refinement
+attained in the research.</p>
+
+<p class='c007'>The “long-period hypothesis” has, however, almost ceased
+to need such support. Schiaparelli’s facts are inconsistent
+with any other; and they are scarcely controvertible. They
+have besides, as in the case of Mercury, been verified by
+Mr. Lowell’s recent observations. Assuming, then, its truth,
+we may consider what it implies. Since the rotation
+and revolution of Venus synchronise, she always looks
+inwards toward the sun, perpetual day reigning on one
+hemisphere, perpetual night on the other. And these regulations
+are much more strictly conformed to than on Mercury.
+For the orbital motion of Venus is so nearly uniform that
+libratory effects count for very little. The equatorial breadth
+of the libration-zones, where light alternates with darkness, is
+only thirty-three miles. On the other hand, the atmospheric
+diffusion of sunshine is a powerful illuminating agency. The
+meteorology of the planet presents great difficulties. Its
+conditions are so remote from our experience that we can
+barely sketch out their results. The most obvious of these is
+the vehement aerial circulation which must proceed without
+ceasing between the hemisphere upon which the sun never
+rises and the hemisphere upon which the sun never sets. We
+<span class='pageno' id='Page_282'>282</span>should expect it to be accompanied by agitated conflicts of
+winds, and surgings of the atmosphere from its lowest to its
+highest strata, betrayed by rendings of the brilliant condensation-canopy,
+by the rapid transport of torn scuds, and wheeling
+vortices of clouds. But nothing of all this is telescopically
+visible. The aspect of the morning star suggests serenity
+rather than interior tumult.</p>
+
+<p class='c007'>One of the most remarkable instances of persistent optical
+illusion refers to a supposed satellite of Venus. It was first
+seen by Fontana at Naples in 1645; it was last seen by
+Horrebow at Copenhagen in 1768; and the intermediate observations
+were numerous, usually careful, and apparently
+authentic. Yet the body, of which they affirmed the existence,
+was purely fictitious; and it is a suggestive circumstance that
+it never ventured into the field of view of an achromatic
+lens.</p>
+
+<p class='c007'>Comparing the two planets nearest to the sun, the first spontaneous
+impression is of astonishment at their unlikeness. One
+travels in an almost circular, the other in a highly eccentric
+orbit. One possesses a dense and extensive atmosphere; the
+other is barely gauze-clad, and is hence exposed to almost unmitigated
+extremes of temperature, while the conformation of
+its solid surface is left open to telescopic scrutiny, impeded
+only by the inconvenient glare of the sun. That surface is of
+a reddish hue, and absorbs more than four-fifths of the light
+with which it is flooded; the disc of Venus being, on the contrary,
+of a dazzling whiteness, and little less reflective than a
+summer cloud. Yet these two globes, so dissimilar individually,
+have apparently had the same destiny prepared for them.
+Deprived of all but a remnant of their rotation by the frictional
+resistance of sun-raised tides, they were debarred from
+the production of satellites, and subjected to what we, in our
+ignorance, might be apt to call fantastic climatal conditions.
+With due reserve it may be added that they have thus apparently
+been rendered unfit to be the abodes of highly developed
+organisms. Why this has been so ordained we are unable to
+conjecture; we must wait to know.</p>
+
+<div>
+  <span class='pageno' id='Page_283'>283</span>
+  <h3 class='c004'>CHAPTER V.<br> <span class='c014'>THE EARTH AND MOON.</span></h3>
+</div>
+
+<p class='c003'>The earth occupies a critical position in the solar system.
+Its greater distance from the sun preserved it from the fate of
+Mercury and Venus. The influence of solar tidal friction fell
+short of predominance over the terrestrial future. All that it
+could do was to defer to the latest possible moment (so to
+speak) the separation of the moon, the comparatively large
+size of which was doubtless due to this postponement. For
+a viscous body, such as the earth must then have been, can
+bear much more rotational strain than a less coherent mass;
+but when the strain comes to be relieved, the needful sacrifice
+of material is proportionally greater. The process of fission,
+instead of being a mere incident, becomes a catastrophe.
+The most violent explosions are precisely those which are
+longest delayed.</p>
+
+<p class='c007'>Had the earth then been situated a few millions of miles
+nearer to the sun there would have been, so far as we can see,
+no moon; and the terrestrial day and year would have been
+of equal length. This equalisation was rendered impossible
+by lunar influence.<a id='r28'></a><a href='#f28' class='c027'><sup>[28]</sup></a> We are indebted to our satellite for the
+alternations of day and night which make life possible. How
+this came about is quite clear upon some brief consideration.
+Lunar tides are now about three times more effective than
+solar tides, and at their origin the disproportion was enormous.
+Their power might be called exclusive. Now, how
+was that power exercised? Primarily, in compelling an
+agreement between the duration of the month and day—that
+duration, to begin with, being of only a few hours. The day
+might, and in the long run did, fall short, but it could not
+possibly get ahead of the month. Hence the earth’s rotation
+<span class='pageno' id='Page_284'>284</span>was for ages protected against the destructive agency of solar
+tidal friction. By the time that the moon left it, as it were,
+to take care of itself, the plastic stage, during which alone
+rapid change could take place, had passed, and the earth was
+solid and secure.</p>
+
+<p class='c007'>Thus, the axial rotation of our planet in twenty-four sidereal
+hours is the outcome of a delicate balance of relations established
+in the “deep backward and abysm of time.” Its shape
+matches, or has accommodated itself to the period, which
+has perhaps not varied much since the epoch when interior
+fires were first banked in by the formation of a rigid crust.
+The compression of rotating globes is so connected with the
+quickness of their spinning that one can be calculated from
+the other; and the earth’s theoretical compression, or ellipticity,
+is found to be practically identical with its measured
+ellipticity of about ¹⁄₂₉₃. Its mean diameter is 7,927 miles;
+the equatorial is 26 miles longer than the polar diameter;
+so that the globe is belted with a protuberance, 13 miles
+high, corresponding to the excess of centrifugal force at
+the Equator.</p>
+
+<p class='c007'>The heat by which it was originally maintained in a liquid
+condition is still in process of dissipation. A small part
+escapes year by year, but enough remains to keep the earth
+<i>alive</i> for ages to come. Were the supply exhausted, the
+oxygen of our air, and the water forming our oceans, would be
+rapidly absorbed, chemically and mechanically, and with
+them, vitality should disappear. Volcanic action, in some
+of its many forms, is accordingly a condition of existence.
+One unmistakable symptom of central fires still glowing is
+the increase of subterranean temperature. It averages one
+degree Fahrenheit for fifty-five feet of descent. Below two
+miles then, water can only remain liquid through the compulsion
+of the overlying strata, the slightest relaxation of
+which occasions it to flash explosively into steam; the
+devastating power of “super-heated” water being one of the
+chief causes of volcanic outbreaks. The growth of temperature
+downward cannot be supposed to proceed indefinitely; otherwise,
+<span class='pageno' id='Page_285'>285</span>a fabulous thermal state would be reached long before
+we got near the core of the globe; but the region of maximum
+heat depends upon an unknown quantity—that is, the
+lapse of time since the antique lava-globe began to crust over.
+Assuming it to be fifty million years, Lord Kelvin showed
+that the limiting temperature of about 5,400° F. is located not
+more than fifty miles from the surface. But 5,400° approaches
+the temperature of the electric arc, at which there is an
+all but universal vaporisation of material substances, and
+rocks liquefy while comparatively cool. Diabase, for instance,
+a typical basalt, is completely fluid at 2,200° F. On the other
+hand, the pressure at 50 miles beneath the earth’s surface is
+of inconceivable power; and it is employed in resisting the
+expansive tendency of heat. The condition of matter subjected
+to these opposing and potent influences we are unable
+to divine, and have no means of ascertaining. We do, however,
+know from the results of various astronomical lines of
+enquiry that the earth is effectively as rigid as steel. Its
+mean density is about five and a half times that of water, the
+entire globe being more than twice as heavy as if made of the
+ordinary surface rocks. This, however, is not surprising, since
+oxygen enters largely into the composition of the exterior
+strata, while the subjacent materials are likely to be in large
+measure metallic.</p>
+
+<p class='c007'>The epoch of the earth’s superficial solidification has again,
+quite lately, been under discussion. “The subject,” Lord
+Kelvin wrote, “is intensely interesting. I would rather know
+the date of the <i>Consistentior Status</i> than of the Norman
+Conquest; but it can bring no comfort in respect to the
+demand for time in palæontological geology. Helmholtz,
+Newcomb, and another (Kelvin) are inexorable in refusing
+sunlight for more than a score, or a very few scores of
+millions of years.”<a id='r29'></a><a href='#f29' class='c027'><sup>[29]</sup></a></p>
+
+<p class='c007'>Improved data having been substituted, the problem was
+solved anew, with the result of very notably diminishing the
+“age of the earth.” It is for the present fixed at twenty-four
+<span class='pageno' id='Page_286'>286</span>million years, and upon such strong evidence as to “throw the
+burden of proof upon those who hold to the vaguely vast
+age derived from sedimentary geology.”<a id='r30'></a><a href='#f30' class='c027'><sup>[30]</sup></a></p>
+
+<p class='c007'>The earth is the largest of the terrestrial planets; and it is
+specifically the heaviest of all the planets. Its compactness
+is more likely to be a consequence of a particular relation
+between internal temperature and pressure, than of a difference
+in chemical constitution.</p>
+
+<p class='c007'>The mass of its atmosphere can be directly determined.
+We have only to look at a barometer in order to gain the
+information that our “cloud of all-sustaining air” weighs as
+much as a universal ocean of mercury thirty inches in depth.
+The corresponding depth of air, were it of the same
+density throughout, would be nearly five miles. But it is <i>not</i>
+of the same density throughout. With each three and a
+half miles of ascent, atmospheric pressure is halved; and the
+interval is lessened by making due allowance for decrease of
+temperature upwards. To the succession of these tenuous
+strata, no definite end can be assigned. The duration of
+twilight shows that, above forty-five miles, they cease to
+reflect light; yet meteors can be set ablaze at heights up
+to 120 miles, through the resistance offered to their motion
+by air reduced to 1/250,000,000,000th its density at sea-level!</p>
+
+<p class='c007'>The cloud-bearing capability of the atmosphere has only of
+late been fully recognised. Ordinary cirrus float about five
+miles high. On December 4, 1894, an aeronaut, Dr. A.
+Berson, passed right through a bank of them at an altitude
+of five and a half miles, and was able to verify by actual
+contact their composition out of snow-flakelets.<a id='r31'></a><a href='#f31' class='c027'><sup>[31]</sup></a> But since
+1885, a still more delicate kind of floating formation has come
+within our acquaintanceship. “Luminous night-clouds” were
+first noticed by Ceraski; they have been systematically studied
+by O. Jesse of Berlin.<a id='r32'></a><a href='#f32' class='c027'><sup>[32]</sup></a> They appear long after sunset, between
+<span class='pageno' id='Page_287'>287</span>May and July, and derive their silvery radiance from
+the sun-rays which their elevated situation enables them to
+intercept, while all below is wrapt in darkness. Their height
+has been determined, from the comparison of photographs
+taken simultaneously at different places, to average fifty-one
+miles, and to range from fifty to fifty-four miles. They are
+an entirely new order of phenomenon.</p>
+
+<p class='c007'>This globe upon which we dwell is a great magnet. Its
+directive action upon the compass sufficiently proves the fact.
+But it is a magnet probably only by virtue of the electric
+currents which course round it. And since these currents
+originate from diverse interacting causes, the laws of terrestrial
+magnetism are necessarily complex. They are conditioned,
+yet not prescribed by the earth’s rotation. The magnetic
+and geographical systems of co-ordinates approximate,
+but by no means coincide. The former is, indeed, both
+complex and variable.<a id='r33'></a><a href='#f33' class='c027'><sup>[33]</sup></a> The inclination, or “dip,” of the
+needle does not vary in the same way as the declination,
+or horizontal position. There are two points on the earth’s
+surface, called “poles of verticity,” where a magnetic needle,
+freely swung, points vertically downward. One is situated
+in the arctic peninsula Boothia, the other on the antarctic
+continent within a few hundred miles of Mount Erebus.
+An intermediate line where the needle poises itself
+horizontally, corresponds roughly with the geographical
+equator. Each hemisphere contains besides two centres of
+maximum force, by the joint action of which magnetic
+deviations from true north and south are determined. Their
+mutual relations are highly intricate. The North American
+focus is stationary, the Siberian focus oscillates. Their
+relative and absolute intensity is probably also subject to
+fluctuations. Hence the inconstancy of magnetic directive
+influences. The variation of the compass varies.</p>
+
+<p class='c007'>It varies hour by hour, as well as year by year. The
+needle performs a diurnal oscillation, reaching an eastward
+maximum about eight <span class='fss'>A.M.</span>, and a corresponding westward
+<span class='pageno' id='Page_288'>288</span>maximum towards four <span class='fss'>P.M.</span> Moreover, the range of this
+vibration increases concordantly with the growth of spotted
+area upon the sun, and falls off again as spots diminish (see
+Fig. 2). The cosmical relations of terrestrial magnetism are
+emphasised by the obvious connexion between a disturbed
+state of the sun and the occurrence of “magnetic storms.”
+During these crises, the smooth progression and regression
+of the needle are superseded by violent and irregular movements.
+The photographic tracing in which they are recorded
+presents only a series of lawless zigzags; earth-currents are
+set up; telegraph-wires transmit messages without batteries;
+and the skies are at night draped with auroral streamers.</p>
+
+<p class='c007'>Auroræ are possibly a survival of our planet’s original
+self-luminosity. If so, their dependence upon the terrestrial
+magnetic system is highly significant. They obey the
+magnetic period, they accompany magnetic disturbances,
+they illuminate magnetic lines of force. That they are immediately
+caused by electrical discharges in the high vacua of
+our upper air is no longer doubtful. In these latitudes, the
+auroral arch and crown are formed at a height of ninety
+to one hundred miles, in (about) 1/1,000,000,000th of an
+atmosphere; but in the polar regions they approach much
+nearer to the earth. There, indeed, they more usually assume
+the form of a curtain, undulating in luminous folds, and
+traversed by vertical electric currents. That they are so
+traversed is demonstrated by the behaviour of the magnetic
+needle, the deviations of which change their sign as the
+auroral drapery crosses the zenith.<a id='r34'></a><a href='#f34' class='c027'><sup>[34]</sup></a> Auroræ seem to be confined
+to two zones of the earth, which, like the sun-spot zones,
+approach the equator as the solar cycle advances. Their
+frequency in temperate regions corresponds, accordingly, to a
+scarcity in high latitudes. The auroral spectrum consists of
+a number of bright rays, one of which is invariably present,
+and seems to be essential and fundamental. Its origin is unexplained.</p>
+
+<p class='c007'>The velocity of the earth in its orbit exceeds more than
+<span class='pageno' id='Page_289'>289</span>sixty times that of a cannon ball just leaving the muzzle of an
+eighty-ton gun. In other terms, the third planet from the sun
+travels at an average rate of 18½ miles per second. Its albedo
+has been estimated—probably under-estimated—at 0·30. This
+would leave 70 per cent. of the solar emanations striking the
+upper surface of its atmosphere available for interior consumption.
+Most of this supply is absorbed or scattered in the
+atmosphere. The proportion sent back to space after reflection
+from the actual terrestrial surface must be extremely
+small. Very little topographical detail could be made out by
+telescopic scrutiny from the moon or Venus. At the most,
+the trend of some great mountain ranges, such as the
+Andes and Himalayas, and a dozen snow-clad peaks, could
+be visible. No sign of the teeming organic life brought forth
+by mother earth could be detected from without.</p>
+
+<p class='c007'>The more we know of the moon, the less inviting, from our
+point of view as animated beings, it appears. It is a harsh
+and inhospitable world, from which vital possibilities, if they
+were ever present, have plainly long ago departed. The
+diameter of our satellite is 2,162 miles. Its disc, so far as
+the most exact measurements tell, is perfectly round. This
+in itself indicates a slow rotation; and even casual observations
+suffice to show that they relate to only one lunar hemisphere.
+Rotation and revolution here again synchronise. In
+27 days 8 hours (nearly), the moon executes one circuit of
+the earth, and one gyration on its axis. The coincidence was
+brought about in remote ages by the power of terrestrial tidal
+friction. The averted hemisphere does not, however, remain
+wholly invisible. Two-elevenths of it are, by the effect of
+librations, both in longitude and latitude, brought piecemeal
+into view. But the additional “lunes,” thus thrown open to
+glimpses round the corner, are greatly foreshortened.</p>
+
+<p class='c007'>The area of the moon is somewhat less than one-thirteenth
+that of the earth. Yet room could be found there for the
+entire British Empire, with six million square miles to spare.
+Its volume is ¹⁄₄₉th, its mass ¹⁄₈₂th, the volume and mass of
+the earth. Hence the lunar materials are less dense than
+<span class='pageno' id='Page_290'>290</span>the terrestrial in the proportion of about three to five. But
+this may be because they are under comparatively slight
+pressure.</p>
+
+<p class='c007'>At the moon’s surface, gravity possesses only one-sixth its
+power here, so that a stone thrown upward with equal force
+would reach a six-fold height. Further, a projectile shot
+straight from our satellite with a velocity of one and a half
+miles a second would never return, while a speed of seven
+miles a second is just controllable by the earth, to say nothing
+of the immense efficacy of her dense atmosphere in hindering
+escape from her precincts. No terrestrial bomb, it may
+therefore be safely asserted, has ever been hurled into space,
+although volcanic ejecta may very well, in past ages, have
+made their way hither from the moon.</p>
+
+<p class='c007'>But lunar volcanoes are no longer active. Only their remains
+stand as records of a fiery past. In guiding a telescope
+across the scarred face of our satellite we seem to traverse a
+volcanic charnel-house. The evidence of ancient seismic
+action on the moon is overwhelming. Its surface is pitted all
+over with cones and craters. Nearly 33,000 are marked on
+Schmidt’s map, and the list is very far from being exhaustive.
+The resulting chiaroscuro is obvious to the naked eye.
+Dante tried to explain it in the “Divina Commedia”; Galileo
+detected its cause and manner of composition. The chief
+facts about it are these.</p>
+
+<div id='i_291'  class='figcenter id001'>
+<span class='pageno' id='Page_291'>291</span>
+<img src='images/i_291.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 10.</span>—<i>Map of the Moon.</i> (From Fowler’s “Telescopic Astronomy.”)</p>
+</div>
+</div>
+
+  <dl class='dl_2'>
+    <dt> 1.</dt>
+    <dd>Furnerius
+    </dd>
+    <dt> 2.</dt>
+    <dd>Petavius
+    </dd>
+    <dt> 3.</dt>
+    <dd>Langrenus
+    </dd>
+    <dt> 4.</dt>
+    <dd>Macrobius
+    </dd>
+    <dt> 5.</dt>
+    <dd>Cleomedes
+    </dd>
+    <dt> 6.</dt>
+    <dd>Endymion
+    </dd>
+    <dt> 7.</dt>
+    <dd>Altas
+    </dd>
+    <dt> 8.</dt>
+    <dd>Hercules
+    </dd>
+    <dt> 9.</dt>
+    <dd>Römer
+    </dd>
+    <dt>10.</dt>
+    <dd>Posidonius
+    </dd>
+    <dt>11.</dt>
+    <dd>Fracastorius
+    </dd>
+    <dt>12.</dt>
+    <dd>Theophilus
+    </dd>
+    <dt>13.</dt>
+    <dd>Piccolomini
+    </dd>
+    <dt>14.</dt>
+    <dd>Albategnius
+    </dd>
+    <dt>15.</dt>
+    <dd>Hipparchus
+    </dd>
+    <dt>16.</dt>
+    <dd>Manilius
+    </dd>
+    <dt>17.</dt>
+    <dd>Eudoxus
+    </dd>
+    <dt>18.</dt>
+    <dd>Aristotle
+    </dd>
+    <dt>19.</dt>
+    <dd>Cassini
+    </dd>
+    <dt>20.</dt>
+    <dd>Aristillus
+    </dd>
+    <dt>21.</dt>
+    <dd>Plato
+    </dd>
+    <dt>22.</dt>
+    <dd>Archimedes
+    </dd>
+    <dt>23.</dt>
+    <dd>Eratosthenes
+    </dd>
+    <dt>24.</dt>
+    <dd>Copernicus
+    </dd>
+    <dt>25.</dt>
+    <dd>Ptolemy
+    </dd>
+    <dt>26.</dt>
+    <dd>Alphonsus
+    </dd>
+    <dt>27.</dt>
+    <dd>Arzachel
+    </dd>
+    <dt>28.</dt>
+    <dd>Walter
+    </dd>
+    <dt>29.</dt>
+    <dd>Clavius
+    </dd>
+    <dt>30.</dt>
+    <dd>Tycho
+    </dd>
+    <dt>31.</dt>
+    <dd>Bullialdus
+    </dd>
+    <dt>32.</dt>
+    <dd>Schiller
+    </dd>
+    <dt>33.</dt>
+    <dd>Schickard
+    </dd>
+    <dt>34.</dt>
+    <dd>Gassendi
+    </dd>
+    <dt>35.</dt>
+    <dd>Kepler
+    </dd>
+    <dt>36.</dt>
+    <dd>Grimaldi
+    </dd>
+    <dt>37.</dt>
+    <dd>Aristarchus
+    </dd>
+    </dl>
+
+  <dl class='dl_2'>
+    <dt>A.</dt>
+    <dd>Mare Crisum
+    </dd>
+    <dt>B.</dt>
+    <dd> Mare Fecunditatis
+    </dd>
+    <dt>C.</dt>
+    <dd> Mare Nectaris
+    </dd>
+    <dt>D.</dt>
+    <dd> Mare Tranquilitatis
+    </dd>
+    <dt>E.</dt>
+    <dd> Mare Serenitatis
+    </dd>
+    <dt>F.</dt>
+    <dd> Mare Imbrium
+    </dd>
+    <dt>G.</dt>
+    <dd>Sinus Iridum
+    </dd>
+    <dt>H.</dt>
+    <dd>Oceanus Procellarum
+    </dd>
+    <dt>I.</dt>
+    <dd>Mare Humorum
+    </dd>
+    <dt>K.</dt>
+    <dd> Mare Nubium
+    </dd>
+    <dt>V.</dt>
+    <dd>Altai Mountains
+    </dd>
+    <dt>W.</dt>
+    <dd>Mare Vaporum
+    </dd>
+    <dt>X.</dt>
+    <dd>Apennine Mountains
+    </dd>
+    <dt>Y.</dt>
+    <dd>Caucasus Mountains
+    </dd>
+    <dt>Z.</dt>
+    <dd>Alps
+    </dd>
+    </dl>
+
+<p class='c007'>The general albedo of the lunar surface is 0·17; but portions
+of the disc are as obscure as basalt or obsidian, while
+isolated spots glitter like snow-peaks. The former are usually
+admitted to be the oldest of conspicuous lunar formations,
+the latter to be comparatively recent. The dusky spaces
+too, are dead levels, if not depressions; they were formerly
+taken for seas, and retain the name of “Maria.” One “ocean,”
+extending over two million square miles, is included amongst
+them. This is the “Oceanus Procellarum” (see Fig.
+10), which is five times larger than its nearest rival, the
+“Mare Nubium.” The late Mr. Gwyn Elger regarded the
+lunar “seas” as lava outflows, by which certain earlier formations
+<span class='pageno' id='Page_292'>292</span>were all but obliterated. M. Suess explains them as
+areas where the primitive thin “slag-crust” re-melted. To
+the same category belong the vast “bulwark plains,” the
+ramparts enclosing which are of so wide a sweep as to be,
+not merely “hull-down,” but completely invisible to an imaginary
+spectator placed at their centres. Yet Pelions by the
+dozen are tumbled upon Ossas for their construction, with
+here and there an Olympus flung on the top. Typical examples
+are Ptolemæus, 115 miles across; and Plato (near the
+Northern Pole), “sixty miles in diameter, with its bright
+border and dark steel-grey floor.”<a id='r35'></a><a href='#f35' class='c027'><sup>[35]</sup></a></p>
+
+<p class='c007'>The bottoms of lunar craters and “circuses” are nearly
+always depressed—sometimes thousands of feet—below the
+general level. Thus, the central peak of the great crater
+Copernicus towers to 11,300 feet above the depressed plain
+from which it rises, but surmounts by only 2,600 feet the
+average level of the moon.</p>
+
+<p class='c007'>Successive stages of activity have left ineffaceable marks
+upon this now stereotyped page. Groups of immense craters
+mutually encroach, and seem to have been scooped out of
+each other’s flanks, like Kilauea from Mauna Loa; craters
+occur within craters, as Vesuvius inside the broken rampart
+of Somma; and the most recent are invariably the deepest
+and steepest. Cup-shaped depressions or “crater-pits”
+are innumerable; they result, according to Suess’s theory,<a id='r36'></a><a href='#f36' class='c027'><sup>[36]</sup></a>
+each from a single explosion, the bursting of a “big bubble”
+of gas in a cooling lava-field. Mountain ranges are profusely
+strewn with them. These lunar Alps and Apennines appear
+to be as unmistakably igneous in their origin as Tycho or
+Aristarchus. They are colossal slag-walls. There are apparently
+no sedimentary deposits upon the moon. Aqueous
+action had no concern with its geological history. Yet on
+the earth water is essential to the production of volcanic
+phenomena. If they are to be developed without it, M.
+Angelot concludes, it must be by explosive escapes from
+<span class='pageno' id='Page_293'>293</span>solidifying materials, of gases absorbed by them when in a
+state of fusion.</p>
+
+<p class='c007'>The mountains of the moon are much higher, proportionally,
+than the summits of the Hindu-Kush, or of the Himalayas.
+Mount Everest, reduced to the lunar scale, would be a
+modest elevation of 8,200 feet; while pinnacles in the lunar
+Apennines spring up to 22,000 feet, and crater-peaks of
+eighteen or twenty thousand abound. The disparity is
+scarcely surprising when it is remembered that there the convulsive
+throes of cooling were restrained by gravity reduced
+to one-sixth the power it exerts here.</p>
+
+<p class='c007'>Among the puzzles of selenography are the objects termed
+respectively “rills” and “rays” The former are very numerous.
+Considerably more than a thousand of them have been mapped
+or photographed. They resemble the cañons of Colorado.
+Some few run to 150 miles; most are a couple of miles wide,
+and above a quarter of a mile deep. Their volcanic origin
+cannot be doubted. The “rays” diverge in extensive systems
+from such huge ring-craters as Tycho and Copernicus.
+They cast no shadows, and come out best at full moon,
+circumstances suggestive of their being immemorial lava-streams
+bleached by the chemical action of fumes from the
+interior. The whiteness of Aristarchus has been similarly
+explained; but accumulations of pumice and snow-like
+volcanic ashes perhaps enhance the effect. The flashing back
+by this wonderful peak, of earthshine at determinate angles of
+illumination, has often counterfeited the vivid glow of actual
+eruptions. Their possibility, however, belongs to the past.
+Nor have any of the rumoured alterations in lunar topography,
+which from time to time excited interest and raised
+controversy, made good their footing as solid facts. Agencies
+of change are certainly there, in tidal strains and alternations
+of temperature, but they work very slowly. There is
+no erosion by air or water; no grinding by ice; no transport
+of materials. Repose reigns apparently undisturbed. Lunar
+landscapes exhibit abrupt transitions from the blinding
+glare of crude sunlight to the blackness of absolute shadow.
+<span class='pageno' id='Page_294'>294</span>Their aspect excludes any but the thinnest possible atmospheric
+remnant To all intents and purposes, the
+moon is an airless globe. Occultations of stars afford
+a very refined test of this condition; and their instantaneousness
+alone suffices to demonstrate its reality. Spectroscopic
+evidence is to the same effect. Dr. Huggins watched,
+January 4, 1865, a <i>prismatic</i> occultation of the small star,
+ε Piscium. Had there been the slightest inequality of
+dispersion or absorption at the moon’s limb, it could not
+have failed to be perceived. There was none. The spectrum
+remained unaffected, and vanished abruptly, all the colours
+together. And moonlight, analysed by the most powerful
+apparatus, varies not an iota from sunlight. It is reflected
+without the smallest selective change.</p>
+
+<p class='c007'>The absence of water is equally well attested. There are
+no river-beds to be seen, no rounded surfaces, no alluvial
+plains. A mosquito could not find a moist corner to lay its
+eggs in. There is nothing to show that this was otherwise in
+any past age, although it is not improbable that the lunar
+rocks contain large volumes of oxygen once free. As regards
+the earth, we can entertain no doubt that a goodly proportion
+of its original atmosphere and oceans is now permanently
+lodged in its bedded crust. But the geological histories of
+the earth and moon probably diverged from the first.</p>
+
+<p class='c007'>Indeed water, as such, could probably not exist upon the
+moon’s surface. It would promptly take the form of ice.
+Professor Langley has shown that the temperature prevailing
+there, under vertical sunshine, is about that of frost; while it
+sinks, during the moon’s long night of fourteen days, almost
+to absolute zero. This frigid state is due to the absence of
+atmospheric protection, leaving heat free to depart into space
+as fast as it is received. Thus, of the small quantity of heat
+contained in moonlight, nearly the entire comes to us by
+mere superficial reflection; a minute residuum only is absorbed
+previously to being emitted. The distinction is
+brought into view by comparing the solar and lunar heat-spectra,
+when moonlight is found to contain longer invisible
+<span class='pageno' id='Page_295'>295</span>heat waves than can be detected in sunlight Moreover,
+Professor Frank Very, through his experimental demonstration
+that the equatorial are slightly hotter than the polar regions,
+has established the fact of a slight retention of heat by
+the moon’s substance. How slight the retention is, has been
+proved by Dr. Boeddicker’s observations with the Rosse three-foot
+speculum, showing that, during total eclipses, moon-heat
+vanishes almost completely. Less than 1 per cent,
+survives. The thermal phases are not, however, identical
+with the luminous phases.</p>
+
+<p class='c007'>The eclipsed moon, on June 10, 1816, is said to have been
+utterly lost to sight; but, as a rule, with very few exceptions,
+our satellite traverses visibly the densest part of the earth’s
+shadow. Even during “black eclipses,” such as that of October
+4, 1884, a dusky spot remains as an index to its locality; while
+in “red eclipses,” the great craters and bulwark plains can be
+easily distinguished with an opera-glass. Occasionally, the
+moon seems turned to blood, and the people cry out in the
+streets with fear. Such a phenomenon was witnessed by the
+writer at Florence, February 27, 1877. Its explanation is
+not difficult The refractive power of the earth’s atmosphere
+suffices to bring illumination to the lunar disc at the very
+middle of the shadow-cone. It is shut off from direct solar
+rays, not from those that are bent into convergence by the
+lens of our air. That they must be reddened by the process,
+sunset-effects on the earth tell plainly enough. But when the
+air is vapour, or dust-laden, and consequently opaque, little
+light is transmitted, and a scarcely mitigated eclipse ensues.
+That of 1884 is believed to have been darkened by the outpourings
+from Krakatoa. A photograph by Professor Barnard,
+of the totally eclipsed moon, September 3, 1895, is reproduced
+in Fig. 11. It was one of a <i>search-series</i> for a lunar
+satellite. None was found: but the question of its possible
+existence was set at rest.</p>
+
+<p class='c007'>De la Rue’s and Rutherfurd’s plan of photographing the
+moon as a whole is no longer followed. Bit by bit photography,
+on a large scale, has superseded it. Splendid pictures
+<span class='pageno' id='Page_296'>296</span>of individual formations and separate regions have in this
+way been obtained, both at the Paris and the Lick Observatories;
+and their microscopic study has given some interesting
+results; yet it is undeniable that the “chemical retina”
+cannot here claim its usual superiority. “The best photograph
+of the moon ever taken,” Professor W. H. Pickering
+avers,<a id='r37'></a><a href='#f37' class='c027'><sup>[37]</sup></a> “will not show what can be seen with a six-inch
+telescope, under favourable atmospheric conditions. For
+general outlines, for completeness of the coarser detail, and for
+purposes of future testimony, the photograph evidently stands
+without a rival; but as regards that which is really most
+interesting upon the moon—the finer detail and more delicate
+features—the photograph does not even hint at their existence.”
+One of the most successful specimens of lunar
+photography forms the frontispiece to this volume. It was
+taken by MM. Loewy and Puiseux, with the large Coudé
+equatorial, February 14th, 1894, at 7<sup>h</sup> 27<sup>m</sup> Paris time, and
+cannot easily be surpassed in pictorial effect.</p>
+
+<div id='i_296'  class='figcenter id001'>
+<img src='images/i_296.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 11.</span>—<i>Photograph of the Totally Eclipsed Moon. By Professor Barnard. Exposure, 3 Minutes.</i></p>
+</div>
+</div>
+
+<p class='c007'>Atmospheric agitations are one cause of imperfection in
+<span class='pageno' id='Page_297'>297</span>lunar photographs. The eye can seize the instant of exquisite
+definition; the camera must take what comes. Then the disparities
+of actinic intensity in the various lunar formations are
+so wide that, in order to get an ideal picture, a different length
+of exposure should be given to each. What is enough for a
+plain—to take an example—is too much for the crater rising
+from it, or for the rampart enclosing it. Minute irregularities
+in the following motion of the telescope during the few
+seconds of exposure occasion further difficulties. A momentary
+shifting, by half a millimetre, of the image upon
+the sensitive plate, would suffice to blur the negative seriously,
+if not fatally. For this, as for several other lines of work, the
+instrument of the future may be of a type with which
+the equatorial has little in common. Professor Pickering
+considers it probable that “a horizontal telescope of three or
+four hundred feet focus, and twelve to fifteen inches aperture,
+would give the most satisfactory results. In such a
+case, it might be found best that the mirror should remain
+fixed during the exposure, while the plate was given an
+uniform motion by clock-work.”</p>
+
+<p class='c007'>The suggestion is one among many signs that a revolution
+in the mounting of telescopes is at hand.</p>
+
+<h3 class='c004'>CHAPTER VI.<br> <span class='c014'>THE PLANET MARS.</span></h3>
+
+<p class='c003'>The furthest terrestrial planet from the sun is Mars, the
+“star of strength.” No other heavenly body, except the
+moon, is so well placed for observation from our position in
+space. As a superior planet, it does not merely, like Mercury
+and Venus, oscillate about the sun, but is best seen
+when in opposition. It is then “full”; it crosses the
+<span class='pageno' id='Page_298'>298</span>meridian at midnight, and is at its least distance from the
+earth. These occasions recur every 780 days; but they are
+not all equally favourable. The opposition distance of the
+planet varies, owing to the eccentricity of its orbit, from
+thirty-five to sixty-one million miles; so that the area of
+the disc is three times larger when a perihelion than when an
+aphelion passage coincides with a midnight culmination.
+Under the best circumstances it is of the apparent dimensions
+of a half-sovereign 2,000 yards from the spectator.</p>
+
+<p class='c007'>The diameter of Mars is 4,200 miles; its surface is equal to
+two-sevenths, its volume to one-seventh those of the earth. But,
+in consequence of its inferior mean density, nine such spheres
+would go to make up the mass of our world. The superficial
+force of gravity on Mars, compared with its terrestrial value,
+is as thirty-eight to a hundred. A man could leap there a
+wall eight feet four inches in height with no more effort than
+it would cost him here to spring over a two-foot fence.</p>
+
+<p class='c007'>The planet’s rotation is performed in 24 hours 37 minutes on
+an axis deviating from the vertical by 240° 50′. Hence its
+seasons resemble our own, except in being nearly twice as
+long, for the Martian year is of 687 days. They are modified,
+too, by the considerable elongation of the ellipse traversed
+by Mars, causing a difference of 26½ millions of
+miles in its greatest and least distances from the sun. These
+are respectively 155 and 128½ millions of miles, the mean
+distance being 141½ millions. A polar compression of
+¹⁄₂₂₀ is just what should be expected from its rotatory
+speed. When at quadrature, it is plainly gibbous; but our
+interior position with regard to it makes it impossible that it
+should ever take the crescent form. Its albedo, according to
+Zöllner, is 0·26—a figure intimating that sunlight is reflected
+from no cloud-canopy, but by the soil itself. This atmospheric
+transparency leaves the door open for researches into
+the condition of a very curious little world.</p>
+
+<p class='c007'>The disc of Mars is diversified with three shades of colour—reddish,
+or dull orange, dark greyish-green, and pure white.
+The last shows mainly in two diametrically opposite patches.
+<span class='pageno' id='Page_299'>299</span>Each pole is surrounded by a brilliant cap, suggesting the
+deposition of ice or snow over the chilly spaces corresponding
+to our arctic and antarctic regions. Nor is this all. Each of
+the polar hoods shrinks to a mere remnant as the local
+summer advances, but regains its original size when wintry
+influences are again in the ascendant. Here, and nowhere
+else in the planetary system, we meet evidence of seasonal
+change; and seasonal change is associated with vital
+possibilities. Again, a globe upon which snow visibly melts
+must contain water; hence the green markings cannot but
+image to our minds seas and inlets sub-dividing continents,
+the blond complexion of which may be caused by some native
+peculiarity of the soil. It is in no way connected with vegetation,
+since it neither fades nor flushes with the advent of
+spring; and an atmospheric origin is excluded by the circumstance
+that it becomes effaced by a whitish haze near the
+limb, just where the densest atmospheric strata are traversed
+by the line of sight.</p>
+
+<p class='c007'>The spots on Mars are by no means so sharply defined as
+lunar craters and <i>maria</i>; yet they are fundamentally permanent.
+Some can be recognised from drawings made over
+two hundred years ago; and these antique records have served
+modern astronomers to determine with minute accuracy the
+rotation-period of the planet. There is accordingly no doubt
+that “areography”  has assured facts to deal with, although
+the facts are not quite as “hard” as they might be. Continents
+are somewhat vaguely outlined. Great tracts of them
+are of an uncertain and variable hue, as if subject to inundations.
+This peculiarity, thoroughly certified during the
+favourable opposition of 1892, makes a strong distinction
+between Mars and the Earth. Terrestrial oceans keep within
+the limits assigned to them. On the neighbouring planet—as
+M. Faye observed in 1892—“Water seems to march about
+at its ease,” flooding, from time to time, regions as wide as
+France. The imperfect separation of the two elements recalls
+the conditions prevailing during the terrestrial carboniferous
+era.</p>
+
+<div id='i_300'  class='figcenter id003'>
+<span class='pageno' id='Page_300'>300</span>
+<img src='images/i_300.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 12.</span>—<i>Chart of Mars on Mercator’s Projection.</i><br>  <br>  (From Proctor’s “Old and New Astronomy.”)</p>
+</div>
+</div>
+
+<p class='c007'>The main part of the land of Mars is situated in the
+northern hemisphere. It covers two-thirds of the entire
+globular surface. Rather than land, indeed, it should be
+called a network of land and water. Fig. 12, from a chart
+<span class='pageno' id='Page_301'>301</span>by Schiaparelli, illustrates the remarkable fashion of their
+intermixture. The great continental block—so its orange
+tint declares it to be—is cut up in all possible directions by
+an intricate system of what appear to be waterways, running
+in perfectly straight lines—that is, along great circles of the
+globe—for distances varying from 350 to upwards of 4,000
+miles. They are frequently seen in duplicate, strictly parallel
+companions developing thirty to three hundred miles apart
+from the original formations. This mysterious phenomenon
+is evanescent, or rather periodical. Canal-duplication is a
+recurrent change, depending upon the Martian seasons, and
+becoming obvious, according to Schiaparelli, chiefly near the
+equinoxes.</p>
+
+<p class='c007'>The canals invariably connect two bodies of water; hence
+they need no locks or hydraulic machinery; their course is
+on a dead level. The broadest of them are comparable with
+the Adriatic; those at the limit of visibility, stretching like
+the finest spider-threads across the disc, have a width of
+eighteen miles. “The canals,” Schiaparelli says, “may intersect
+among themselves at all possible angles, but by preference
+they converge towards the small spots to which we
+have given the name of lakes. For example, seven are seen
+to converge in Lacus Phoenicis, eight in Trivium Charontis,
+six in Lunae Lacus, and six in Ismenius Lacus.”<a id='r38'></a><a href='#f38' class='c027'><sup>[38]</sup></a></p>
+
+<p class='c007'>These “lakes” evidently form an integral part of the canal
+system. They resemble huge railway-junctions; and the
+largest of them—the “Eye of Mars” (Schiaparelli’s Lacus
+Solis)—seems, in Mr. Lowell’s phrase, like the hub of a five-spoked
+wheel. It is depicted in Fig. 13 from a drawing
+made by Professor Barnard with the great Lick refractor,
+September 3, 1894. Mr. W. H. Pickering in 1892, and Mr.
+Percival Lowell in 1894, were amazed at their extraordinary
+abundance.</p>
+
+<p class='c007'>“Scattered over the orange-ochre groundwork of the
+continental regions of the planet,” the latter wrote, “are any
+number of dark, round spots. How many there may be it is
+<span class='pageno' id='Page_302'>302</span>not possible to state, as the better the seeing, the more of
+them there seem to be. In spite, however, of their great
+number, there is no instance of one occurring unconnected
+with a canal. What is more, there is apparently none which
+does not lie at the junction of several canals. Reversely, all
+the junctions appear to be provided with spots.”</p>
+
+<div id='i_302'  class='figcenter id003'>
+<img src='images/i_302.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 13.</span>—<i>The “Eye of Mars,” drawn by Prof. Barnard with the great Lick Refractor. The southern snow-cap is visible much shrunken by melting.</i></p>
+</div>
+</div>
+
+<p class='c007'>Most of these foci are
+about 120 miles in diameter,
+and appear most
+precisely circular when
+most clearly seen.
+“Plotted upon a globe,”
+Mr. Lowell continues,
+“they and their connecting
+canals make a most
+curious network over all
+the orange-ochre equatorial
+parts of the planet,
+a mass of lines and knots,
+the one marking being
+as omnipresent as the
+other. Indeed, the spots are as peculiar and distinctive a
+feature of Mars as the canals themselves.”</p>
+
+<p class='c007'>Like the canals, too, they emerge periodically, and in the
+same but a retarded succession. They “are therefore, in the
+first place, seasonal phenomena, and, in the second place,
+phenomena that depend for their existence upon the prior
+existence of the canals.”<a id='r39'></a><a href='#f39' class='c027'><sup>[39]</sup></a></p>
+
+<p class='c007'>Mr. Lowell terms them “oases” (see Fig. 14), and does
+not shrink from the full implication of the term.</p>
+
+<p class='c007'>The most important result of the numerous observations
+of Mars, made during the oppositions of 1892 and 1894, was
+the recognition of a regular course of change dependent upon
+the succession of its seasons. Schiaparelli had long anticipated
+this result; he is commonly in advance of his time.
+These changes, moreover, when closely watched, are really
+<span class='pageno' id='Page_303'>303</span>self-explanatory. The alternate melting of the northern and
+southern snow-caps initiates, and to some extent determines
+them. As summer advances in either hemisphere, the wasting
+of the corresponding white calotte can be followed in
+every minute particular. “The snowy regions are then seen
+to be successively notched at their edges; black holes and
+huge fissures are formed in their interiors; great isolated
+fragments many miles in extent stand out from the principal
+mass, dissolve, and disappear a little later. In short, the
+same divisions and movements of these icy fields present
+themselves to us at a glance that occur during the summer of
+our own arctic regions.”<a id='r40'></a><a href='#f40' class='c027'><sup>[40]</sup></a></p>
+
+<p class='c007'>Indeed, glaciation on Mars is much less durable than on
+the earth. In 1894, the southern snow-cap vanished to the
+last speck 59 days after the solstice; and the remnant usually
+left looks scarcely enough to make a comfortable cap for Ben
+Nevis. An immense quantity of water is thus set free. The
+polar seas overflow; gigantic inundations reinforced, doubtless,
+from other sources, spread to the tropics; Syrtis regions
+of marsh or bog deepen in hue, and become distinctly
+aqueous; canals dawn on the sight, and grow into undeniable
+realities. We seem driven to believe that they discharge the
+function of flood-emissaries.</p>
+
+<p class='c007'>Mr. Lowell does not hesitate to pronounce them of artificial
+formation, and, on that large assumption, the purpose of their
+connexion with his “oases” becomes transparently clear.
+They bring to these Tadmors in the wilderness the water
+supply by which they are made to “blossom as the rose.”
+The junction-spots, we are told, do not enlarge when the
+vernal freshet reaches them; they only darken through the
+sudden development of vegetation. These circular “districts,
+artificially fertilised by the canal system,” are strewn broadcast
+over vast desert areas, the orange-ochreous sections
+of Mars, covering the greater part of its surface, but deep
+buried in the millennial dust of disintegrated red sandstone
+strata.</p>
+
+<p class='c007'><span class='pageno' id='Page_304'>304</span>“Here, then,” Mr. Lowell remarks,<a id='r41'></a><a href='#f41' class='c027'><sup>[41]</sup></a> “we have an end and
+reason for the existence of canals, and the most natural conceivable—namely,
+that the canals are constructed for the express
+purpose of fertilising the oases. When we consider the
+amazing system of the canal lines, we are carried to this
+conclusion as forth-right as is the water itself; what we see
+being not the canal itself, indeed, but the vegetation along its
+banks.”</p>
+
+<div id='i_304'  class='figcenter id001'>
+<img src='images/i_304.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 14.</span>—<i>The Oases of Mars. Drawn by Percival Lowell.</i><br>  <br>  (From “Popular Astronomy,” April, 1895.)</p>
+</div>
+</div>
+
+<p class='c007'>The idea that we see the water only by its effects along the
+shores of these prodigious troughs, originated with Professor
+W. H. Pickering. It is strikingly illustrated by the aspect of
+rivers from a balloon. Thus the Rhine, as M. Flammarion
+<span class='pageno' id='Page_305'>305</span>attests,<a id='r42'></a><a href='#f42' class='c027'><sup>[42]</sup></a> seen from a perpendicular altitude of 8,000 feet,
+shows like a green thread drawn in the midst of a ribbon of
+meadow. The Martian canals, it is suggested, correspond
+to the “ribbon of meadow.”</p>
+
+<p class='c007'>The hypothesis is seductive, but should not be hastily
+adopted. It gives no account of the doubling of the canals,
+yet the process takes place on a grand scale, at determinate
+epochs, and under fairly well ascertained conditions. It undoubtedly
+belongs to the series of vernal changes going forward
+upon the planet, and is accomplished with amazing rapidity.
+A single canal may be transformed into a double canal within
+twenty-four hours, and that simultaneously along its whole
+course. The two stripes, so curiously substituted for one, “run
+straight and equal with the exact geometrical precision of the
+two rails of a railroad.”<a id='r43'></a><a href='#f43' class='c027'><sup>[43]</sup></a> The tendency is shared by the lakes
+or “oases.” “One of these,” we learn from the same authority,
+“is often seen transformed into two short, broad dark lines
+parallel to one another, and traversed by a yellow line.”</p>
+
+<p class='c007'>This singular principle of subdivision offers at present no
+hold for profitable speculation. Schiaparelli trusts to the
+“courtesy of nature” for some ray of light by which, in the
+future, to penetrate the mystery; but wisely deprecates recourse
+being had to the intervention of intelligent beings.
+Such arbitrary modes of dealing with perplexing problems
+constitute, as he says, a grave obstacle to the acquisition of
+just notions concerning them. They raise prepossessions by
+which the progress of genuine research is impeded.</p>
+
+<p class='c007'>The proportion of water to land is much smaller on Mars
+than on the earth. Only two-sevenths of the disc are
+covered by the dusky areas, and of late the aqueous nature
+of some, if not all of these, has been seriously called in
+question. Professor Pickering was convinced by his observations,
+in 1892 and 1894, “that the permanent water area upon
+Mars, if it exist at all, is extremely limited in its dimensions.”<a id='r44'></a><a href='#f44' class='c027'><sup>[44]</sup></a>
+<span class='pageno' id='Page_306'>306</span>He estimated it at about half the size of the Mediterranean.
+Professor Schaeberle is similarly incredulous. If the dark
+markings are seas, he asks, how explain the irregular gradations
+of shade in them?<a id='r45'></a><a href='#f45' class='c027'><sup>[45]</sup></a> How, above all, explain their
+apparent intersection by well-marked canals? Professor
+Barnard, observing with the Lick thirty-six inch in 1894,
+discerned on the Martian surface an astonishing wealth of
+detail, “so intricate, small, and abundant, that it baffled all
+attempts to properly delineate it.”<a id='r46'></a><a href='#f46' class='c027'><sup>[46]</sup></a> It was embarrassing
+to find these minute features belonging more characteristically
+to the “seas” than to the “continents.” Under the
+best conditions, the dark regions lost all trace of uniformity.
+Their appearance resembled that of a mountainous country,
+broken by cañon, rift, and ridge, seen from a great elevation.
+These effects were especially marked in the “ocean” area of
+the hour-glass sea.</p>
+
+<p class='c007'>Evidently the relations of solid and liquid in that remote
+orb are abnormal; they cannot be completely explained by
+terrestrial analogies. Yet a series of well-attested phenomena
+are intelligible only on the supposition that Mars is, in some
+real sense, a terraqueous globe. Where snows melt there
+must be water; and the origin of the Rhone from a great
+glacier is scarcely more evident to our senses than the dissolution
+of Martian ice-caps into pools and streams.</p>
+
+<p class='c007'>The testimony of the spectroscope is to the same effect.
+Dr. Huggins found, in 1867, the spectrum of Mars impressed
+with distinct traces of aqueous absorption, and the fact,
+although called in question by Professor Campbell of Lick,
+in 1894, has been re-affirmed both at Tulse Hill and at
+Potsdam. That clouds form and mists rise in the thin
+Martian air, admits of no doubt. During the latter half of
+October, 1894, an area much larger than Europe remained
+densely obscured. Whether or no actual rain was at that
+time falling over the Maraldi Sea and the adjacent continent,
+it would be useless to conjecture. We only know that with
+<span class='pageno' id='Page_307'>307</span>the low barometric pressure at the surface of Mars, the boiling
+point of water must be proportionately depressed (Flammarion
+puts it at 115° Fahrenheit), which implies that it
+evaporates rapidly, and can be transported easily.</p>
+
+<p class='c007'>If the Martian atmosphere be of the same proportionate
+mass as that of our earth, it can possess no more than
+one-seventh its superficial density. That is to say, it is
+more than twice as tenuous as the air at the summits of
+the Himalayas.<a id='r47'></a><a href='#f47' class='c027'><sup>[47]</sup></a> The corresponding height of a terrestrial
+barometer would be four and a half inches. Owing, however,
+to the reduced strength of gravity on Mars, this slender
+envelope is exceedingly extensive. In the pure sky scarcely
+veiled by it, the sun, diminished to less than half his size at
+our horizons, probably exhibits his coronal streamers and
+prominences as a regular part of his noontide glory;
+atmospheric circulation proceeds so tranquilly as not to
+trouble the repose of a land “In which it seemeth always
+afternoon”; no cyclones traverse its surface, only mild trade-winds
+flow towards the equator to supply for the volumes
+of air gently lifted by the power of the sun, to carry reinforcements
+of water-vapour north and south. Aerial movements
+are, in fact, by a very strong presumption, of the
+terrestrial type, but executed with greatly abated
+vigour.</p>
+
+<p class='c007'>Brilliant projections above the terminator of Mars were
+first distinctly perceived at the Lick Observatory in 1890.
+They have been re-observed at Nice, Arequipa, and Flagstaff
+(Mr. Lowell’s Observatory), coming into view, as a rule, when
+circumstances concur to favour their visibility. They strictly
+resemble lunar peaks and craters, catching the first rays of the
+sun, while the ground about them is still immersed in darkness;<a id='r48'></a><a href='#f48' class='c027'><sup>[48]</sup></a>
+and Professor Campbell<a id='r49'></a><a href='#f49' class='c027'><sup>[49]</sup></a> connects them with “mountain
+chains lying <i>across</i> the terminator of the planet,” and in
+some cases possibly snow-covered. He calculates their height
+<span class='pageno' id='Page_308'>308</span>at about ten thousand feet. Their presence was unlooked-for,
+since a flat expanse is a condition <i>sine quâ non</i> for the
+minute intersection of land by water, which seems to prevail
+on Mars.</p>
+
+<p class='c007'>Although the sun is less than half as powerful on Mars as
+it is here, the Martian climate, to outward appearance, compares
+favourably with our own. Polar glaciation is less extensive
+and more evanescent, and little snow falls outside the
+arctic and antarctic regions. Yet the theoretical mean temperature
+is minus 4°C., or 61° of Fahrenheit below freezing.
+This means a tremendous ice-grip. The coldest spot on the
+earth’s surface is considerably warmer than this cruel average.
+Fortunately, it exists only on paper. Some compensatory store
+of warmth must then be possessed by Mars, and it can scarcely
+be provided by its attenuated air. Possibly, internal heat may
+still be effective, and we see exemplified in Mars the geological
+period when vines and magnolias flourished in Greenland, and
+date-palms ripened their fruit on the coast of Hampshire.</p>
+
+<p class='c007'>The climate of Mars, according to Schiaparelli,<a id='r50'></a><a href='#f50' class='c027'><sup>[50]</sup></a> “must
+resemble that of a clear day upon a high mountain. By day
+a very strong solar radiation hardly at all mitigated by mist
+or vapour; by night a copious radiation from the soil towards
+celestial space, and hence a very marked refrigeration; consequently,
+a climate of extremes, and great changes of
+temperature from day to night, and from one season to
+another. And as on the earth, at altitudes of from 17,000 to
+20,000 feet, the vapour of the atmosphere is condensed only
+into the solid form, producing those whitish masses of suspended
+crystals which we call cirrus-clouds, so in the atmosphere
+of Mars it would be rarely possible to find collections
+of cloud capable of producing rain of any consequence. The
+variation of temperature from one season to another would
+be notably increased by their long duration, and thus we can
+understand the great freezing and melting of the snow,
+renewed in turn at the poles at each complete revolution of
+the planet round the sun.”</p>
+
+<p class='c007'><span class='pageno' id='Page_309'>309</span>But the anomalies in the Martian domestic economy cannot
+thus easily be removed, and the only safe conclusion is
+Flammarion’s, that “the general order of things is very
+different on Mars and on the earth.”</p>
+
+<p class='c007'>The German astronomer, Mädler, searched in 1830 for a
+Martian satellite, and although his telescope was of less than
+four inches aperture, he satisfied himself that none with a
+diameter of as much as twenty-three miles could be in
+existence. As it happened, he was right. The pair of moons
+detected by Professor Asaph Hall with the Washington
+twenty-six refractor, August 11 and 17, 1877, are unquestionably
+below that limit of size. Neither of them can well be
+more than ten miles across. Their names, “Deimos” and
+“Phobos,” are taken from the <cite>Iliad</cite>, where Fear and Panic
+are introduced as attendants upon the God of War. Deimos
+revolves in 30 hours and 18 minutes at a distance of 14,600
+miles from the centre of Mars. And, since the planet rotates
+in 24 hours 37 minutes, the diurnal motion of the sphere
+from east to west is so nearly neutralised by the orbital
+circulation of the satellite from west to east that nearly 132
+hours elapse between its rising and its setting. During the
+interval, it changes four times from new to full, and <i>vice versâ</i>.
+Professor Young estimates that Mars receives from it when
+full only ¹⁄₁₂₀₀th of full moonlight.</p>
+
+<p class='c007'>Phobos is more effective in illumination, both because it is
+larger, and because it is less distant. At the Martian equator,
+its brightness is equal to ¹⁄₆₀th that of our moon, but beyond
+69° of latitude it is permanently shut out from view by the
+curvature of the globe. This exclusion is an effect of its
+uncommon closeness to its surface, the interspace being only
+3,700 miles, while its distance from the centre is 5,800. Moreover,
+the period of Phobos being only 7 hours 39 minutes, or
+less than ⅓ the time of rotation of its primary, it rises in the
+west, sets in the east, and courses across the heavens in 11
+hours, during which interval it accomplishes one entire cycle
+of its phases, and gets through half another. This is an
+unique phenomenon, and points to an unique origin for the
+<span class='pageno' id='Page_310'>310</span>little moon. No other known satellite revolves more quickly
+than its primary rotates, and the discovery of the fact has
+dealt a fatal blow to Laplace’s method of planetary evolution.
+Were Phobos capable of raising any appreciable tide on Mars,
+its frictional effects would hence be of an opposite character
+to those of other tidal waves; and instead of being pushed
+outward, it would be drawn inward, and finally precipitated
+upon the planet. But it derives safety, on the one hand, from
+its small mass; on the other, from the insensibility of Mars to
+tidal action. The satellite is incapable of exerting the
+required influence; the planet is not in a state to respond to
+it, were it exerted. For the configuration of land and water
+upon its surface is such as effectually to prevent the flow of
+tides, were the compulsive power a thousand-fold that possessed
+by its pair of diminutive satellites.</p>
+
+<h3 class='c004'>CHAPTER VII.<br> <span class='c014'>THE ASTEROIDS.</span></h3>
+
+<p class='c003'>Between the orbits of Mars and Jupiter is interposed a huge
+gap. On one side of it lie the terrestrial planets; on the
+other, the “major planets”—orbs belonging to a different
+order, both as to magnitude and as to constitution. The hiatus
+marks a change of front in planetary development, and its
+existence gravely compromises the symmetry of the solar
+system. Its inconsistency with Bode’s law of planetary distances
+long troubled investigators. A member of the series
+had somehow dropped out; it was sought for under the form of
+a planet, and found, apparently, as its disintegrated constituents.
+The discovery of Uranus nearly at the distance indicated
+<span class='pageno' id='Page_311'>311</span>for it by the law roused astronomers to the necessity
+for a systematic chase; but before their organisation had got
+into full working order, the missing occupant of the vacant
+zone presented itself spontaneously. This was Ceres, the first
+asteroid, discovered by Piazzi at Palermo, January 1, 1801,
+the opening day of the present century.</p>
+
+<p class='c007'>A series of surprises followed. While watching its path,
+Dr. Olbers, March 28, 1802, came across an associated body.
+He named it Pallas, and it was at once proved by the calculations
+of Gauss to revolve practically at the same distance
+from the sun as Ceres. <i>Both</i> occupied nearly the position required
+by Bode’s law. This double fulfilment was more
+than was bargained for; it was unprecedented and perplexing;
+but the anomaly was temporarily removed by Olbers’
+daring hypothesis of an exploded planet. The prediction
+based upon it that the acquaintance made with two specimen-products
+of the catastrophe would be followed by an introduction
+to many more, was strikingly verified by Harding’s
+discovery of Juno, September 1, 1804, and by Olbers’ of
+Vesta, March 29, 1807. By a further coincidence, both were
+at the time situated in the positions suggested as the most
+promising for a successful search—that is, near the line of
+intersection which should necessarily be common to orbits
+described by fragments of a single original mass.</p>
+
+<p class='c007'>The four asteroids received for many years no accession to
+their numbers. They were found to deviate, in several
+respects, from the example set them by the planets, properly
+so-called. They revolve, indeed, from west to east, thus
+following the current of systemic movement; but their paths
+are considerably eccentric and highly tilted. Each one of
+the quartette transgresses the zodiacal limits; and Pallas
+travels at an angle of no less than thirty-five degrees to the
+plane of the ecliptic.</p>
+
+<p class='c007'>Vesta, the brightest asteroid, can occasionally be seen
+with the naked eye; but the natural inference that it is
+the largest has lately been disproved. No trustworthy
+measurements of the real <i>discs</i> of the asteroids had been
+<span class='pageno' id='Page_312'>312</span>made until Professor Barnard in 1894 successfully performed
+the feat with a power of 1000 on the Lick refractor. The upshot
+has been to substitute Ceres for Vesta as the leading
+member of the group. Its diameter proved to be 485 miles,
+Pallas coming next with 304, while those of Vesta and Juno
+are respectively 243 and 118 miles. Now, Professor Edward
+Pickering, by comparing the brightness of the same bodies,
+and assuming for all indiscriminately an albedo equal to that
+of Mars, had arrived at a diameter for Vesta of 319, for Pallas
+of 169 miles. The disparity between his results and Barnard’s
+can be reconciled only on the supposition of marked
+differences in reflective power. Their reality was established
+by G. Müller’s photometric observations at Potsdam.<a id='r51'></a><a href='#f51' class='c027'><sup>[51]</sup></a> Thus
+Ceres is large and dull, Vesta comparatively small, but
+exceedingly bright—almost incredibly bright, indeed, since
+its albedo is estimated at 0·72, which represents a lustre midway
+between those of white paper and fresh-fallen snow.
+Ceres, on the other hand, is as obscure as Mercury, while
+Pallas throws back proportionately somewhat less, and Juno
+considerably more light than Mars.<a id='r52'></a><a href='#f52' class='c027'><sup>[52]</sup></a> The phases of these
+last two bodies progress besides in such a manner as to
+show that they are superficially uneven, and at quadratures
+flecked with profound shadows.</p>
+
+<p class='c007'>The facts thus arrived at are disconcerting to the views
+previously entertained. Few expected to meet with so much
+individuality in the asteroids. They were looked upon rather
+as loaves from the same batch. But now we find among
+them bodies as physically unlike as Venus and the moon.
+Ceres must be composed of rugged and sombre rock, unclothed
+probably by any vestige of air. Vesta displays a
+brilliant shell of clouds. And from Vesta alone among the
+asteroids, Vogel derived in 1873 some uncertain indications
+of atmospheric action upon the sun-rays reflected by it.
+There is, nevertheless, great difficulty in supposing a body of
+no more than one-thousandth the mass of Mars endowed with
+<span class='pageno' id='Page_313'>313</span>a dense atmosphere. Yet it must be dense and extensive in
+order to maintain the heavy cloud-layer implied, so far as
+our present knowledge goes, by an unusually high albedo.
+The difficulty is this. All gases tend, by their nature, to
+become indefinitely diffused through space. They can be
+restrained within a sphere of finite radius only through
+the exertion of some force capable of holding their
+elasticity in check. This force is gravity; none other suitable
+for the purpose is known. It acts as a counterpull to
+the translational velocities of the gaseous particles which,
+according to the dynamical theory of gases, constitute
+their elasticity. But if the confining power be insufficient,
+the roving particles will dart away, each on its own account,
+and will cease to form an atmosphere. This condition
+was adverted to some years ago by Dr. Johnstone Stoney,
+and he calculated the mass needed to secure to a heavenly
+body the lasting possession of an aerial envelope. It differs
+naturally for different gases; the lightest particles being
+affected by the swiftest movements, and hence being the
+readiest to escape. The earth, on this view, is impotent to
+retain hydrogen; since the critical velocity at its surface is
+seven miles a second, and hydrogen-molecules can, now and
+again, attain 7·4 miles, so that they would dribble away, one
+after another, until the whole original supply was exhausted.
+Mars (a projectile fired from which, with a speed exceeding
+three miles a second, would depart irrevocably), can but just
+hold oxygen, nitrogen, and water-vapour, all with more
+massive and sluggish molecules than those of hydrogen;
+while the moon has long ago been forsaken by whatever
+gaseous substances primitively belonged to it. The mass of
+Vesta, however, is only ¹⁄₃₁₂ the lunar mass (supposing their
+mean densities the same); hence, if the relation just described
+holds good under all circumstances, its surface <i>ought</i> to be as
+bare and dry as any lunar volcano. The albedoes of the
+asteroids raise, then, questions of fundamental importance in
+planetary physics.</p>
+
+<p class='c007'>Endeavours to add to the asteroidal group, after having
+<span class='pageno' id='Page_314'>314</span>been relinquished for over a score of years, were resumed, in
+1830, by a retired Prussian post-master named Hencke.
+His watch was rewarded with the discoveries of Astraea,
+December 8, 1845, and of Hebe eighteen months later.
+Since then, every year has regularly brought its quota of
+detections. About forty astronomers devoted themselves
+systematically to the search, and some of them reckoned
+their trophies by the score. No less than eighty-five were
+credited, in 1893, to Palisa of Vienna; Peters of Clinton
+(N.Y.), whose career closed in 1890, owned forty-eight;
+Watson, another American professor, made testamentary
+provision for his twenty-two clients, lest, for lack of computational
+care, they should relapse into their former outcast
+condition. The task is, indeed, a heavy one of keeping
+guard over some hundreds of minute objects threading their
+way through a maze of orbits, amid throngs of stars, from
+which they are indistinguishable except by continuous observation,
+and the question, <i>Cui bono?</i> has been asked, and
+has only with hesitation been answered. But the business
+has, up to the present, been kept going; the registry and
+inquiry asteroidal office remains open at Berlin, and the
+almost overwhelming mass of calculations, necessary for
+identification, is punctually dealt with.</p>
+
+<p class='c007'>The work and responsibilities of this department have,
+of late, been alarmingly augmented. Until five years ago
+the telescope was the sole implement of research in connection
+with it, but on December 22, 1891, Professor Max Wolf of
+Heidelberg, discovered No. 323, afterwards named Brucia, on
+a sensitive plate exposed with a six-inch portrait lens, of
+thirty inches focus, and a field of seventy square degrees.
+Before the year 1892 had closed, his photographic discoveries
+of the same kind numbered eighteen, and they had, in
+January, 1897, run up to fifty-six, of which five were
+recorded on the same night. He picked up, besides,
+several “lost” or strayed asteroids. M. Charlois of
+Nice immediately adopted Wolf’s method, and emulated
+his success. About ninety of these objects have already
+<span class='pageno' id='Page_315'>315</span>fallen to his share by telescopic and photographic means.
+In either case they are discriminated from stars solely
+by their motion; but on sensitive plates its effects are directly
+visible, fixed objects being represented by round dots, travelling
+objects by lines, the length of which is proportionate to
+the amount of displacement during the hour, or hours, of
+exposure.</p>
+
+<p class='c007'>About 440 asteroids are now established members of the
+solar system. It has long been thought that numerical identification
+is as much as they can properly claim; but the old and
+inconvenient system of mythological nomenclature is still
+pursued. Indeed, the supply of goddesses is running out,
+and has to be reinforced by apotheosis or invention. Already,
+to some extent, as Professor Holden remarks, the asteroidal
+catalogue “reads like the Christian names at a girls’ school.”
+Needless to say that the brightness of the objects annually
+registered is in steady course of decline. Very few of those
+now drawn to shore in the photographic net are likely to exceed
+twenty miles in diameter. Yet although mere planetary
+shreds, they are probably large compared with the grains of
+planetary dust, numberless as the sands of the seashore,
+which indiscernably revolve round the sun under analogous
+conditions.</p>
+
+<p class='c007'>Their aggregate mass is very small. Leverrier assigned
+for its superior limit one-fourth that of the earth, but the
+limit, we may rest assured, is very far from being attained.
+M. Niesten of Brussels estimated that the first 216 asteroids,
+including all the larger ones, amounted to ¹⁄₁₀₀₀th the
+earth’s volume, and we may add, since they are beyond
+doubt specifically lighter, to about ¹⁄₈₀₀₀th the earth’s mass.
+Mr. Roszl finds for the mass of 311 asteroids one-fortieth that
+of the moon.<a id='r53'></a><a href='#f53' class='c027'><sup>[53]</sup></a> Still later, M. Gustave Ravené has attempted
+to account for the superfluous movement of the perihelion of
+Mars by the gravitational influence of these bodies.<a id='r54'></a><a href='#f54' class='c027'><sup>[54]</sup></a> He
+computes the required mass to be two-thirds that of the moon.
+<span class='pageno' id='Page_316'>316</span>In other words, he assumes the group to be fairly represented
+by 500 globes as large as Juno (124 miles in diameter), and
+of terrestrial density. But he obviously puts some constraint
+on nature in order to secure the desired agreement.</p>
+
+<p class='c007'>The distribution of these dwarfed globes is not without significant
+features. It is such, at any rate, as absolutely to negative
+Olbers’s hypothesis of their origin through the explosion of an
+already formed planet. They represent, on the contrary, the
+materials of a planet that never was, and never will be formed.
+They follow paths curiously intertwined. D’Arrest noticed
+forty-five years ago, as a proof of the intimate relation subsisting
+among the members of what was then a small group,
+“that, if their orbits are figured under the form of material
+rings, these rings will be found so entangled that it would be
+possible, by means of one among them taken at hazard, to
+lift up all the rest.” They are not, however, scattered at
+random over the wide zone appropriated to them which, at its
+extreme limits, measures three times the radius of the earth’s
+orbit. It includes blank spaces which seem as if cleared by
+some expulsive agency. That agency, as Professor Kirkwood
+divined in 1866, is the disturbing power of Jupiter.
+For the blank spaces occur where there would be commensurability
+of periods, and whence, accordingly, revolving
+particles should be ejected by accumulated perturbations.
+The clearing power was not exerted once for all; it is still
+active. But its effectiveness in modifying distribution is now
+perceived to be less complete than it seemed when our
+acquaintance with the bodies in question was more limited.
+It has produced in general only partial vacancies. M.
+Parmentier<a id='r55'></a><a href='#f55' class='c027'><sup>[55]</sup></a> analysed in 1895 the arrangement in space of
+390 orbits, with the result of finding that some of the
+originally noted gaps had ceased to exist. The mean
+distances, for instance, corresponding to periods two-sevenths
+and three-sevenths the Jovian period, are fairly well
+frequented; while, on the other hand, there is an unmistakable
+thinning out where five revolutions are performed
+<span class='pageno' id='Page_317'>317</span>while Jupiter accomplishes two. He found again that no
+asteroid circulates either in half, or in one-third the same
+dangerous period. Yet, even since he wrote, No. 401 has
+been detected occupying the former of these prohibited
+spaces. But this apparent breach of rule may turn out to result
+from a miscalculation, as in the case of Menippe, which
+has in consequence never been recaptured since she first presented
+herself in 1878, and was erroneously assigned
+a period two-fifths that of Jupiter. There is no doubt
+that the asteroids are collected most densely about the
+mean distance 2·8 of the earth’s, just where conformity to
+Bode’s law would place them. Nor is it less certain that
+Kirkwood’s “rule of commensurability” has fundamentally
+influenced their distribution.</p>
+
+<p class='c007'>He further discerned among them groups of two or three
+moving in closely-related orbits. Additional examples of
+this sort of connexion, which is far too close to be casual,
+have been pointed out by M. Tisserand and Mr. Monck, and
+eighty asteroids are at present known to have companions,
+their actual ties with which indicate, as Kirkwood held,
+original identity. Each group consists of fragments of a
+primitive nebular mass torn asunder by the unequal attraction
+of Jupiter shortly after its detachment from the great parent
+sphere eventually condensed to form the sun. As an example,
+we may take Juno and its twin Clotho. Both revolve
+at a mean distance from the sun 2·67 times that of the earth,
+in orbits of sensibly the same eccentricity, and of nearly the
+same inclination to the ecliptic, their major axes diverging,
+however, to the extent of ten degrees, obviously through unequal
+perturbations. As surely as corresponding scars on
+opposite cliffs vouch for their antique disruption, do these
+concurrent paths attest the primitive unity of the pair of
+planetules traversing them. And bodies similarly connected
+occur not in pairs only, but in triplets as well.</p>
+
+<p class='c007'>From whatever point of view the “planetary cluster”
+composed by the asteroids is regarded, the influence of
+Jupiter is perceived as dominant in the background.
+<span class='pageno' id='Page_318'>318</span>The manner of planetary production underwent a marked
+change subsequently to the separation of his mighty mass.
+No interval of repose followed; but a constant shredding off
+of chips and shavings. This may safely be attributed (in
+accordance with Professor Kirkwood’s surmise) to the tide-raising
+power of Jupiter at close quarters, by which strain in
+the central rotating mass was almost prevented, through the
+facility with which it was relieved. Hence the parent nebula
+long remained incapable of parting with any appreciable portion
+of its substance, and never resumed planet-making on
+the ancient scale. The asteroids then came into existence
+under Jupiter’s auspices; they were, while still in an inchoate
+state, subdivided, or even pulverised by his disruptive
+influence, and scattered over the zone allotted to them under
+the compulsion of his perturbing power.</p>
+
+<h3 class='c004'>CHAPTER VIII.<br> <span class='c014'>THE PLANET JUPITER.</span></h3>
+
+<p class='c003'>Jupiter is by far the most important member of the solar
+family. The aggregate mass of all the other planets is only
+two-fifths of his, which 316 earths would be needed to
+counter-balance. His size is on a still more colossal scale
+than his weight, since in volume he exceeds our globe 1,380
+times. His polar and equatorial diameters measure respectively
+84,570 and 90,190 miles,<a id='r56'></a><a href='#f56' class='c027'><sup>[56]</sup></a> giving a mean diameter of
+88,250 miles, and a polar compression of ¹⁄₁₆th. The corresponding
+equatorial protuberance rises to 2,000 miles, so that
+the elliptical figure of the planet strikes an observer at the
+first glance. This at once indicates rapid axial movement;
+and Jupiter’s rotation is accordingly performed in nine hours
+<span class='pageno' id='Page_319'>319</span>and fifty-five minutes, with an uncertainty of a couple of
+minutes. The cause of this uncertainty will presently appear.</p>
+
+<p class='c007'>The numbers just given imply that this great planet is of
+somewhat slight consistence, and its mean density is in
+fact, a little less than that of the sun. The sun is heavier
+than an equal bulk of water in the proportion 1·4 to 1,
+Jupiter in the proportion of 1·33 to 1. The earth is thus
+more than four times specifically heavier than the latter
+globe. Three Jupiters would keep in equipoise four equal
+globes of water, while the earth would turn the scale against
+five and a half aqueous models of itself. This low density,
+an unfailing characteristic of all the giant planets, is
+charged with meaning. It at once gives us to understand
+that, in crossing the zone of asteroids, we enter upon a
+different planetary region from that left behind. The bodies
+revolving there are on an immensely larger scale of magnitude
+than those on the hither side; they are of solar, rather
+than terrestrial, density; they rotate much more rapidly, and
+are in consequence of a more elliptical shape; they display,
+and most likely possess, no solid surface; they are attended
+by retinues of satellites.</p>
+
+<p class='c007'>Jupiter circulates round the sun in 11·86 years, in an orbit
+deviating by less than one and a half degrees from the plane
+of the ecliptic, but of thrice the eccentricity of the ellipse
+traced out by the earth. With a mean distance from the sun
+of 483 millions of miles, it accordingly approaches within
+462 at perihelion, and withdraws to 504 millions of miles
+at aphelion. And since the heat and light received from the
+sun are inversely as the squares of these numbers, it follows
+that Jupiter is better warmed and illuminated when at the
+near than when at the far extremity of its orbit, in the proportion
+of 109 to 100. Seasons it has none worth mentioning;
+nor could they be of much effect even if they were
+better marked. At its mean distance of 5·2 “astronomical
+units”—that is, radii of the earth’s orbit—the sun’s potency
+is reduced to ¹⁄₂₇th what it is here; we might accordingly
+have expected to meet in this planet the conditions of a
+<span class='pageno' id='Page_320'>320</span>frozen world. But this anticipation has been singularly
+falsified.</p>
+
+<p class='c007'>Under propitious circumstances Jupiter comes within 369
+million miles of the earth. These occur when he is in opposition
+nearly at the epoch of his perihelion passage. His maximum
+opposition distance, on the other hand, is 411 million
+miles. He is then at aphelion. Thus, at the most favourable
+opposition, he is 42 million miles nearer to us than at
+the least favourable. The effect on his brightness is evident
+to the eye. When his midnight culmination takes place
+in October, he in fact sends us one and a half times more
+light than when the event comes round to April. We need
+only recall the unusual splendour of his appearance in September
+and October, 1892, when his lustre was double that of
+Sirius. His opposition period, as we may call it, is 399 days.</p>
+
+<p class='c007'>The intrinsic brilliancy of his surface is surprising, especially
+when we consider that it is somewhat deeply tinged with
+colour. According to Müller’s determination (relative to
+Mars), it actually returns 78 per cent. of the incident light.
+But this would imply self-luminosity, the presence of which is
+negatived by trustworthy evidence. Hence Zöllner’s absolute
+albedo of 0·62 seems preferable. In either case, Jupiter does
+not fall far short of being as reflective as white paper.</p>
+
+<p class='c007'>The minimum diameter of the visible disc considerably
+exceeds the maximum of that of Mars. The latter never
+measures more than 25″; Jupiter at conjunction, when (in
+round numbers), 600 million miles distant from us, presents a
+surface 32″ in diameter, widened at a favourable opposition
+to 50″. Even with a low power it thus makes a beautiful and
+interesting telescopic object Its distinctive aspect is that of
+a belted planet, the belts varying greatly in number and
+arrangement. As many as thirty have, on occasions, been
+counted, delicately ruling the disc from pole to pole. They
+are always parallel to the equator, but are otherwise highly
+changeable, and cannot be too closely studied as an index
+to the planet’s physical constitution. Two in particular are
+remarkable. They are called the north and south equatorial
+<span class='pageno' id='Page_321'>321</span>belts, and enclose a lustrous equatorial zone. The poles are
+shaded by dusky hoods.</p>
+
+<p class='c007'>This general scheme of markings, however, when viewed
+with one of the great telescopes of the world, is so overlaid
+with minor particulars as sometimes to be scarcely recognisable.
+One cannot see the wood for the trees. Lovely colour-effects,
+too, come out under the best circumstances of definition
+and aerial transparency. The tropical belts may be
+summarily described as red; but they are of complex
+structure, and their subordinate features and formations are
+marked out, under the sway of a ternating and tumultuous
+activities, by strips and patches of vermilion, pink, purple,
+drab and brown. The intermediate space is divided into two
+bands by a line, or narrow riband, pretty nearly coinciding with
+the equator, and rosy, or vivid scarlet in hue. The polar caps
+are sometimes of a delicate wine-colour, sometimes pale grey.</p>
+
+<p class='c007'>Professor Keeler made an elaborate study of the planet
+with the Lick 36-inch in 1889, and executed a series of
+valuable drawings, one of which we are privileged to reproduce
+(Fig. 15). With a power of 320, the disc, he tells us,
+“was a most beautiful object, covered with a wealth of detail
+which could not possibly be accurately represented in a drawing.”
+Most of the surface was then “mottled with flocculent
+and irregular cloud-masses. The edges of the equatorial
+zone were brilliantly white, and were formed of rounded,
+cloud-like masses, which, at certain places, extended into the
+red belt as long streamers. These formed the most remarkable
+and curious feature of the equatorial regions. They are
+the cause of the double or triple aspect which the red belts
+present in small telescopes.”<a id='r57'></a><a href='#f57' class='c027'><sup>[57]</sup></a></p>
+
+<p class='c007'>Near their starting-points the streamers were white and
+sharply defined, but became gradually diffused over the ruddy
+surface of the belts. When at all elongated, they invariably
+flowed backward <i>against</i> the rotational drift, and were inferred
+to be cloud-like masses expelled from the equatorial
+region, and progressively left behind by its advance. This
+<span class='pageno' id='Page_322'>322</span>hypothesis was confirmed by the motion of some bright points,
+or knots, on the streamers. “The portions of the equatorial
+zone surrounding the roots of well-marked streamers were
+somewhat brighter,” Professor Keeler continues, “than at
+other places, and it is a curious circumstance that they were
+almost invariably suffused with a pale olive-green colour,
+which seemed to be associated with great disturbance, and
+was rarely seen elsewhere.”</p>
+
+<div id='i_322'  class='figcenter id001'>
+<img src='images/i_322.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 15.</span>—<i>Jupiter, October 3, 1890. Drawn by Professor Keeler with the great Lick Refractor. The Red Spot is visible.</i></p>
+</div>
+</div>
+
+<p class='c007'>Now, if the material of the streamers had been simply a
+superficial overflow, it should have carried with it into higher
+latitudes an excess of linear rotational speed, and should hence
+have pushed its way onwards as it proceeded north and south.
+But, instead, it fell behind; its velocity was less, not greater
+than that of the belts with which it eventually became incorporated.
+<span class='pageno' id='Page_323'>323</span>What are we to gather from this fact? Evidently
+that the currents issuing north and south were of eruptive
+origin. Their motion, in miles per second, was slow, because
+they belonged to profound strata of the planet’s interior.
+Their backward drift measured the depth from which they
+had been flung upward.</p>
+
+<p class='c007'>The spots, red, white, and black, constantly visible on the
+Jovian surface, excite the highest curiosity. They are of all
+kinds and qualities, and their histories and adventures are as
+diverse as they are in themselves. Some are quite evanescent;
+others last for years. At times they come in undistinguished
+crowds, like flocks of sheep, then a solitary spot will
+acquire notoriety on its own account. White spots appear in
+both ways; black spots more often in communities; and it is
+remarkable that the former frequent distinctively, though not
+exclusively, the southern, the latter the northern hemisphere.
+Red spots, too, develop pretty freely; but the attention
+due to them has been mainly absorbed by one striking
+specimen.</p>
+
+<p class='c007'>The Great Red Spot has been present with us for at
+least nineteen years; and it is a moot point whether its beginnings
+were not watched by Cassini more than two centuries
+ago. Its modern conspicuousness, however, dates from 1878.
+Then of a full brick-red hue, and strongly-marked contour, it
+measured 30,000 by nearly 7,000 miles, and might easily have
+enclosed three such bodies as the earth. It has since faded
+several times to the verge of extinction, and partially recovered;
+but there has never been a time when it ceased to
+dominate the planet’s surface-configuration. More than once
+it has been replaced by a bare elliptical outline, as if through
+an effusion of white matter into a mould previously filled with
+red matter; and just such a sketch was observed by Gledhill
+in 1870. The red spot is attached, on the polar side, to the
+southern equatorial belt. It might almost be described as
+jammed down upon it; for a huge gulf, bounded at one end
+by a jutting promontory, appears as if scooped out of the
+chocolate-coloured material of the belt to make room for it.
+<span class='pageno' id='Page_324'>324</span>Absolute contact, nevertheless, seems impossible. The spot
+is surrounded by a shining aureola, which seemingly defends
+it against encroachments, and acts as a <i>chevaux de frise</i> to preserve
+its integrity. The formation thus constituted behaves
+like an irremovable obstacle in a strong current. The belt-stuff
+encounters its resistance, and rears itself up into a promontory
+or “shoulder,” testifying to the solid presence of the
+spot, even though it be temporarily submerged. The great
+red spot, the white aureola, and the brownish shoulder are
+indissolubly connected.</p>
+
+<p class='c007'>The spot is then no mere cloudy condensation. Yet it
+has no real fixity. Its period of rotation is inconstant. In
+1879–80, it was of 9 hours, 55 minutes, 34 seconds; in
+1885–86, it was longer by 7 seconds. The object had retrograded
+at a rate corresponding to one complete circuit of
+Jupiter in six years, or of the earth in seven months.<a id='r58'></a><a href='#f58' class='c027'><sup>[58]</sup></a> It is
+not then fast moored, but floats at the mercy of the currents
+and breezes predominant in the strange region it navigates.
+A quiescent condition is implied by the approximate constancy
+of its rotation-period during the last ten years. With
+the paling of its colour, its “proper motion” slackens or
+ceases. This must mean that, at its maxima of agitation, it
+is the scene of uprushes from great depths, which, bringing
+with them a slower linear velocity, occasion the observed
+laggings. It is not self-luminous, and shows no symptom of
+being depressed below the general level of the Jovian surface.
+A promising opportunity was offered in 1891 of determining
+its altitude relative to a small dark spot on the same parallel,
+by which, after months of pursuit, it was finally overtaken.
+An occultation appeared to be the only alternative from a
+transit; yet neither occurred. The dark spot chose a third.
+It coasted round the obstacle in its way, and got damaged
+beyond recognition in the process. Its material, as Mr.
+Stanley Williams observed, “was diverted and forced bodily
+southwards, and obliged to pass round the southern side of
+the red spot as if it were an island projecting above a stream.”</p>
+
+<p class='c007'><span class='pageno' id='Page_325'>325</span>Jupiter has no certain and single period of rotation. Nearly
+all the spots that from time to time come into view on its disc
+are in relative motion, and thus give only individual results.
+The great red spot has the slowest drift of all (with the rarest
+exceptions), while the black cohorts of the northern hemisphere
+outmarch all competitors. Mr. Stanley Williams,<a id='r59'></a><a href='#f59' class='c027'><sup>[59]</sup></a> as
+the upshot of long study, has delimitated nine atmospheric
+surfaces with definite periods. They are well marked, and
+evidently have some degree of permanence, yet the velocities
+severally belonging to them are distributed with extreme
+irregularity. Thus, two narrow, adjacent zones differ in movement
+by 400 miles an hour. This state of things must
+obviously be maintained by some constantly acting force, since
+friction, if unchecked, would very quickly abolish such enormous
+discrepancies. The rotational zones are unsymmetrically
+placed; there is no correspondence between those north and
+south of the Jovian equator; and, although the equatorial
+drift is quicker than that of either tropic, it is outdone in 20°
+to 24° north latitude. The stability of this anomalous mode
+of rotation was remarkably illustrated by Dr. Rambaud’s
+measurements of the “Garnet Spot” of October, 1895. Its
+movement proved to be strictly conformable to that of the
+zone in which it was situated (10° to 20° north latitude), and
+to agree, moreover, within a fifth of a second with the value
+deduced by Schröter in 1787 for that of a spot in the same
+“zenographical” district.<a id='r60'></a><a href='#f60' class='c027'><sup>[60]</sup></a></p>
+
+<p class='c007'>Jupiter’s equatorial rotation, as indicated by observations of
+spots, is accomplished in 9 hours 50 minutes; but Bélopolsky’s
+and Deslandres’ spectrographic determinations gave rates of
+approach and recession falling somewhat short of the corresponding
+velocity.<a id='r61'></a><a href='#f61' class='c027'><sup>[61]</sup></a> Possibly the spots forge ahead in the
+medium that sustains them; or it may be, as M. Bélopolsky
+suggests, that the planetary sphere itself has been measured
+too large, owing to refraction in its atmosphere.</p>
+
+<p class='c007'><span class='pageno' id='Page_326'>326</span>However this be, the rotation of the great planet, albeit ill-regulated
+(if the expression be permissible), is distinctly of the
+solar type. It is itself a “semi-sun,” showing no trace of a
+solid surface, but a continual succession of cloud-like masses
+belched forth from within. Each series, in fact, of certain
+classes of markings, such as the equatorial “port-holes,”
+plainly owes its origin to the rhythmical activity of
+a solitary, deep-buried focus.<a id='r62'></a><a href='#f62' class='c027'><sup>[62]</sup></a> Jupiter’s low mean density,
+considered apart from every other circumstance, suffices to
+demonstrate the primitive nature of his state. Under the
+enormous pressure reigning in his interior, the same materials
+should be vastly more massive, specifically, than within our
+own small globe; their fourfold expansion gives us to understand
+the intensity of that heat by which pressure has been so
+much more than neutralised. Moreover, the agitations due to
+the cooling of a fluid globe make their mark on its turbulent
+surface. On a solidified body like the earth, circulation is
+kept up by heat received from without, and is purely atmospheric,
+and essentially horizontal. In a sun-like body, the
+circulation is bodily and vertical. That the processes going
+on in Jupiter are of this kind is beyond question. Exchanges
+of hot and colder substances are effected, not by surface-flows,
+but by up and down rushes. The parallelism of his belts to
+his equator makes this visible to the eye. An occasional
+oblique streak<a id='r63'></a><a href='#f63' class='c027'><sup>[63]</sup></a> betokens a current in latitude, but it is exceptional,
+and might be called out of character.</p>
+
+<p class='c007'>Jupiter’s true atmosphere encompasses the disturbed shell
+of vapours observed telescopically. Its general absorptive
+action upon light is betrayed by the darkening of the planet’s
+limb—another point of resemblance to the sun; while its
+special, or selective, absorption can only be detected with the
+spectroscope. The arresting effect of water-vapour was early
+noticed by Huggins and Vogel, and they measured a strong
+line in the red of unknown origin, but contained in banded
+<span class='pageno' id='Page_327'>327</span>star spectra. Atmospheric absorption is strongest above the
+ruddy equatorial belts, which are hence concluded to be
+placed at a lower level than the white surface.</p>
+
+<p class='c007'>Planetary photography was set on foot by Dr. Gould of
+Boston, in 1879, when he obtained some promise of success
+with Mars, Jupiter, and Saturn; and Dr. Lohse prosecuted
+the subject in 1883. The actinic power of Jupiter’s light is
+very remarkable. It surpasses that of moonlight nine times,
+and that of Mars twenty-four times. Dr. Lohse further ascertained
+that the southern hemisphere is twice as chemically
+effective as the northern.<a id='r64'></a><a href='#f64' class='c027'><sup>[64]</sup></a> This superiority is doubtless connected
+with the greater physical agitation of the same region.
+A series of photographs of Jupiter, taken in 1891 with the
+great Lick refractor, were the first of any value for purposes
+of investigation. Each is one inch in diameter; the image
+of the planet having been enlarged eight times before being
+received upon the plate. Mr. Stanley Williams found them
+full of interesting detail. Figure 16 shows an enlargement of
+a striking photograph taken by Professor E. C. Pickering.</p>
+
+<p class='c007'>Jupiter’s satellites were the first trophies of telescopic observation.
+They are, indeed, bright enough for naked eye
+perception, could they be removed from the disc which obscures
+them with its excessive splendour; and the first and
+third have actually been seen, in despite of the glare, by a few
+persons with phenomenally good eyesight. The mythological
+titles of the Galilean group—Io, Europa, Ganymede, and
+Calypso (proceeding from within outward) have been superseded
+by prosaic numbers. The change was unlucky,
+but is now probably irremediable.</p>
+
+<p class='c007'>The Jovian family presents an animated and attractive
+spectacle. The smallest of its original members (No. II.) is almost
+exactly the size of our moon; the largest (No. III.), with
+its diameter of 3,550 miles, considerably exceeds the modest
+proportions of Mercury. Satellite I. revolves in 42½ hours at
+the same average distance from Jupiter’s surface that our
+moon does from that of the earth. No. II. has a period of
+<span class='pageno' id='Page_328'>328</span>3 days 13 hours, and its distance from Jupiter’s centre is
+415,000 miles. Both these orbits are sensibly circular; and
+Nos. III. and IV. travel in ellipses of very small eccentricity,
+the one at a mean distance of 664,000, the other at 1,167,000
+miles, in periods respectively of 7 days 4 hours, and 16 days
+16½ hours. All four revolve strictly in the plane of Jupiter’s
+equator.</p>
+
+<div id='i_328'  class='figcenter id001'>
+<img src='images/i_328.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 16.</span>—<i>Photograph of Jupiter. Exposure, 87 seconds.</i><br>  <br>  (From <cite>Knowledge</cite>, November, 1889.)</p>
+</div>
+</div>
+
+<p class='c007'>They constitute a system bound together by peculiar
+dynamical relations, in consequence of which they can never
+be all either eclipsed, or seen aligned at one side of
+their primary, at the same time. They can all, however, be
+<span class='pageno' id='Page_329'>329</span>simultaneously hidden behind it, or in its shadow; although
+this moonless condition is looked out for as a telescopic
+rarity.</p>
+
+<p class='c007'>The varied phenomena of eclipses, occultations, and transits,
+offer the interest, not only of predictions fulfilled, but sometimes
+of discrepancies detected. The three inner satellites
+plunge through the huge neighbouring shadow-cone at every
+revolution; the fourth, owing to its greater distance, escapes
+eclipse when the shadow makes an appreciable angle with
+the plane of its orbit. When Jupiter is in opposition or
+conjunction, occultations, but no eclipses, of his moons take
+place; at other periods, the two kinds of obscuration merge
+into, or succeed each other. “Time cannot stale their infinite
+variety.”</p>
+
+<p class='c007'>From observations of the eclipses of Jupiter’s satellites,
+Olaus Römer gathered, in 1675, the first intimations of the
+finite velocity of light. He noticed that their visibility was
+alternately retarded and accelerated as the earth withdrew
+from, and approached the scene of their occurrence; and he
+designated half the extreme difference, or the time occupied
+by light in travelling from the earth to the sun, the “equation
+of light.” Its value is 500 seconds; and until recently, no
+other measure was available of that fundamental constant of
+nature—the rate of luminous transmission.</p>
+
+<p class='c007'>The transits of the satellites across the Jovian disc present
+many curious appearances, due to complicated and changeable
+effects of light and shade both upon the planetary background,
+and upon the little circular objects self-compared
+with it. These, in the ordinary course, show bright while
+near the dusky limb, then vanish during the central passage,
+and re-emerge again bright at the opposite side. But, instead
+of duly vanishing, they now and then darken even to
+the point of becoming indistinguishable from their own
+shadows, by which they are preceded or followed. This
+difference of behaviour cannot be attributed wholly to
+varieties of lustre in the sections of the disc transited; otherwise,
+it could be predicted. But this has never been
+<span class='pageno' id='Page_330'>330</span>attempted; “black transits” come when least expected.
+The third and fourth satellites are those chiefly subject
+to these phases; the second has never been known to
+exhibit them; and they but slightly affect the first. A
+drawing by Professor Barnard of one of its bright transits
+with an attendant shadow that Peter Schlemyl might have
+envied, is reproduced in Figure 17. Its belted appearance,
+detected by that eminent observer, will be noted. Indeed,
+all the satellites, except perhaps No. II. are striped or
+spotted; and this leads to seeming deformations in their
+shape, as well as fluctuations in their brightness, the markings
+being evidently of atmospheric origin, and hence changeable.
+Their distinct and accurate perception has been made possible
+by the excellence of the Lick thirty-six inch refractor.</p>
+
+<div id='i_330'  class='figcenter id001'>
+<img src='images/i_330.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 17.</span>—<i>Transit of Jupiter’s first Satellite, with Shadow, drawn by Prof. Barnard, November 19, 1893.</i> (From <cite>Monthly Notices</cite>, January, 1894.)</p>
+</div>
+</div>
+
+<p class='c007'>Jupiter’s moons seem to resemble him in constitution.
+The three first possess the same high reflective power. No.
+II. is as bright as the planet’s brightest parts, so that
+its albedo cannot fall short of 0·70. And even No. IV.
+(formerly designated “Calypso” in reference to its frequent
+obscurations) exactly matches, during its darkest phases, the
+blue-grey polar hoods of its primary. On an average, too,
+the satellites seem to be of about the same mean density as
+Jupiter, No. I. being considerably the lightest for its bulk;
+and their spectra, according to Vogel’s observations in 1873,
+<span class='pageno' id='Page_331'>331</span>are composed of solar rays modified in precisely the same
+way as those reflected by the planet. Nothing is known
+quite certainly about their rotation-periods. Sir William
+Herschel concluded them to be of the same length with
+their periods of revolution; but recent work throws some
+doubt upon the reality of this agreement.</p>
+
+<p class='c007'>The discovery, September 9, 1892, of Jupiter’s “fifth
+satellite” was one of the keenest astronomical surprises on
+record. An accession to a system so symmetrically arranged,
+so complete, to our judgment, as it stood, appeared superfluous,
+and, considering the eager scrutiny devoted to it during
+282 years, well-nigh incredible. But the extra member was
+in truth out of reach until it was found; original discovery
+being, as every one knows, a greatly more arduous feat than
+subsequent verification. Nor could it have been casually
+detected. Professor Barnard seized the opportunity, lent by
+the specially favourable opposition of 1892, to rummage the
+system for novelties. Keeping the telescopic field dark by
+means of a metallic bar placed so as to occult the gorgeous
+planetary round, he sought, night after night, for what might
+appear. At length, on September 9, he caught the glimmer
+he wanted, and made sure, September 10, that it truly intimated
+the presence of a new satellite.</p>
+
+<p class='c007'>This small body revolves in a period of 11 hours, 57
+minutes, 23 seconds, at a mean distance of 112,160 miles
+from Jupiter’s centre, or 67,000 from his bulged equatorial
+surface. Hence, it should by right be called “No. I.” instead
+of “No. V.” The major axis of the ellipse in which it
+circulates advances so rapidly, owing to the disturbance
+caused by Jupiter’s spheroidal figure, as to complete a
+revolution in five months. The implied eccentricity of its
+orbit, as M. Tisserand has shown,<a id='r65'></a><a href='#f65' class='c027'><sup>[65]</sup></a> very slightly exceeds
+that of the orbit of Venus, yet it has been made obvious
+by Barnard’s observations of the differences between
+its east and west elongations. Its orbital velocity of 16½
+miles a second far surpasses that of any other satellite in the
+<span class='pageno' id='Page_332'>332</span>solar system. Close vicinity to a mass so vast as Jupiter’s
+demands counter-balancing swiftness. Its period of revolution
+being, however, longer by one hour than Jupiter’s period
+of rotation, it so far conducts itself normally as to rise in the
+east and set in the west. On the other hand, since its progress
+over the sphere is measured by the difference between
+the two periods, it spends five Jovian days in journeying from
+one horizon to the other, running, in the meantime, four times
+through all its phases. Yet it never appears full. Jupiter’s
+voluminous shadow cuts off sunlight from it during nearly
+one-fifth of each circuit.</p>
+
+<p class='c007'>It is an exceedingly elusive telescopic object. There is no
+chance of catching a glimpse of it except with a powerful and
+perfect telescope at its “elongations,” or furthest excursions
+of about eight seconds of arc on either side of the planet
+For the most part, it lurks within the blaze as closely as
+Teucer behind the shield of Ajax. It is far too small to be
+discerned in projection upon the disc, which, viewed from it
+in mid-transit, is <i>full</i> with a diameter of 42° 2′, and an area
+6,440 times that of our moon. Yet, since its intrinsic lustre
+is less in the proportion of 2 to 15, the light shed by Jupiter
+upon the “fifth satellite” equals the joint radiance of no
+more than 860 full moons.</p>
+
+<p class='c007'>The new satellite is indistinguishable in aspect from a star
+of the thirteenth magnitude. And its neighbour No. I. being
+of 5·6 magnitude, we receive from it 910 times more light
+than from the stranger. If both be equally reflective, the
+diameter of the latter is ¹⁄₃₀th the diameter of the former, or,
+approximately, 80 miles. But its albedo is unlikely to exceed
+that of Mars. By a rough estimate, therefore, this interesting
+object measures 120 miles across, and 9000 such miniature
+globes would go to the making of one full-sized Jovian
+attendant. Instead of being a late addition to the system, or,
+so to speak, an afterthought, it may be presumed, from the perceptible
+eccentricity of its path, to be the senior member of
+the family. But the subject of its origin is not yet ripe for
+discussion.</p>
+
+<div>
+  <span class='pageno' id='Page_333'>333</span>
+  <h3 class='c004'>CHAPTER IX.<br> <span class='c014'>THE SATURNIAN SYSTEM.</span></h3>
+</div>
+
+<p class='c003'>Nearly twice as far from the sun as Jupiter revolves a planet,
+the spacious orbit of which was, until 1781, supposed to mark
+the uttermost boundary of the solar system. The mean radius
+of that orbit is 886 millions of miles; but in consequence
+of its eccentricity, the sun is displaced from its middle point
+to the extent of 50 million miles, and Saturn is accordingly
+100 million miles nearer to him at perihelion than at aphelion.
+The immense round assigned to the “saturnine” planet is
+traversed in 29½ years, at the tardy pace of six miles a second.
+His seasons are thus twenty-nine times more protracted than
+ours, and are nominally more accentuated, since his axis of
+rotation deviates from the vertical by 27°. But solar heat,
+however distributed, plays an insignificant part in his internal
+economy. In the first place, its amount is only ¹⁄₉₁th its
+amount on the earth; in the second, Saturn, like Jupiter—even
+more than Jupiter—is thermally self-supporting. The
+bulk of his globe comparatively to its mass suffices in itself to
+make this certain. The mean diameter of Saturn is 71,000
+miles, or nine times (very nearly) that of the earth; if of
+equal density, its mass should then be nine cubed, or 729
+times the same unit The actual proportion, however, is 95;
+hence the planet has a mean density of only ⁹⁵⁄₇₂₉, or between
+⅐th and ⅛th the terrestrial, and being thus composed of
+matter as light as cork, would float in water. Professor G.
+H. Darwin has moreover demonstrated, from the movements
+of its largest satellite, that its density gains markedly with
+descent into the interior, so that its surface-materials must be
+lighter than any known solid or liquid.</p>
+
+<p class='c007'>When at its nearest to the earth, Saturn is as large as a
+sixpence held up at a distance of 210 yards.<a id='r66'></a><a href='#f66' class='c027'><sup>[66]</sup></a> But instead of
+<span class='pageno' id='Page_334'>334</span>being round like a sixpence, it is strongly compressed—more
+compressed even than Jupiter. The spectra of the two planets
+are almost identical. Both are impressed with traces of
+aqueous absorption, and include the “red star line.” About
+the albedo of Saturn there is some uncertainty. Zöllner
+made it 0·50, a very probable value; Müller of Potsdam
+determined it at 3·3 times that of Mars, the unit of his scale.
+For the value of the unit, the only authority is Zöllner, who
+found Mars to give back 0·26 of the light dispensed to him.
+Multiplying then 0·26 by 3·3 we get for the albedo of Saturn
+0·86, an impossible number for a non-luminous body, the
+albedo of “untrodden snow” being, as already stated,
+0·78.</p>
+
+<p class='c007'>Saturn resembles to the eye a large, dull star; its rays are
+entirely devoid of the sparkling quality which distinguishes
+those of Jupiter. But it shows telescopically an analogous
+surface-structure. Its most conspicuous markings are tropical
+dark belts of a greyish or greenish hue; the equatorial
+region is light yellow, diversified by vague white spots;
+while the poles carry extensive pale blue canopies. The apparent
+tranquillity of the disc may be attributed in part to the
+vast distance from which it is viewed; yet not wholly. For lack
+of fiducial points, no attempt was made to determine the
+planet’s rotation until 1794, when the elder Herschel, by following
+an identified irregularity in a complex banded formation,
+arrived at a period of 10 hours 16 minutes. The first possibility
+of checking this result offered itself to Professor Hall of
+Washington, after fourteen years of vain expectation, in the
+emergence of a white spot just north of the equator, the movement
+of which gave for the length of the Saturnian day, 10 hours,
+14 minutes, 24 seconds. In 1891–2, Mr. Stanley Williams
+made observations upon a good many such objects; and
+their discussion by Mr. Denning afforded a mean period two
+seconds longer than Hall’s. Individual variations, however, to
+the extent of 14 seconds were brought out by it, proving that
+Saturnian, like Jovian, spots have “proper motions,” and cannot
+be depended upon to give the true rotation of the planet.
+<span class='pageno' id='Page_335'>335</span>Its compound nature may be suspected, but has not yet been
+proved.</p>
+
+<p class='c007'>From measures executed by Barnard in 1895, it appears
+that the equatorial diameter of Saturn is 76,470, its
+polar diameter 69,770 miles, giving a mean diameter of
+74,240, and a compression of about ¹⁄₁₂. Gravity, at its surface,
+is only one-fifth more powerful than on the earth.</p>
+
+<p class='c007'>Thus, Saturn not only belongs to the same celestial species
+as Jupiter, but is a closely-related individual of that species.
+There is no probability that either is to any extent solid.
+Both exhibit the same type of markings; both betray internal
+tumults by eruptions of spots which, by their varying
+movements, supply a measure for the profundity of their
+origin; both possess identically constituted atmospheres, and
+are darkened marginally by atmospheric absorption.</p>
+
+<div id='i_335'  class='figcenter id003'>
+<img src='images/i_335.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 18.</span>—<i>Saturn and its Rings. Drawn by Prof. Barnard, July 2, 1894.</i></p>
+</div>
+</div>
+
+<p class='c007'>Saturn is, however, distinguished by the possession of an
+unique set of appendages. Nothing like them is to be seen
+elsewhere in the heavens; and when well opened (as in Fig.
+18) they form, with the globe they enclose, and the retinue of
+satellites in waiting outside, a strange and wonderful telescopic
+<span class='pageno' id='Page_336'>336</span>object. The rings, since they lie in the plane of Saturn’s
+equator, are inclined 27° to the Saturnian orbit, and 28° to the
+ecliptic. The earth is, however, comparatively to Saturn,
+so near the sun, that their variations in aspect, as
+viewed from it, may in a rough way be considered the same
+as if seen from the sun. They correspond exactly with the
+Saturnian seasons. At the Saturnian equinoxes, the rings are
+illuminated edgewise, and disappear, totally or approximately;
+at the Saturnian solstices, sunlight strikes them nearly at the
+full angle of 27°, first from <i>below</i>, then from <i>above</i>. At these
+epochs, we perceive the appendage expanded into an ellipse
+about half as wide as it is long. Two concentric rings
+(generally called A and B) are then very plainly distinguishable,
+the inner being the brighter. The black fissure which
+separates them is called “Cassini’s division,” because that
+eminent observer was, in 1675, the first to perceive it. A
+chasm known as “Encke’s division,” in the outer ring (A), is
+a thinning out rather than an empty space; and temporary
+gaps frequently appear in A, while B is entirely exempt from
+them. There are then two definite and permanent bright
+rings, and no more; but with them is associated the dusky
+formation discovered by W. C. Bond, November 15, 1850,
+and described by Lassell as “something like a crape veil
+covering a part of the sky within the inner ring.” It is
+semi-transparent the limb of Saturn showing distinctly
+through it.</p>
+
+<p class='c007'>The exterior diameter of the ring-system is 172,800, while
+its breadth is 42,300 miles.<a id='r67'></a><a href='#f67' class='c027'><sup>[67]</sup></a> The rings A and C are each
+11,000 miles wide; while B measures 18,000, Cassini’s
+division 2,270, and the clear interval between C and the
+planetary surface somewhat less than 6,000 miles. Each
+ring, C included, is brightest at its outer edge; but there is
+no gap between the shining and the dusky structures, B
+shading by insensible gradations up to C, yet maintaining
+distinctness from it. The earliest exact determinations of
+the former were made by Bradley in 1719, since when they
+<span class='pageno' id='Page_337'>337</span>have been affected by no appreciable change.<a id='r68'></a><a href='#f68' class='c027'><sup>[68]</sup></a> The theoretically
+inevitable subversion of the system is progressing with
+extreme slowness.</p>
+
+<p class='c007'>The thickness of the rings is quite inconsiderable. They
+are flat sheets, without (so to speak) a third dimension. For
+this reason, they disappear utterly in most telescopes, when
+their plane passes through the earth, as it does twice in each
+Saturnian year. Only under exceptional conditions, a narrow,
+knotted, often nebulous, streak survives as an index to their
+whereabouts. On October 26, 1891, Professor Barnard,<a id='r69'></a><a href='#f69' class='c027'><sup>[69]</sup></a>
+armed with the Lick refractor, found it impossible to see
+them projected upon the sky, notwithstanding that their
+shadow lay heavily on the planet It was not until three
+days later, that “slender threads of light” came into
+view. The corresponding thickness of the formation was
+estimated at less than fifty miles. The phenomenon of
+the disappearance of the rings will not recur until July 29,
+1907.</p>
+
+<p class='c007'>The constitution of this marvellous structure is no longer
+doubtful. It represents what might be called the fixed form
+of a revolving multitude of diminutive bodies. This was
+demonstrated by Clerk Maxwell in the Adams Prize Essay
+of 1857. His conclusion proved irreversible. The pulverulent
+composition of Saturn’s rings is one of the acquired
+truths of science. An incalculable number of tiny satellites,
+revolving independently in distinct orbits, in the precise
+periods prescribed by their several distances from the planet,
+are aggregated into the unmatched appendages of Galileo’s
+<i>tergeminus planeta</i>. The local differences in their brightness
+depend upon the distribution of the component
+satelloids. Where they are closely packed, as in the outer
+margins of rings A and B, sunlight is copiously reflected;
+where the interspaces are wide, the blackness of the sky is
+barely veiled by the scanty rays thrown back from the
+thinly scattered cosmic dust. The appearance of the crape
+<span class='pageno' id='Page_338'>338</span>ring as a <i>dark</i> stripe on the planet results—as M. Seeliger
+has pointed out—not from the transits of the objects themselves,
+but from the flitting of their shadows in continual
+procession across the disc.</p>
+
+<p class='c007'>The albedo of these particles is so high as to render it
+improbable that they are of an earthy or rocky nature, such
+as the meteorites which penetrate our atmosphere. The
+rings they form are, on the whole, more lustrous than Saturn’s
+globe; but this superiority is held to be due to the absence
+of atmospheric absorption. Their spectrum is that of unmodified
+sunlight.</p>
+
+<p class='c007'>An eclipse of Japetus, the eighth Saturnian moon, by the
+globe and rings, November 1, 1889, was highly instructive as
+to the nature of the dusky appendage. The satellite was
+never lost sight of during its passage behind it; but became
+more and more deeply obscured as it travelled outward;
+then, at the moment of ingress into the shadow of ring B,
+suddenly disappeared. Certainty was thus acquired that
+the particles forming the crape ring are most sparsely
+strewn at its inner edge—which is, nevertheless, perfectly
+definite—and gradually reach a maximum of density at its
+outer edge. Yet, while there is not the smallest clear interval,
+a sharp line of demarcation separates it from the contiguous
+bright ring. Professor Barnard was the only observer of
+these curious appearances. The distribution of the ring-constituents,
+like that of the asteroids, was governed by the law of
+commensurable periods, Saturn’s moons replacing Jupiter as
+the perturbing and regulating power. Kirkwood showed in
+1867, that Cassini’s division represents a region of peculiarly
+strong disturbance; since a body revolving there would have
+a period connected by a simple relation with the periods of
+no less than <i>four</i> satellites. Encke’s division, too, as Dr.
+Meyer has indicated, and other lines of scanty occupation
+and occasional vacancy, coincide with districts of space where
+similar combinations occur.</p>
+
+<p class='c007'>The “satellite-theory” of Saturn’s rings has received
+confirmation from apparently the least promising quarters.
+<span class='pageno' id='Page_339'>339</span>Professor Seeliger of Munich showed, from photometric
+experiments in 1888, that their constant lustre under angles
+of illumination ranging from 0° to 30° was proof positive of
+their composition out of discrete small bodies.<a id='r70'></a><a href='#f70' class='c027'><sup>[70]</sup></a> And Professor
+Keeler of Alleghany, by a beautiful and refined application of
+the spectroscopic method, arrived at the same result in April,
+1895.<a id='r71'></a><a href='#f71' class='c027'><sup>[71]</sup></a> “Under the two different hypotheses,” he remarked,
+“that the ring is a rigid body, and that it is a swarm of
+satellites, the relative motion of its parts would be essentially
+different.” The former would necessarily involve increasing
+velocity <i>outward</i>, the latter, increase of velocity <i>inward</i>,
+just for the same reason that Mercury moves more swiftly
+than the earth, and the earth than Saturn; while the sections
+of a solid body, which could have but one period of rotation,
+should move faster, <i>in miles per second</i>, the farther they were
+from the centre of attraction. The line of sight test is then
+theoretically available; but it was an arduous task to render
+it practically so. The difficulties were, however, one by one
+overcome; and a successful photograph of the spectra of
+Saturn and its rings gave the required information in unmistakable
+shape. From measurements of the inclinations of
+five dusky rays contained in it with reference to a standard
+horizontal line, rates of movement were derived of 12½ miles
+per second for the inner edge of ring B, and of 10 miles for
+the outer edge of ring A. The agreement with theory was, as
+nearly as possible, exact; the components of the rings were
+experimentally demonstrated to be moving, each independently
+of every other, under the dominion of Kepler’s
+laws.</p>
+
+<p class='c007'>For the globe of Saturn, Professor Keeler obtained, by
+the same exquisite method, a rotational period of 10 hours,
+14 minutes, 24 seconds, in precise accordance with that indicated
+by the white spot of 1876, which thus seems to have
+had no proper motion, but to have floated on the ochreous
+equatorial surface as tranquilly as a water-lily upon a stagnant
+<span class='pageno' id='Page_340'>340</span>pool. The result, so far as it goes, hints that Saturn may be
+really, as well as apparently, less ebullient than Jupiter.</p>
+
+<p class='c007'>Seers into the future of the heavenly bodies consider that
+the rings of Saturn, like the gills of a tadpole, are symptomatic
+of an early stage of development; and will be disposed
+of before he arrives at maturity. They cannot be regarded
+otherwise than as abnormal excrescences. No other planet
+retains matter circulating round it in such close relative
+vicinity. It was proved by Roche of Montpellier that no
+secondary body of importance can exist within less than
+2·44 mean radii of its primary; inside of that limit, it would
+be rent asunder by tidal strain. But the entire ring-system
+lies within the assigned boundary; hence, being <i>where</i> it is,
+it can only exist <i>as</i> it is—in flights of discrete particles.
+Will it, however, always remain where it is?</p>
+
+<p class='c007'>“Clerk Maxwell,” wrote Mr. Cowper Ranyard,<a id='r72'></a><a href='#f72' class='c027'><sup>[72]</sup></a> “used to
+describe the matter of the rings as a shower of brickbats,
+amongst which there would inevitably be continual collisions.
+The theoretical results of such impacts would be a spreading
+of the ring both inwards and outwards. The outward spreading
+will in time carry the meteorites beyond Roche’s limit,
+where, in all probability, they will, as Professor Darwin
+suggests, slowly aggregate, and a minute satellite will be
+formed. The inward spreading will in time carry the meteorites
+at the inner edge of the ring into the atmosphere of the
+planet, where they will become incandescent, and disappear
+as meteorites do in our atmosphere.”</p>
+
+<p class='c007'>Yet it may be that collisions are infrequent in this conglomeration
+of “brickbats.” There is the strongest presumption
+that they all circulate in the same direction, in orbits
+nearly circular, and scarcely deviating from the plane of the
+Saturnian equator. Those pursuing markedly eccentric tracks
+must long ago have been eliminated. Thus, encounters can
+only occur through gravitational disturbances by Saturn’s
+moons, and they must be of a mild character, depending upon
+very small differences of velocity. The first sign of a
+<span class='pageno' id='Page_341'>341</span>“spreading outwards” should be the formation of an exterior
+“crape ring,” of which no faintest trace has yet been
+perceived.</p>
+
+<p class='c007'>Saturn’s rings are entirely invisible from its polar regions,
+but occasion prolonged and complex eclipse-effects in its
+temperate and equatorial zones. They have been fully
+treated of from the geometrical point of view by Mr.
+Proctor in “Saturn and its System.”</p>
+
+<p class='c007'>Of this planet’s eight satellites, the largest, Titan (No. VI.),
+was discovered first (by Huygens in 1655), and the smallest,
+Hyperion (No. VII.), last (by Lassell and Bond in 1848). The
+five others were detected by J. D. Cassini and William
+Herschel. Titan, alone of the entire group, equals our moon
+in size. It measures, according to Professor Barnard, 2,720
+miles across. Its period of revolution is nearly sixteen days,
+its distance from Saturn’s centre, 771,000 miles. The orbit
+of Japetus (No. VIII.) is the largest, and its period the longest
+of any secondary body in the solar system. It circulates in
+79⅓ days at a distance of 2,225,000 miles, equal to 59½ of
+Saturn’s equatorial radii. Hence its path is of about the
+same <i>proportional</i> dimensions as that of our moon. Japetus
+is remarkable for its variability in light. It is capable of
+tripling or quadrupling its minimum lustre. Sir William
+Herschel noticed that these maxima coincided with a position
+on the western side of the planet, and inferred rotation of the
+lunar kind. “From the changes in this body,” he argued in
+1792,<a id='r73'></a><a href='#f73' class='c027'><sup>[73]</sup></a> “we may conclude that some part of its surface, and
+this by far the largest, reflects much less light than the rest;
+and that neither the darkest nor the brightest side is turned
+towards the planet, but partly one and partly the other,
+though probably less of the bright side.”</p>
+
+<p class='c007'>This explanation, however, he admitted to be incomplete.
+There was, and is, outstanding variability, which seems to
+intimate the presence of an atmosphere and the formation of
+clouds. But no positive knowledge has yet been gained regarding
+the physical state of Saturn’s moons. We may
+<span class='pageno' id='Page_342'>342</span>nevertheless conjecture that, since tidal friction has destroyed
+the rotation (as regards Saturn) of the remotest member of
+the family, it has not spared those more exposed to its
+grinding-down action. All presumably rotate in the same
+time that they revolve.</p>
+
+<p class='c007'>The five inner satellites move in approximately circular
+orbits; the three outer in ellipses about twice as eccentric as
+the terrestrial path. All, Japetus only excepted, keep strictly
+to the plane of the rings. And since this makes an angle
+of 270 with the planet’s orbit, eclipses are much less frequent
+here than in the Jovian system. They can only occur when
+Saturn is within a certain distance (different for each) from
+the node of the satellite-orbit. Even Mimas (No. I.), although
+it wheels round the ring at an interval of only 34,000 miles,
+often slips outside the obliquely-projected shadow-cone. Its
+distance from Saturn’s centre is 118,000 miles, and it completes
+a circuit in 22½ hours. Perpetually wrapped in the
+glare of its magnificent primary, it is a very shy object, only
+to be caught sight of in its timid excursions by the very finest
+telescopes. Like all the Saturnian moons, except Titan, and,
+by a rare conjuncture, Japetus, it is far too much contracted
+to be visible in transit across the disc.</p>
+
+<p class='c007'>The movements of these bodies have been carefully
+studied, and their mutual perturbations to some extent
+unravelled. They have proved exceedingly interesting to
+students of celestial mechanics. Titan has, in this department,
+chiefly to be reckoned with. He exercises in the
+Saturnian system a similar overpowering influence to that
+wielded by Jupiter in the solar system. Mr. Stone finds its
+mass to be ¹⁄₇₆₀₀th that of Saturn, showing that its density is
+nearly equal to that of our moon. This seems to indicate an
+advanced stage of cooling. On the other hand, its albedo is
+evidently very high. The other satellites appear in the
+largest telescopes as mere stellar points.</p>
+
+<div>
+  <span class='pageno' id='Page_343'>343</span>
+  <h3 class='c004'>CHAPTER X.<br> <span class='c014'>URANUS AND NEPTUNE.</span></h3>
+</div>
+
+<p class='c003'>The four giant planets, closely allied as they are, and strongly
+distinguished in physical constitution from the terrestrial
+planets, divide again of themselves into two sub-groups.
+Jupiter and Saturn have much more in common than either has
+with Uranus or Neptune; while Uranus and Neptune present
+peculiar analogies. Conclusions concerning one may
+almost be said to apply to the other. Their enormous distance,
+it is true, tends to efface minor differences; yet it is
+insufficient to obliterate similarities of a peculiar kind.</p>
+
+<p class='c007'>Uranus is a globe 32,000 miles in mean diameter, and
+decidedly elliptical in shape. Mädler and Schiaparelli
+agreed in assigning to it a compression of ¹⁄₁₁; Barnard, in
+1894, uninformed of their results, noticed the disc to be more
+oval than Saturn’s. The indicated rotational movement must
+be very swift; and a lucid spot watched by MM. Perrotin and
+Thollon at Nice in 1884, seemed to fix it at about ten hours.
+This was, however, only a vague estimate. Faint equatorial
+belts, too, have with difficulty been seen. Remembering,
+indeed, that the object they diversify is just large enough to
+be <i>annularly eclipsed</i> by a cricket ball two miles off, there is
+little cause for surprise at the indistinctness of its surface-markings.
+They probably consist, like those of Jupiter and
+Saturn, in dusky polar hoods, a brilliant equatorial zone, and
+obscure intermediate bands. The last were seen as “the
+merest shades on the planet’s surface,” and under a somewhat
+deformed aspect, by the Lick observers in 1890 and 1891.<a id='r74'></a><a href='#f74' class='c027'><sup>[74]</sup></a>
+By Professor Young in 1883, on the other hand, and by the
+MM. Henry at Paris in 1884, they were observed to be
+symmetrically placed, parallel one to the other, and of what
+might be called the normal type for great planets. That they
+<span class='pageno' id='Page_344'>344</span>constitute, with the bright space they enclose, an equatorial
+scheme of marking, was proved by Barnard’s comparison of
+the trend (or position angle), determined for them by Young,
+with the direction of the shortest axis of the little disc they
+traverse.<a id='r75'></a><a href='#f75' class='c027'><sup>[75]</sup></a> Their considerable foreshortening in 1894 was,
+doubtless, the reason why Barnard, with his acute vision, was
+compelled to rely upon earlier observations, brought up to
+date by computation. Unless, indeed, the markings are
+intrinsically variable.</p>
+
+<p class='c007'>This was suspected at Nice in 1889, when a thirty-inch refractor
+was available for their scrutiny.<a id='r76'></a><a href='#f76' class='c027'><sup>[76]</sup></a> Dusky rulings were
+obvious on a strongly compressed spheroid; and they ran
+parallel to the major axis of the spheroid—that is, to the
+planet’s equator. But their appearance varied, and their
+width seemed irregular. At the same establishment, but
+with a fourteen-inch telescope, Uranus was observed, under
+particularly favourable circumstances, March 18, 1884.<a id='r77'></a><a href='#f77' class='c027'><sup>[77]</sup></a>
+An unexpected resemblance to Mars was apparent. The
+ordinarily sea-green disc was divided into a sombre north-western
+and a bluish-white south-eastern hemisphere. Dark
+spots were visible, and a conspicuous white one at the limb
+simulated a snow-cap. But ulterior observations resolved the
+spots into belts, and showed the shining patch to be, not
+polar, but equatorial. It was presumably of an eruptive
+nature.</p>
+
+<p class='c007'>The axis upon which Uranus rotates is very much bowed
+towards the plane of its orbit. Its seasons are hence abnormal;
+but their vicissitudes can scarcely be sensible at a
+distance from the sun more than twice that of Saturn. This,
+as Mr. Proctor noticed, is the only case in which the ratio of
+one to two is exceeded in the radii of two adjacent planetary
+orbits. The radius of the Uranian track, pursued at the
+leisurely pace of 4⅕ miles a second, is 1,782 millions of miles,
+or more than 19 astronomical units. It consequently receives
+<span class='pageno' id='Page_345'>345</span>from the sun 370 times less warmth and light than the earth
+does. Area for area, it is true, the sun shines with the same
+intensity there as here; the difference lies in its apparent size.
+Instead of the broad eye of day to which we are accustomed,
+the luminary of Uranus presents a surface only 2¼ times that of
+Jupiter, as seen from the earth at an <i>unfavourable</i> opposition;
+and although Uranus is 166 millions of miles nearer to the
+sun at perihelion than at aphelion, no conspicuous difference
+would mark the passage from one to the opposite point.
+This is accomplished in 42, the entire round in 84 years.</p>
+
+<p class='c007'>In point of size, as Professor Young remarks, Uranus compares
+with the earth very much as the earth compares with
+the moon. For its surface exceeds the terrestrial surface
+about sixteen times, and its volume amounts to sixty-six
+times the terrestrial volume. Its mass, however, is less than
+fifteen times that of the earth, whence its density is represented
+(in round numbers) by the fraction ¹⁵⁄₆₆. The large
+globe is then nearly five times less dense than the small one,
+its materials exceeding the weight of an equal bulk of water
+by only one-fifth. Gravity is actually less at its surface than
+at the sea-level on the earth. Every ton of coal, for instance,
+delivered in that remote globe would fall short by two
+hundred pounds. The albedo of Uranus differs little from
+that of Jupiter; if anything, it is somewhat higher, and is
+nearly represented by the brilliancy of white paper.</p>
+
+<p class='c007'>The spectrum of Uranus indicates an emphatic departure
+from the planetary conditions so far met with. This body is
+obviously surrounded by a powerfully absorptive atmosphere,
+of a constitution foreign to our experience. The greenish hue
+of the light which has traversed some of its strata gives a
+preliminary indication of the manner in which it has been
+affected. This its spectrum, first inspected by Secchi in 1869,
+expounds in detail. He noticed a number of heavy dark bands
+in the red, while the green and blue sections remaining open
+gave to the planet its characteristic colour. A couple of years
+later, Huggins and Vogel executed concordant measurements
+of six pronounced bands, besides some faint streaks; and
+<span class='pageno' id='Page_346'>346</span>on June 3, 1889, the former obtained, with two hours’
+exposure, a beautiful spectrographic impression extending
+far up into the ultra-violet. A corroborative,
+though less comprehensive, photograph was taken by Mr.
+Frost at Potsdam, April 23, 1892. Both included many
+Fraunhofer lines, the presence of which demonstrates that the
+light of Uranus, although more powerfully stamped with
+original absorption than that of the rest of the planets, consists
+essentially of reflected solar rays. Professor Keeler’s
+admirable series of visual observations with the Lick refractor
+were undertaken in 1889 to test the truth of a suggestion
+that this peculiar spectrum consisted of bright bands upon a
+dark ground, and not of dark bands upon a bright ground.
+His decision in favour of the latter alternative was without
+appeal.</p>
+
+<p class='c007'>Of the six principal dark bands representing the arresting
+action upon light of the planetary atmosphere, four are quite
+distinctive; the fifth is the “red star line” common to the
+spectra of Jupiter and Saturn; the sixth is the hydrogen
+“F” (Hβ)—not definite and narrow as it is seen in the
+solar spectrum, but hazy, and graduating in darkness towards
+the middle, an undoubted outcome of native absorption.<a id='r78'></a><a href='#f78' class='c027'><sup>[78]</sup></a>
+Now, this is a fact that implies a great deal. It gives direct
+evidence of a very high temperature. Free hydrogen ceases
+to be present in a body upon which water can form—given,
+of course, the presence of oxygen, which it would be in the
+highest degree arbitrary to exclude. At one epoch of its
+development, the earth must have been surrounded by
+immense volumes of hydrogen. But with the diminution of
+heat, union with oxygen became possible, and the gas
+vanished to reappear in the form of liquid oceans, with their
+related hydrographic and cloud-systems. Uranus is presumably—almost
+certainly—still too hot to permit the
+combination of hydrogen and oxygen; and the absence from
+its spectrum of the slightest trace of aqueous absorption
+strengthens this inference. Doubtless, the time will come
+<span class='pageno' id='Page_347'>347</span>when the two elements will no longer be held at arms’
+length; their affinities will come into play; the familiar, all-important
+terrestrial liquid will be formed, and the geological
+history of Uranus will begin.</p>
+
+<p class='c007'>Uranus is attended by four moons. They are named Ariel,
+Umbriel, Titania and Oberon. Titania—the third in order of
+distance from the primary—is the brightest of the group, and
+has a diameter of possibly one thousand miles. Oberon is
+slightly inferior. Both were detected by Herschel in 1787.
+Ariel and Umbriel, captured by Lassell at Malta in 1851, are
+insignificant bodies in themselves—their dimensions probably
+differing but slightly from those of Hyperion, the seventh and
+least Saturnian moon, estimated to measure five hundred miles
+across. They are among the most difficult of telescopic
+objects, since they circulate about as close to Uranus as
+Mimas and Enceladus do to Saturn, are physically smaller,
+and more than twice as remote from the earth. Both were
+believed variable by Lassell, and Newcomb obtained in 1875
+plausible, though not convincing, evidence that Ariel, at any
+rate, is subject to light changes in the period of its orbital
+circulation, showing that, here again, tidal friction has done
+its work of synchronising rotation and revolution.<a id='r79'></a><a href='#f79' class='c027'><sup>[79]</sup></a> None of
+the four orbits are appreciably eccentric; they all lie in the
+same plane, and are described in periods ranging from 2½ to
+13½ days.</p>
+
+<p class='c007'>The position of that plane is, however, exceedingly remarkable.
+It is tilted at an angle of 98° to the ecliptic. This
+means that the satellites move <i>backward</i>, against the succession
+of the zodiacal signs. For direct becomes retrograde
+motion automatically, so to speak, by turning the plane in
+which it is performed beyond the limit of the vertical. The
+same fact is merely expressed in two different ways by saying
+that the bodies in question travel from west to east at an
+angle of 98°, or from east to west at an angle of 82° to the
+ecliptic. The planes of the ecliptic and of the Uranian orbit
+deviate, it should be mentioned, by only two-thirds of a
+<span class='pageno' id='Page_348'>348</span>degree. The disturbance by which the Uranian system was
+set topsy-turvy did not in the least affect the motion of
+Uranus itself.</p>
+
+<p class='c007'>Another unusual circumstance about that system is that
+the satellite-plane departs widely from the equatorial plane.
+Our own moon, it is true, is similarly circumstanced; but, on
+the Uranian scale, it is nearly eight times farther from its
+primary than Ariel, and 2·6 times farther than Oberon; while
+the enormous equatorial protuberance of Uranus almost
+seems to impose conformity upon bodies revolving so
+close to it. Conformity, none the less, is absent. The
+direction taken by the equator of Uranus, as we have seen, is
+indicated in a two-fold manner: first, by the trend of the
+belts; secondly, by the lie of the major axis. And these
+indications agree. Supposed discrepancies between them
+have been reconciled by improvements in the conditions of
+observation. But with the equatorial line the plane of
+satellite-revolution cannot be brought to coincide. The
+angle of divergence is uncertain, but may be put roughly at
+20°. This would give 78° for the inclination of the Uranian
+equator, so that the rotation of the planet is likely to be
+direct. If so, the extraordinary anomaly is here met
+with of a satellite-system circulating in a direction opposite
+to that of its primary’s rotation.</p>
+
+<p class='c007'>Uranus can at times be perceived with the naked eye.
+Indian traditions of an eighth “dark” planet have been thought
+to refer to it, and its slow course among the stars had been
+noted by savage tribes long before Herschel singled it out from
+them by its tiny disc. It is about three times brighter than
+Vesta; and Mr. Proctor stated that “in the summer of 1887
+they were comparable under favourable conditions,” when
+both, in the transparent skies of Florida, were “quite conspicuous
+without telescopic aid.” Twenty chances of discovering
+Uranus were missed before it came to Herschel’s
+turn. So many times it had been located or catalogued as a
+fixed star by astronomers far from indifferent to immortal
+fame.</p>
+
+<p class='c007'><span class='pageno' id='Page_349'>349</span>Neptune is much nearer to the sun than it ought to be.
+Both Leverrier and Adams assumed that Bode’s law would
+hold good for the planet still below the horizon of knowledge;
+they could do no otherwise; yet the rule played them false.
+Some have even asserted paradoxically that the planet found
+was not the planet sought. In point of fact, the distance of
+the theoretical Neptune is thirty-eight, that of the real
+Neptune thirty astronomical units. The mean radius of its
+orbit measures 2,792 million miles. Hence the sun is
+reduced to ¹⁄₉₀₀th its terrestrial brilliancy, and could be
+replaced by 687 full moons. “As seen from Neptune,”
+Professor Young remarks, “the sun would look very much
+like a large electric arc lamp at a distance of a few feet. It
+would give about forty-four millions the light of a first-magnitude
+star.”<a id='r80'></a><a href='#f80' class='c027'><sup>[80]</sup></a> Accordingly, Neptune does not circulate
+by any means in outer darkness. His orbit, although very
+slightly eccentric, brings him at perihelion fifty millions of
+miles nearer to the sun than at aphelion. It makes an angle
+of less than 2° with the ecliptic, and is traversed, at the rate of
+3⅓ miles a second, in a period of 165 years.</p>
+
+<p class='c007'>Neptune, being fainter than the eighth stellar magnitude,
+is quite inaccessible to unaided vision. But a good telescope
+at once displays the seeming star in the guise of a small
+planetary nebula with a diameter of 2″·433. This mean
+value, reduced to the mean distance of the planet from the
+sun, was afforded by Barnard’s measures in 1895 with a
+power of 1,000 on the Lick refractor.<a id='r81'></a><a href='#f81' class='c027'><sup>[81]</sup></a> It corresponds to a
+linear diameter of 32,900 miles. Neptune accordingly, although
+only 17 times more massive than the earth, is 72 times more
+bulky, and composed of materials 4·2 times specifically lighter.
+Gravity at its surface has almost precisely its terrestrial
+power. The albedo of Neptune, combining Zöllner’s with
+Müller’s results, is 0·65; and its spectrum appears identical
+with that of Uranus. It may be inferred that this planet also
+is too hot to contain water.</p>
+
+<p class='c007'><span class='pageno' id='Page_350'>350</span>Its satellite is believed to be of about the size of the
+moon; but since it is 12,000 times more distant, it can be
+distinguished only with the most powerful telescopes as a star
+of the fourteenth magnitude. The radius of its orbit measures
+225,000, that of our moon 238,000 miles; but Neptune’s attendant
+completes a circuit in 5 days 21 hours; and it is through
+this rapidity of movement that the large mass of its primary
+has been learned. It resembles the moon besides in being
+solitary, so far as can be ascertained by the most diligent
+researches; and it is beyond doubt that if any companion-bodies
+exist they are comparatively small or obscure. That
+they do exist, appears probable on the face of it.</p>
+
+<p class='c007'>The one Neptunian satellite emphasises the problems set
+by the Uranian four. These problems are concerned with the
+origin and early mechanical relations of the solar system.
+Here, at its utmost verge, we encounter a decided reversal in
+the direction of systemic motion—a reversal prepared for, as
+it might seem, by the nearly vertical position of the Uranian
+plane of satellite-revolution. This diversity is in no sense
+“accidental,” as some have unwisely asserted, invoking impacts
+of comets, and such like futile devices, to account for
+it; it belongs fundamentally to the design of planetary evolution.
+Laplace’s scheme has no room for it; Faye’s, constructed
+expressly to include it, requires that Uranus and
+Neptune, instead of being the first, should have been the
+latest formed of all the solar train. And their obviously rudimentary
+condition favours the suggestion. Neptune’s satellite
+revolves from east to west in a quasi-circular path, inclined
+to the ecliptic at an angle of 35°; or, putting it otherwise, it
+revolves from west to east at an angle of 145°.</p>
+
+<p class='c007'>As the only member of the solar system exempt from perturbations
+by a third body (the sun being too remote to cause
+perceptible deflections), it seemed admirably fitted to discharge
+the functions of a standard celestial clock, greatly
+needed, but nowhere to be found in our system.<a id='r82'></a><a href='#f82' class='c027'><sup>[82]</sup></a> But in 1886
+Mr. Marth drew attention to certain divagations of this
+<span class='pageno' id='Page_351'>351</span>“ideal time-keeper” resulting from conspicuous changes
+in the position and plane of its orbit. They were explained
+almost simultaneously in 1888 by M. Tisserand,<a id='r83'></a><a href='#f83' class='c027'><sup>[83]</sup></a> late
+director of the Paris Observatory, and by Professor Newcomb.<a id='r84'></a><a href='#f84' class='c027'><sup>[84]</sup></a>
+The disturbance, which, in its mode of production, is analogous
+to the precession of the equinoxes, results from the polar
+compression of the Neptunian globe combined with a deviation
+of the satellite’s motion from its equatorial plane. By
+the action of the protuberant girdle, a slow gyration of the
+secondary body’s orbital plane is produced, its inclination to
+the primary’s equator remaining unchanged. Viewed under a
+different aspect, the same phenomenon may be described as a
+retrograde movement, in a period of at least five hundred years,
+of the pole of the satellite’s orbit round the pole of the
+planet’s equator. The radius of the circle described cannot
+be less than 20°, implying a flattening of the Neptunian globe
+of ¹⁄₈₅th, and may easily amount to 30°, with which an ellipticity
+of ¹⁄₁₁₅ should be associated. But before the centre of this
+circle—that is, the pole of Neptune’s axial movement—can
+be satisfactorily located, several centuries must elapse. At
+present we may affirm with reasonable certainty: first, that the
+rotation in question is retrograde, like the satellite’s revolution;
+secondly, basing the inference upon the comparatively slight
+ellipticity of Neptune’s figure, that it is much slower than the
+vertiginous spinning of Jupiter, Saturn, and Uranus.</p>
+
+<p class='c007'>Uranus and Neptune are, as has been said, companion globes.
+In bulk and density they differ very slightly; their albedoes are
+virtually the same, their spectra indistinguishable. They seem
+perfectly alike in chemical and physical constitution, and to
+be situated at precisely the same stage of development.
+Both govern retrograde systems. In Uranus the peculiarity
+appears as if in an incipient form; in Neptune, strongly
+accentuated.</p>
+
+<p class='c007'>Viewed from the position of Neptune, all the planets are
+morning and evening stars. They are tethered to the
+<span class='pageno' id='Page_352'>352</span>chariot-wheels of the sun, instead of having the run of the
+sky. “The four terrestrial planets,” Professor Young writes,
+“would be hopelessly invisible, unless with powerful telescopes,
+and by carefully screening off sunlight. Mars would
+never reach an elongation of three degrees from the sun; the
+maximum elongation of the earth would be two, and that of
+Venus about one and a half degrees. Jupiter, attaining an
+elongation of about ten degrees, would probably be easily
+seen somewhat as we see Mercury. Saturn and Uranus
+would be conspicuous, though the latter is the only planet of
+the whole system that can be better seen from Neptune than
+it can be from the earth.”<a id='r85'></a><a href='#f85' class='c027'><sup>[85]</sup></a></p>
+
+<p class='c007'>To a spectator retreating with the velocity of light, all the
+planetary cortège would in a few hours disappear, and the
+sun would shine alone. No sign would remain that his office
+is purely ministerial—that he exists only to enlighten, rule,
+and vivify the relatively minute globes shred from his mass
+in the beginning, maintaining by his attractive power the
+adjusted movements of the complicated piece of mechanism
+they constitute. The skies perhaps hold millions of his
+stamp; every solitary star telescopically visible may be the
+centre of a planetary scheme like our own; or, on the other
+hand, our own may, quite conceivably, have no counterpart
+in the wide universe.</p>
+
+<h3 class='c004'>CHAPTER XI.<br> <span class='c014'>FAMOUS COMETS.</span></h3>
+
+<p class='c003'>In the fourth year of the 101st Olympiad (373 <span class='fss'>B.C.</span>), the
+Greeks were startled by a celestial portent. They did not,
+at that time, draw fine distinctions, and posterity would have
+remained ignorant that the terrifying object was a great
+<span class='pageno' id='Page_353'>353</span>comet but for the description of it left by Aristotle, who saw
+it as a boy at Stagira. It was mid-winter when it flared up
+from due west at sunset, its narrow, definite tail running
+“like a road through the constellations” over a third of the
+heavens. Diodorus relates that it cast shadows like the
+moon, which implies a very unusual, yet not impossible,
+degree of brightness. The prompt engulfment by an earthquake
+and its attendant tidal wave of the Achaean towns,
+Helice and Bura, justified the apprehensions it aroused.
+It never came back to retrieve its reputation. During at
+least two thousand subsequent years, such objects lay
+under the ban of popular superstition; and the counts
+upon which they were accused of malefic influence were
+so many and so vague that acquittal was impossible.
+Their respect of persons was notorious; nor were they
+consistent in their dealings with the great, to whom
+alone they paid individual attention. A comet marked
+the apotheosis of the great Julius; a comet announced
+the death of Constantine; a comet illuminated the cradle
+of Napoleon.</p>
+
+<p class='c007'>The very word “comet” takes us back to the Stagyrite;
+for it is derived from the Greek word κόμη, hair, and signifies
+a <i>hirsute</i> star. Shakspeare’s “crystal tresses” represent what
+we now, in homely fashion, call the “tail,” while the “nucleus”
+and “coma” make up the “head.” The nucleus, in great
+comets, shines like a star of the first magnitude, sometimes
+indeed surpassing the brilliancy of Jupiter. It is
+usually of measurable dimensions, often of granular texture.
+The planetary disc, round which the filmy appendages
+of the comet of December 1618 were displayed,
+was observed by Cysatus, a Jesuit astronomer at Ingolstadt,
+to become transformed into the semblance of a
+star cluster; Hevelius noticed a double nucleus in the comet
+of 1652; and modern instances of the same kind abound.
+There is indeed no likelihood that substantial globes are ever
+included in the construction of comets.</p>
+
+<p class='c007'>The coma is of immense volume, and extreme tenuity.
+<span class='pageno' id='Page_354'>354</span>The rays of faint stars traverse, undimmed and unrefracted,
+strata of it tens of thousands of miles in thickness. Yet strong
+lines of structure develop in it through the influence of forces
+emanating from the sun. As they approach our system out
+of the depths of space, comets are scarcely distinguishable
+from round nebulæ, and they relapse into a similar quiescent
+condition on leaving it. Their temperature must then be
+very near the absolute zero of cold, since they cannot be supposed
+either to contain stores of native heat, or to retain
+stores of borrowed heat. Thus the rapidly augmenting
+power of solar radiation, as they rush with accelerated
+velocity nearer and nearer to its source, produce upon them
+stupendous effects. The nucleus blazes out into a coruscating
+star; the coma, violently driven off from it, forms multiple
+envelopes like thin gauze veils, one outside the other, flung
+round the nucleus on the side next the sun, separated by
+intervening dark spaces, and diversified by brilliant jets and
+sectors. The tail is the outcome of a double repulsion.
+Matter expelled by the nucleus towards the sun is, at a
+certain point, thrown back to form an immense, oppositely
+directed appendage, usually convex on the forward
+side. Some tails resemble hollow cones, being bright at the
+edges, and dark within: others are traversed by a shining
+<i>backbone</i>; many, perhaps all, are composite. The magnificent
+object first seen by Klinkenberg at Haarlem, December 9,
+1743, was supplied with six, varying in length from 30° to 44°,
+each, according to the extant representations, being separately
+<i>rooted</i> in the head. Grouped into a lustrous fan, they presented
+a very beautiful and surprising appearance, not again
+to be displayed until the world and humanity have undergone
+some unlooked-for changes. For the period of the comet
+was computed to be one hundred thousand years! Tails,
+less obviously and splendidly multiplex, are rather the rule
+than an exception. Or rather, closer observations, chiefly
+photographic, have made it manifest that the single efflux
+of nebulous stuff generally designated as a comet’s tail can
+be analysed into bundles of fibres, into straight rays and curved
+<span class='pageno' id='Page_355'>355</span>plumes of light, or into knotted and branching emanations.
+Homogeneous outflows, such as are seen in drawings, do not
+really exist. Tails pointing <i>towards</i> the sun have also been
+occasionally noticed; but they are always feeble. Olbers recorded,
+however, that, during eight days of January, 1824, the
+comet then visible had a solar tail of 7°, while its anti-solar
+tail was only 3½° long.</p>
+
+<p class='c007'>The great comet of 1680 will always be memorable for
+having had its orbit calculated by Newton on gravitational
+principles. It was not unworthy of the distinction. Approaching
+the sun almost in a straight line, it penetrated the
+corona at the rate of 370 miles a second, and passing within
+140,000 miles of the photosphere, escaped by means of its
+extraordinary velocity from those perilous precincts. Resulting
+internal commotions became evident through the rapid
+development of a tail more than a hundred million miles in
+length. Newton calculated that particles from the head
+reached its extremity in two days. He assigned to the
+comet a highly elliptical orbit traversed in six centuries. But,
+since its speed might be called parabolic, millenniums may
+be nearer the mark than centuries. It cannot, therefore, be
+identified with any earlier apparition.</p>
+
+<p class='c007'>The comet of 1682 was Halley’s, the predicted return of
+which, in 1759, was unprecedented and memorable. At
+its apparition in 1835, valuable observations of a physical
+kind were made upon it by Bessel at Königsberg, and by
+Sir John Herschel at the Cape. They were facilitated
+by the circumstance that this far-travelling body, the
+perihelion distance of which is 55 million miles, and the
+aphelion-distance 2½ times that of Neptune, approached the
+earth on this occasion within 4½ million miles. It was remarkable
+for singular and sudden changes of aspect. To
+Bessel the nucleus seemed like a burning rocket. Divergent
+flames issued from it towards the sun, and he took especial
+note of a blazing “sector,” which swung like a pendulum to
+and fro, in a period of 4⅗ days. These emanations, accumulating
+at the surface where the solar balanced the cometary
+<span class='pageno' id='Page_356'>356</span>repulsive force, were then swept back, as if by a tempestuous
+wind, to form a tail, which, on October 15, measured at least
+24°. The conviction was forced upon him that the body in
+which these wonderful processes were going on was affected
+by opposite polarities; and he fully concurred with Olbers in
+the opinion that tail-production was a purely electrical
+phenomenon.</p>
+
+<p class='c007'>During some time before and after its perihelion passage
+on November 16, the comet wore the disguise of a star.
+All its hairy appendages had vanished. On the 23rd of
+January, 1836, it was sharply stellar; twenty-four hours later
+it had acquired, besides a twenty-fold increase of light, a disc
+like that of the planet Neptune, enclosed in a nebulous sheath
+of about fourfold breadth. Later in its career, Sir John
+Herschel<a id='r86'></a><a href='#f86' class='c027'><sup>[86]</sup></a> observed the nucleus under the form of “a miniature
+comet, having a nucleus, head, and tail of its own,
+perfectly distinct, and considerably exceeding in intensity of
+light the nebulous disc or envelope” containing it, which was,
+properly speaking, the “head” of the comet. At last, on
+May 5, through the progress of distension, the last thin
+shred of its substance melted into the sky. The next return
+of Halley’s comet, somewhat accelerated by Jupiter’s influence,
+is looked for in the year 1910.</p>
+
+<p class='c007'>The “vintage comet” lingered in northern skies during
+510 days—from March 26, 1811, until August 17, 1812. It
+was attentively observed by Sir William Herschel, who
+gathered from it the then new truth that comets are self-luminous
+bodies. “The quality of giving out light,” he
+acutely remarked, “is immensely increased by an approach
+to the sun.” But he failed to persuade his contemporaries or
+successors. His inference had to wait for spectroscopic
+demonstration. The nucleus of the comet of 1811 he found
+to measure 428 miles. It showed a ruddy hue, and was
+eccentrically placed within a greenish-blue “planetary body”
+127,000 miles in diameter. This was again enclosed in a
+shining atmosphere about four times as wide, round which
+<span class='pageno' id='Page_357'>357</span>was flung an envelope of a yellow tint, forming a thin hemispherical
+shell on the side next the sun, and continued indefinitely
+away from the sun as the hollow cone of the tail.
+Owing to this mode of construction, the space between the
+head and the hemispherical sheath, as well as the central part
+of the tail, appeared dark. The latter extended, in October,
+over 100 million miles of space, and was 15 million miles
+broad. Its soft radiance resembled that of the Milky Way,
+side by side with which it ran on November 9, 1811.
+The comet’s path lay entirely outside the earth’s orbit,
+and Argelander assigned to it a period of 3,065 years.
+The restriction was needless. Between a period of infinite
+length, and one of 3,000, or 1,000 years, no valid distinction
+can, where comets are in question, be drawn. The short
+sections of their tracks observable from the earth might
+belong equally well to parabolas or to the far-stretching
+ellipses which such protracted periods imply.</p>
+
+<p class='c007'>The apparition of 1811 suggested to Olbers the “electrical
+theory” of comets’ tails. The uncommon impressiveness
+with which it displayed not uncommon phenomena, was
+perhaps a result of its considerable distance from the sun,
+owing to which the <i>interior</i> force obtained an advantage over
+the <i>exterior</i>, and the locus of equilibrium between solar and
+cometary repulsion was pushed back further than usual from
+the nucleus.<a id='r87'></a><a href='#f87' class='c027'><sup>[87]</sup></a> He calculated that the materials of the tail
+spent 11 minutes in the journey from its root to its tip, indicating
+ejection by a force greatly more powerful than the
+opposing force of gravity. Olbers anticipated the modern
+view that chemical differences determine the shapes of
+comets’ tails, the various species of matter being diversely
+acted upon by electrical repulsion. The long, straight ray,
+for instance, issuing from the comet of 1807, must, he perceived,
+have been composed of particles much more energetically
+repelled than those aggregated in the inflected plume
+with which it was associated. The curvature of these appendages,
+in fact, depends upon the relation between the orbital
+<span class='pageno' id='Page_358'>358</span>velocity of the comet and the velocity of ejection imparted
+to their constituent molecules. It has to be borne in mind,
+however, that while curved tails may appear straight in projection,
+straight tails can never appear curved</p>
+
+<p class='c007'>Olbers’ classification of comets is still of great significance.
+He divided them into:</p>
+
+<p class='c007'>(1.) Comets which develop no matter subject to solar repulsion.
+These are without tails, and may be regarded as
+simple nebulosities devoid of solid nuclei.</p>
+
+<p class='c007'>(2.) Comets showing no trace of nuclear, while subject to
+solar repulsion. They throw out no matter <i>towards</i> the sun;
+the heads are consequently left bare of envelopes, and are of
+simple structure. The comet of 1807 was of this kind.</p>
+
+<p class='c007'>(3.) Comets manifesting the effects of both species of action.
+They are characterised by the presence of a dark hoop round
+the head, and of a dark rift in the tail, by which it may be
+judged to be a hollow conoid.</p>
+
+<p class='c007'>On February 28, 1843, a “short, dagger-like object” blazed
+out at an interval of only fifty-two minutes of arc from the
+sun’s limb. It was viewed with amazement in various parts
+of the world; and spectators in Italy, by shielding their eyes
+from the direct mid-day glare, were able to discern a tail already
+several degrees long. The proportions of the appendage rapidly
+grew. On March 3, it measured twenty-five degrees; on
+March 11, an adjunct to it shot out, within twenty-four hours,
+to nearly twice the apparent length of the main structure,
+conveying, as Sir John Herschel said, “an astounding impression
+of the intensity of the forces at work.” It was first
+seen in this country after sunset on March 17, as “a perfectly
+straight, narrow band of white cloud, thirty degrees in length,
+and about one and a half in width.” On the following night,
+Sir John identified this “luminous appearance” as the tail of
+a grand comet, stretching over an extent of space (as it afterwards
+proved) of no less than two hundred millions of miles.</p>
+
+<p class='c007'>The movements of this body were as surprising as its
+aspect. It rushed past perihelion with a speed of 366 miles a
+second, leaving an interval of 100,000 miles between its
+<span class='pageno' id='Page_359'>359</span>centre and the sun’s surface, and swinging through two right
+angles in two hours and eleven minutes. The northern part
+of its course was finished in two hours and a half; hence, it
+was a “southern” comet. Very curiously, it seems to have
+remained obscure throughout its journey towards the sun, reserving
+its outburst for the day <i>after</i> perihelion. Periods
+were assigned to it ranging from seven to six hundred years.</p>
+
+<p class='c007'>Strangest of all, it turned out to be but one member of a
+whole family of similarly-conditioned bodies. The “great
+southern comet” of February, 1880, seemed like its ghost.
+It had no perceptible nucleus, but an inordinately extended
+train, which rapidly faded; and it scarcely deviated by a
+hair’s breadth from the track of its predecessor. That is to
+say, so far as could be ascertained; for the object was so indefinite
+as to elude exact observation. Its period could not
+even be conjectured. The nature of the relationship between
+the comets was thus left uncertain.</p>
+
+<p class='c007'>But after the lapse of two years and a half, the question
+was reopened by the appearance of the leading constituent of
+the group. Like the comet of 1843, the “great September
+comet” of 1882, was first seen close beside the sun. At
+Ealing, shortly before noon, on September 17, Dr. Common
+was struck with the astonishing spectacle of a brilliant comet
+hurrying up to perihelion. A transit was evidently imminent,
+but clouds veiled the scene. Its completion was, however,
+fortunately witnessed six thousand miles away by Mr. Finlay
+and Dr. Elkin at the Cape Observatory. The comet was
+watched by them “right into the boiling of the limb,” which
+it had no sooner touched, than it utterly disappeared. This
+cannot have been through the absence of contrast; for
+although its intrinsic brilliancy was excessive, it must either
+have shown bright against the sun’s dusky margin, or dark
+when projected upon his dazzling centre. Since neither
+effect was produced, it can only be inferred that the object
+was translucent owing to insubstantiality. That it had not
+passed <i>behind</i> the sun was later fully ascertained. During
+three subsequent days the “blazing star near the sun” drew
+<span class='pageno' id='Page_360'>360</span>popular attention in the southern hemisphere, and many parts
+of Europe. Nothing quite so extraordinary had ever been
+seen before. The spectacle of 1843 was renewed, but outdone.</p>
+
+<p class='c007'>Meanwhile, an astonished public hung on the dicta of perplexed
+astronomers. The speculation which obtained most
+currency was that the three successive southern comets were
+accelerated returns of the same body, destined, after a few
+short, spiral circuits, to make fiery shipwreck in the glowing
+solar ocean. The effects upon terrestrial life were unwarrantably
+described as likely to prove disastrous; but only
+an abortive panic ensued. Data, however, to serve as the
+basis of a determinate conclusion, were on this occasion
+collected in abundance. The comet of 1882 was not lost
+sight of until June 1, 1883, when its distance from the earth
+was more than five astronomical units—the greatest at which
+any previous comet except that of 1729 had been observed.
+Hence the general character of its orbit became thoroughly
+known. It proved to deviate somewhat from the tracks
+pursued by the comets of 1843 and 1880; it gave the sun a
+slightly wider berth; above all, its period had unmistakably
+a duration of several centuries. There could then be no
+further question of its being a return of either, or both of
+those bodies, although its close connexion with them
+was assured. This can be most rationally explained by
+supposing them to have primitively constituted a single body.
+According to Professor Kreutz’s able and exhaustive research,
+the period of the September comet is 772, that of the
+comet of 1843, between five and six hundred years; and the
+relative situation of their orbits indicates that the supposed
+catastrophe of their disruption took place at perihelion, where
+a large incoherent mass could scarcely fail to yield to the
+strain of the sun’s unequal attraction at the excessively close
+quarters it was brought into by the conditions of its movement.
+The comet of 1880 is another splinter from the same
+trunk; and yet one more fragment presented itself to M.
+Thome at Cordoba, January 18, 1887, when he observed
+<span class='pageno' id='Page_361'>361</span>literally a “nine days’ wonder” in the guise of a shadowy
+ray, thirty-five degrees in extent, following the lead of the
+other “southern comets,” and taking rank (so far) as the last
+and least of their company.</p>
+
+<p class='c007'>A tendency to still further disaggregation was evident in
+the comet of 1882. It did not pass with impunity through
+the fiery ordeal of its visit to the sun; internal agitations
+supervened; abnormal appendages of rarefied texture, but
+prodigious dimensions, issued from it sunward; the nucleus
+broke up into six spherules like strung pearls; and it was
+noticed in October to be surrounded by detached nebulous
+masses, just launched perhaps on independent cometary
+careers. The tail was two-fold. It consisted of a dim, straight
+ray which temporarily attained a length of a couple of hundred
+millions of miles, and a massive forked appendage,
+strongly luminous and unusually permanent. Fig. 19 shows
+one of a series of photographs of this comet taken with an
+ordinary portrait lens under Dr. Gill’s direction in October,
+1882. The observations of its transit proved to be of great
+importance. Having been made just before perihelion, they
+availed to demonstrate that no loss of motion had been
+suffered in its plunge through the corona. This incontrovertible
+fact implies an inconceivable degree of rarity in the
+solar surroundings.</p>
+
+<div id='i_361'  class='figcenter id003'>
+<img src='images/i_361.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 19.</span>—<i>Great Comet of September, 1882. Photographed at the Royal Observatory, Cape of Good Hope.</i> (From Clerke’s “History of Astronomy,” 3rd ed.)</p>
+</div>
+</div>
+
+<p class='c007'>So long ago as 1831, Clausen pointed out that many comets
+are grouped together after the manner incomparably exemplified
+<span class='pageno' id='Page_362'>362</span>later by the southern comets. An analogous system,
+composed of only two known members, is formed by the
+comet of 1807, and Tebbutt’s comet of 1881. The former,
+made by Bessel the subject of a masterly investigation, was
+not again due at perihelion until the remote epoch 3346 <span class='fss'>A.D.</span>,
+so that the announcement of a reappearance so exceedingly
+premature was startling. But when the new comet was
+also found to have a period of several thousand years, it became
+clear that no return had been observed, but only a companion
+recognised. Tebbutt’s comet was a beautiful object.
+Its head, adorned with interlacing arcs of light, was an overmatch
+for Capella, while so translucent that a star of the
+seventh magnitude seemed rather to gain than to lose brightness
+by shining centrally through it. As the upshot of these
+singular experiences, the difficulty of identifying comets has
+been increased tenfold. Their aspects were always perceived
+to be well-nigh interchangeable, but their movements were
+held to be distinctive; now their very orbits are found to be,
+to a considerable extent, common property.</p>
+
+<p class='c007'>A small, glimmering nebulosity descried at Florence by
+Donati, June 2, 1858, gave little promise of coming splendour.
+Yet few more picturesque celestial effects have been witnessed
+than it presented, October 5, when Arcturus blazed
+undimmed through the denser part of the tail, in brilliant
+conjunction with the equal splendour of the nucleus. The ineffable
+grace with which the comet spread its luminous plumage
+was set off by the juxtaposition, as if for the purpose of
+determining the amount of its curvature, of a long, perfectly
+straight ray. The aspect of this beautiful object on October
+3, is represented in Fig. 20; some idea of its rapid development
+in size and brilliancy can be gathered from an inspection
+of the Frontispiece to this Section. The apparition lasted,
+to the naked eye, for 112 days, and will not again be visible
+for 2,000 years. So that Donati’s comet may be reckoned
+an “irrevocable traveller.”</p>
+
+<div id='i_363'  class='figcenter id003'>
+<span class='pageno' id='Page_363'>363</span>
+<img src='images/i_363.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 20.</span>—<i>Donati’s Comet, October 3, 1858.</i> (From Langley’s, “New Astronomy.”) <i>The Star to the left of the Comet’s head is Arcturus.</i></p>
+</div>
+</div>
+
+<p class='c007'>Twice during the present century the earth has traversed,
+with impunity, the tail of a comet. First, on June 26, 1819,
+<span class='pageno' id='Page_365'>365</span>when a comet passed invisibly between us and the sun, sending
+its tail our way. Again on June 30, 1861. The sun had
+scarcely set that evening when a yellowish disc became apparent
+at the horizon, from which issued an enormous double
+train, enclosing our planet within its folds. The closing-up
+and withdrawal of the “outspread fan” to which they were
+compared was accomplished in a few hours. The head of the
+comet had as many envelopes as a Chinese puzzle.</p>
+
+<p class='c007'>The first recognised “short-period” comet approached
+within one and a half million miles of the earth, July 1, 1770.
+Had it possessed ¹⁄₅₀₀₀th the mass of the globe which rushed by
+it with entire indifference, a perceptible lengthening of the year
+should have ensued; and its gravitational insignificance was
+confirmed by the fact that it passed, in 1779, right through
+the Jovian system without troubling the mutual relations of
+its members. Lexell (with whose name it has continued to
+be associated) fixed its period of revolution at five and a half
+years; yet it had never been seen before. Astronomers, in
+fact, caught it on its trial trip along a fresh orbit to which
+it had been transported in 1767 by the disturbing power of
+Jupiter, and whence it was removed by the same influence in
+1779. An intermediate return in 1776 had doubtless occurred;
+but circumstances precluded its observation. Further
+encounters with the giant planet may, however, bring back
+the vagrant, and the possibility was thought to have been
+realised when the history of a comet discovered by Mr.
+Brooks of Geneva, N.Y., July 6, 1889, came to be inquired
+into. Its return about the predicted time in 1896 afforded
+an opportunity for revising the laborious inquiry, with the
+result of disproving the case for identity.</p>
+
+<p class='c007'>A comet, lost under very different circumstances, was
+picked up February 27, 1826, by an Austrian officer, Wilhelm
+von Biela. His calculations led him to the unlooked-for
+discovery that it travelled in an orbit with a period of
+6½ years, and had already been observed in 1772 and in 1805.
+On its return in 1832, when it had become reduced to the
+status of a telescopic object, Sir John Herschel watched its
+<span class='pageno' id='Page_366'>366</span>conjunction with a knot of minute stars, the rays of which
+traversed it without the smallest obstruction. It had neither
+tail nor nucleus; its aspect was that of the commonest type
+of nebula. On December 29, 1845, however, a curious change
+was seen to have affected it. The comet had split into two,
+each of which immediately assumed the characteristic
+cometary shape, by providing itself with a tail and bright
+nucleus. Thus divided and regenerated, the pair advanced
+side by side, 157,000 miles apart, without the least trace of
+mutual action through gravity, but displaying vivid interchanges
+of brightness, reasonably attributed to the play of
+electrical forces.<a id='r88'></a><a href='#f88' class='c027'><sup>[88]</sup></a> They re-visited the sun in 1852, but have
+never since, and most probably will never again, be seen.
+Their end came through senile decay. It was that predicted
+by Newton for all such bodies. <i>Diffundi tandem et spargi
+per universos cœlos.</i></p>
+
+<p class='c007'>The most rapidly-revolving comet of our acquaintance was
+investigated in 1819 by Johann Franz Encke, of the Seeberg
+Observatory, who assigned to it a period of 3½ years, and
+predicted its return in May, 1822. It was punctually
+recaptured at Sir Thomas Brisbane’s Observatory in New
+South Wales. Encke traced back its appearances to 1786,
+and identified it with a comet detected by Caroline Herschel
+in 1795. At its last return in 1894–5, it was just at the limit
+of naked eye visibility. It fluctuates, however, considerably,
+at successive apparitions. M. Berberich<a id='r89'></a><a href='#f89' class='c027'><sup>[89]</sup></a> has sought to
+associate these perplexing changes with solar vicissitudes;
+but his arguments are not entirely convincing. Encke’s
+comet, even if 45,000 billion times less dense than air at
+atmospheric pressure—the consistence attributed by Babinet
+to cometary matter—would still weigh twelve hundred tons.<a id='r90'></a><a href='#f90' class='c027'><sup>[90]</sup></a>
+Its excessive rarefaction is a matter of ocular proof. On
+October 21, 1881, Barnard observed a central passage of this
+comet, then more than usually bright and condensed, over a
+<span class='pageno' id='Page_367'>367</span>ninth magnitude star, which “remained so remarkably
+distinct during the entire progress of occultation, that it
+formally impressed me with the idea of a transit of the star
+<i>across</i> the comet—a pearly point floating between me and the
+bright mass of vapour.”<a id='r91'></a><a href='#f91' class='c027'><sup>[91]</sup></a></p>
+
+<p class='c007'>This object signally exemplifies the cometary peculiarity
+of contracting near perihelion, and re-expanding after the
+critical point has been passed. Thus, it measured 312,000
+miles across, October 28, 1828, when 135 million miles from
+the sun, but only 14,000 on December 24, when its distance
+had been reduced to 50 millions; and in passing perihelion,
+December 17, 1838, at an interval of 32 millions, its diameter
+had shrunk to 3,000 miles. It fulfils, as regards Mercury,
+the function of spying upon the planets, assigned to
+comets by Airy; for, only through the Mercurian disturbances
+of its motion has the Mercurian mass been at all definitely
+ascertained; and a residual acceleration, which, at each
+circuit, brings it back to perihelion a couple of hours before
+the appointed time, has long been regarded as an index to
+the condition of planetary space. Encke explained this
+shortening of period by the action of an hypothetical “resisting
+medium” augmenting in density towards the sun; but
+accumulated facts have swept it out of existence. The
+southern comets performed for our benefit, one after the
+other, an <i>experimentum crucis</i> in the matter. The chief of
+them, on September 17, 1882, swept through a region where
+Encke’s medium should be <i>two hundred thousand</i> times
+denser than it is at the perihelion distance of Encke’s comet;
+yet suffered no appreciable loss of motion. Nor has the
+comet itself of late complied with the requirements of the
+theory it suggested. At its return to the sun in 1868, the
+acceleration had fallen to one-half its customary, and until
+then, constant value. And the change has proved to be
+permanent. But the influence of the postulated medium is
+evidently incapable of diminution. Thus, the movements of
+Encke’s comet still remain problematical.</p>
+
+<div>
+  <span class='pageno' id='Page_368'>368</span>
+  <h3 class='c004'>CHAPTER XII.<br> <span class='c014'>NATURE AND ORIGIN OF COMETS.</span></h3>
+</div>
+
+<p class='c003'>Comets reflect sunlight, and also emit light of their own.
+But the combination was scarcely thought of as possible
+until the spectroscope gave its verdict. The first analysis of
+cometary rays was made by Donati at Florence, August 5,
+1864. They were dispersed by his prisms into a yellow, a
+green, and a blue band, with wide intervals between. Their
+chemical interpretation was afforded by Dr. Huggins in 1868.
+The subject of his experiments was Winnecke’s comet, an
+insignificant object with a period of five and a half years.
+He found it to be composed—at least in part—of acetylene,
+or some other hydro-carbon gas. The coloured bands agreed
+precisely in position with those in the spectrum of the blue
+light at the base of a candle-flame, or of a gas-jet. The
+spectra of the immense majority of comets is of this pattern,
+with more or less of continuous light added. A portion of
+this is borrowed, a portion inherent. A photograph of the
+spectrum of Tebbutt’s comet (1881, III.), taken by Dr.
+Huggins, June 24, 1881, demonstrated by its distinct impression
+with several Fraunhofer lines the presence of solar
+radiance; the association of which with native emissions of
+the continuous sort has been made evident in various comets
+by sudden outbursts of white light.</p>
+
+<p class='c007'>Comets do not then consist entirely of carbon-compounds;
+but their remaining constituents make no distinctive show in
+their spectra unless when sun-raised agitation is particularly
+vehement. Thus, an approach within five million miles of the
+sun evoked in comet Wells (1882, I.), sodium-luminosity, detected
+by Dr. Copeland at Dunecht, June 17, 1882. The blaze
+was so vivid that a crocus-tinted image of the entire head with
+the beginning of the tail was visible, like a solar prominence,
+through the open slit of the spectroscope. The same observer
+witnessed an outbreak of both sodium and iron lines
+<span class='pageno' id='Page_369'>369</span>in the September comet (1882, II.). In both cases, the newly-kindled
+emissions effaced the old, and, after a time, were
+replaced by them. This mode of procedure is characteristic
+of electrical action, and combines with other symptoms to
+assure us that cometary illumination is produced by interior
+electrical disruptive discharges due to solar induction.</p>
+
+<p class='c007'>Olbers’s felicitous conjecture has been developed into a
+plausible theory of comets’ tails by M. Bredichin, late director
+of the Pulkowa Observatory. He divided them into three
+“types,” distinguished by the values of the repulsive forces
+employed severally in their production. Those belonging to
+type I. imply the exertion of a counter-influence fourteen
+times stronger than gravity. They are long, straight rays,
+the constituent particles of which are carried, in a torrent too
+swift to be deflected, to the observed extraordinary distances.
+Their outward velocity of five miles a second to start with is,
+we must remember, constantly accelerated, and finally becomes
+enormous. Halley’s comet and the great comets of 1811 and
+1861 had tails of this type. Donati’s great plume exemplified
+the second, in which the average strength of repulsion exceeds
+that of gravity one and a half times. Tails of the
+third type correspond to a ratio varying from three-tenths
+to one-tenth. Solar attraction is, in them, only partially
+neutralised. They are short, strongly-bent, brush-like appendages,
+seldom seen apart from those of a more striking
+kind.</p>
+
+<p class='c007'>These three types have a physical meaning of great interest.
+The attractive force of gravity varies as the mass, the repulsive
+force of electricity as the surface of the molecules
+they sway; hence the ratio of repulsion is inversely as the
+ratio of molecular weight, the lightest particles being the
+most violently driven away from the sun. Assuming them
+to be hydrogen-molecules, Bredichin found that the atomic
+weights of hydro-carbon gases and iron would correspond
+fairly well with the speed of projection signified respectively
+by the curvatures of the second and third types of tail.
+Materials of other kinds are not excluded; their presence is,
+<span class='pageno' id='Page_370'>370</span>indeed, demanded by the width of these appendages, which
+obviously consist of bundles of emanations differently influenced,
+and presumably of a different chemical nature.
+Bredichin’s theory works admirably from a geometrical point
+of view. All the varieties of cometary trains can be constructed
+by strict calculation from the basis it supplies. Yet
+there are spectroscopic difficulties in the way of accepting it
+unreservedly. No evidence is at present forthcoming of any
+connexion between the chemistry of tails and their shapes;
+and hydrogen rays are conspicuously absent from cometary
+spectra.</p>
+
+<p class='c007'>“Short period,” or “planetary” comets may be defined as
+those revolving in periods of less than eight years. They
+have much more in common, however, than the quickness of
+their successive returns to the sun. All move from west to
+east; they show some preference for the plane of the ecliptic;
+and none of their orbits are excessively elongated. Thus,
+they tend towards conformity with the regular ordinances
+of the solar system, which its less accustomed visitants
+completely ignore. All, too, have a <i>used-up</i> appearance.
+This is easily understood. They have wasted their substance
+spinning out nebulous appendages—<i>sicut bombyces filo
+fundendo</i>, as Kepler said—at their frequent returns to perihelion.
+They are thus visibly effete bodies. Before long,
+they will drop out of individual existence, and survive
+obscurely, reduced to the “dust of death.” Yet the supply
+is not likely to become exhausted. Discovery proceeds
+faster than disappearance.</p>
+
+<p class='c007'>“Lost comets” belong, without exception, to this class.
+Two typical instances have already been mentioned in the
+disaggregation of Biela’s, and the removal of Lexell’s comet.
+The fate of Biela may have been shared by Brorsen’s, a
+comet with an established period of five and a half years,
+which has, nevertheless, remained submerged since 1879. It
+is believed by Dr. Lamp to have exploded through internal
+forces in 1881, and he recognises as one of its fragments a
+faint comet detected by Mr. Denning at Bristol, March 26,
+<span class='pageno' id='Page_371'>371</span>1894. The adventures of displaced comets, such as Lexell’s
+can be traced only by arduous and delicate inquiries. They
+depend upon a single cause. Unsettled comets are those
+which pass near Jupiter’s orbit, and are subject to encounters
+with his mighty mass. And since they must necessarily return
+to the point of disturbance, the series of their vicissitudes
+can come to an end only by their being driven off finally from
+the solar system along a hyperbolic path.</p>
+
+<p class='c007'>The condition of these bodies might be described by saying
+that, in the regular course of things, they revolve round the
+sun disturbed by Jupiter; while, during brief but energetic
+crises, they revolve round Jupiter disturbed by the sun. Their
+abnormal condition results from the situation of their aphelia
+close to the Jovian track. This is the case, in a minor degree,
+with many comets of comparatively settled habits. They
+escape eviction and exile, and suffer only disquietment.
+Such are Winnecke’s, D’Arrest’s, Faye’s comets, which, having
+been continuously observed during half a century, are, as
+Mr. Plummer expresses it, “well under control.”<a id='r92'></a><a href='#f92' class='c027'><sup>[92]</sup></a></p>
+
+<p class='c007'>Short-period comets, with the solitary exception of Encke’s,
+appear to be inevitably connected with Jupiter. The
+peculiarity is rendered more significant by the circumstance
+that the other great planets are also provided with cometary
+clients. The Jovian group is the largest; it includes more
+than two dozen recognised individuals. Saturn claims nine,
+Uranus eight, and Neptune five. Halley’s comet belongs to the
+Neptunian family. Another of its members was discovered
+by Pons in 1812, and re-discovered by Brooks in 1883, so
+that it has a period of 71 years. And the reappearance in
+1887 of a comet first seen by Olbers in 1815, bore reassuring
+testimony to the regularity with which Neptune’s comets
+conduct themselves during their long periods of invisibility.</p>
+
+<p class='c007'>The nature of these planetary relationships was at once
+conjectured. It seemed an open secret that the comets had
+been taken prisoners by the attractive force of the great
+globes they flitted past on their way to the sun. But
+<span class='pageno' id='Page_372'>372</span>astronomers can take nothing for granted; and preliminary
+mathematical inquiries served rather to discredit the first and
+easy surmise. The case had to be thoroughly sifted; and it
+was only through the profound researches of Tisserand,
+Callandreau, and Newton of Yale, that the “capture-theory”
+has taken its place as a highly probable truth. With an unstinted
+allowance of time and <i>comets</i>, it can perform all that is
+required of it. “Captures” are not effected all at once; the
+lasso is thrown many times over the escaping body before it
+is definitively secured. Moreover, at each such effort, the
+chances are even of its being made in the wrong direction.
+We observe only the outcome of the hits; the misses are
+beyond our reckoning. A multitude of happy accidents
+have led to the domestication in our system of Faye’s,
+Tuttle’s, Winnecke’s, D’Arrest’s comets. Mr. Plummer has
+adverted to the likelihood that we are indebted to some
+slight but well-directed pulls from Mercury for the permanent
+addition of Encke to the solar company; and
+Neptune exerted itself ages ago with similar success as regards
+Halley’s comet, yet under great difficulties, since
+retrograde comets, and those with highly inclined orbits are,
+as a rule, exempt from capture. This is one of the reasons
+why short-period comets show some degree of conformity to
+planetary modes of motion.</p>
+
+<p class='c007'>These investigations remove all doubt as to the foreign
+origin of comets. Those that are in the solar system are not
+of it. They assuredly remained unaffected by the gradual
+processes of its development. Yet they, as well as the multitude
+of parabolic comets, belong to it in a wider sense. That
+is to say, they accompany its march through space. Otherwise,
+as M. Fabry has demonstrated, most of their orbits
+should be strongly hyperbolic; and no such cometary orbits
+are known. They should, besides, if casually encountered,
+present themselves chiefly along the line of the sun’s way;
+they arrive, on the contrary, indifferently from all quarters of
+the heavens. They are then subject to the same mysterious
+influences which govern his motion, and drift with the
+<span class='pageno' id='Page_373'>373</span>cosmic current which bears the solar family along, we know
+not how or whither.</p>
+
+<div id='i_374'  class='figcenter id001'>
+<img src='images/i_374.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 21.</span>—<i>Photograph of Swift’s Comet. Taken by Prof. Barnard, April 6, 1892. Exposure, 1h. 5m.</i></p>
+</div>
+</div>
+
+<div id='i_375'  class='figcenter id001'>
+<img src='images/i_375.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 22.</span>—<i>Photograph of Swift’s Comet. Taken by Prof. Barnard 24h. later. Exposure, 50m.</i></p>
+</div>
+</div>
+
+<p class='c007'><span class='pageno' id='Page_377'>377</span>Comet-photography became possible only through the
+introduction of highly-sensitive gelatine plates; and even
+with them, exposures of an hour and upwards are necessary
+in order to obtain the desired results. But these results are
+of such importance as to deserve the closest attention. For
+investigating either the forms or the spectra of comets, the
+camera is unrivalled. Its systematic employment for these
+purposes dates from 1892. It can also serve as an engine of
+discovery. On October 12, 1892, a comet so faint that, had
+it not been photographed, it would most likely never have
+been seen, appeared as a nebulous trail on a plate exposed by
+Professor Barnard to the Milky Way in Aquila. It proved
+to be one of Jupiter’s dependents, pursuing, in a period of 6·3
+years, a track so closely resembling the orbit of Wolf’s comet
+in 1884, that Schulhof regarded them as the offspring of one
+parent body.</p>
+
+<p class='c007'>In the year 1892, seven comets were detected; and all, by
+one of those picturesque coincidences with which nature
+loves to entertain her devotees, were, towards its close, visible in
+the sky together. One of them was first noticed by Lewis
+Swift—a specialist in that line—and passed perihelion April
+6.<a id='r93'></a><a href='#f93' class='c027'><sup>[93]</sup></a> The head competed in brightness with a third-magnitude
+star; the tail was 20° long, and came out, in a photograph
+taken by Mr. Russell at Sydney, on March 22, self-analysed
+into eight perfectly distinct rays. <i>No such structure could be
+seen with the telescope.</i> Figs. 21 and 22 reproduce two pictures
+of this object obtained by Professor Barnard, April 6 and 7
+respectively. During the interval, a striking change had occurred.
+In the first photograph, the tail is sharply separated
+into two branches, and shows traces of further indefinite subdivisions.
+The uneven, knotty texture of the main stream is
+obvious. The matter composing it seems as if it had rushed in
+a torrent over a rocky bed, whirling and foaming round the
+<span class='pageno' id='Page_378'>378</span>obstacles it encountered. Twenty-four hours later, this
+powerful emanation left scarcely a trace on the plate. Its
+dwindled remnant had split up into two faint streaks, while
+the almost negligeable offset of the previous night had sprung
+into unlooked-for prominence. A unique feature was added
+in the apparent development of a secondary comet two
+degrees behind the head. The anomalous enlargement
+brightened gradually inwards, and can readily be seen upon
+the plate to be the centre of an entirely new system of
+tails.<a id='r94'></a><a href='#f94' class='c027'><sup>[94]</sup></a></p>
+
+<p class='c007'>Owing to moonlight and clouds, the autobiography of this
+planetary <i>bud</i> unfortunately remained a fragment; and since
+Swift’s comet has an indefinitely long period, it will never
+again exhibit for our benefit any of its caprices of change.</p>
+
+<div id='i_378'  class='figcenter id001'>
+<img src='images/i_378.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 23.</span>—<i>Photograph by Prof. Barnard of Holmes’ Comet near the Andromeda Nebula.</i></p>
+</div>
+</div>
+
+<p class='c007'>On November 8, 1892, Professor Barnard secured a very
+perfect representation (shown in Fig. 23) of a peculiar-looking
+comet grouped with the great Andromeda and its attendant
+nebula. Discovered only two days previously by Mr.
+<span class='pageno' id='Page_379'>379</span>Edwin Holmes of London, it presented a great round disc
+with definite edges visible to the naked eye. This contained
+a tail in embryo, which subsequently opened out into a feeble
+brush, the head being then pear-shaped, and granulated like
+a remote star cluster.<a id='r95'></a><a href='#f95' class='c027'><sup>[95]</sup></a> A strictly continuous spectrum was
+derived from it. “Its appearance,” Professor Barnard wrote,
+“was absolutely different from that of any comet I had ever
+seen. It was a perfectly circular and clean-cut disc of dense
+light, almost planetary in outline. There was a faint, hazy
+nucleus.”<a id='r96'></a><a href='#f96' class='c027'><sup>[96]</sup></a> A photograph taken by him, November 10,
+showed, distant about one degree to the south-east, “a large
+irregular mass of nebulosity covering an area of one square
+degree or more, and noticeably connected with the comet by
+a short, hazy tail.”</p>
+
+<p class='c007'>This object underwent extraordinary vicissitudes of aspect.
+From a seeming planet it quickly degenerated by distension
+into the thinnest of nebulosities; then suddenly, on January
+16, 1893, gathered itself together into an ill-defined star of
+the eighth magnitude. This evanescent outburst was
+simultaneously observed in several parts of the world. After
+some minor rallies and relapses, the comet finally, on April 6,
+1893, melted into the sky-ground. Jupiter is responsible for
+its introduction into the solar system, and it will again be
+due at perihelion in May, 1899. Yet its reappearance is
+considered doubtful.</p>
+
+<p class='c007'>It was perhaps caught sight of during a temporary crisis
+of internal agitation, which may not recur. Certainly it
+could not, if as bright as when discerned by Mr. Holmes,
+have remained many nights unnoticed. Nevertheless, it had
+passed the sun five months previously. Its orbit is more
+nearly circular than that of any previously observed comet,
+and it revolves wholly within the asteroidal zone. That is to
+say, its perihelion lies outside the orbit of Mars, its aphelion
+inside that of Jupiter. Hence, it ought to be visible like a
+planet, at every opposition. Professor Barnard, however,
+<span class='pageno' id='Page_380'>380</span>sought vainly for it, when thus situated. The apparition was
+in many ways enigmatical.</p>
+
+<p class='c007'>A comet discovered by Brooks, October 16, 1893, was
+photographed by Barnard three nights later, when a tail was
+disclosed, 3½° long, and flowing off in two branches with a
+spine-like ray attached to each. A series of impressions
+were fortunately taken, and that of October 21 (reproduced
+in Fig. 24) proved to be of peculiar interest. Since the
+night before, the tail had apparently met with an accident.
+It imprinted itself upon the plate shattered, deformed, and
+affected by a double curvature. A collision with some
+external body was at first suggested as the cause of this untoward
+state of things; but, knowing all that we do about
+the violent interior paroxysms of comets, it seems more
+rational to attribute it to extreme irregularities in the
+quantity and direction of effluences from the nucleus. The
+following night’s photograph gave evidence of a partial return
+to normal conditions. Yet the appendage still looked
+badly damaged; and an elliptical fragment, wrenched from
+it during the convulsion, showed no tendency towards reunion.
+At the time of this incident, Brooks’ comet was
+situated well outside the orbit of the earth.</p>
+
+<p class='c007'>The facts already collected by the photographic study of
+comets are concordant, and easily interpreted. One obvious
+inference from them is “that the matter of a comet’s tail is
+driven away from the nucleus in a very irregular and
+spasmodic manner.”<a id='r97'></a><a href='#f97' class='c027'><sup>[97]</sup></a> At certain crises, outflows are only
+accomplished by convulsions, compared by Mr. Ranyard to
+the explosions of terrestrial volcanoes, or solar prominences.
+Moreover, capricious as cometary forms are to the eye, they
+are still more inconstant as recorded chemically. “The appearance
+one day,” Professor Hussey says, “affords no indication
+as to what it may be the next. The most radical
+changes of form have been observed in almost every reasonably
+bright comet that has been photographed; and they
+<span class='pageno' id='Page_381'>381</span>sometimes take place so rapidly as to become conspicuous in
+an hour or two.”<a id='r98'></a><a href='#f98' class='c027'><sup>[98]</sup></a></p>
+
+<div id='i_381'  class='figcenter id003'>
+<img src='images/i_381.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 24.</span>—<i>Brooks’ Comet, photographed by Prof. Barnard, October 21, 1893. Exposure, 35m.</i></p>
+</div>
+</div>
+
+<p class='c007'><span class='pageno' id='Page_383'>383</span>Comets’ tails appear very different in structure photographically
+and visually. On the sensitive plate, they are
+perceived to be composed of innumerable, distinct filaments,
+sometimes tied up, as it were, into sheaves. The filaments,
+or streamers may, however, according to the same authority,
+“leave the coma in a single compressed bundle, or they may
+spring from it in widely divergent and loosely connected
+groups; they may be smooth, and straight, and distinct, or they
+may be lumpy, crooked, interlacing, and spirally twisted; or
+again, they may be broken into fragments, and scattered as
+though they were smoke driven by the wind.” And these
+effects often swiftly succeed each other in the same comet.</p>
+
+<p class='c007'>In photographs of Swift’s and Rordame’s comets in 1892
+and 1893 (taken by Barnard and Hussey respectively), the
+effects of a spiral outward movement in the grouped streamers
+of the tail can be plainly recognised. They are indistinguishable
+from “the twisted forms produced by an electrical discharge
+in a magnetic field.”<a id='r99'></a><a href='#f99' class='c027'><sup>[99]</sup></a> Another much more common
+peculiarity of such appendages brought into prominence by
+chemical portraiture, is the occurrence upon them of knots,
+or condensations. These are evidently accumulations of
+outflowing matter. Again, in most of the comets recently
+photographed, the tails start directly from the nuclei, which
+appear destitute of genuine envelopes. This is the precise
+criterion of Olbers’ first cometary division, in which solar
+repulsion acts alone, nuclear repulsion being ineffective, or
+non-existent. It comes out remarkably in Barnard’s photographs
+of Gale’s comet in 1894.</p>
+
+<p class='c007'>We may now resume in a few words what we have learned
+about comets. To begin with, they are of such small mass
+that no gravitational effects from their closest vicinity have ever
+yet been detected. Their bulk, on the other hand, is enormous.
+The great comet of 1811 comprised a nebulous
+<span class='pageno' id='Page_384'>384</span>globe 2½ times larger than the sun, with a tail many thousand
+times more voluminous. Hence the extraordinary tenuity
+of such bodies. They must indeed contain solid matter;
+otherwise they could not hold together even in the imperfect
+way that they do; but it is probably in a state of very loose
+aggregation. Their permeability to light may thus be
+accounted for. The visibly granular texture of their nuclei
+is confirmatory of the supposition. If, then, the nuclei of
+comets are essentially “meteor-swarms,” all the constituent
+particles must revolve round the centre of gravity of the
+whole, in a common period, but with a velocity directly
+proportional to distance from the centre—that is, increasing
+outward. And the joint mass being so small, the utmost
+speed attained would perhaps rarely exceed a couple of
+hundred yards a second. Moreover, towards the centre,
+where the components of the swarm would crowd most closely
+together, motion would become so slow as to be scarcely
+perceptible. Hence collisions would be infrequent and of
+slight effect; while the probability of their occurrence should
+diminish with the comet’s approach to the sun, which, by
+its unequal attraction, would draw the revolving particles
+asunder, and amplify their allowance of space. Internal
+collisions may then fairly be left out of the account in considering
+the phenomena of comets. The expansion of their
+nuclear parts, due to tidal forces, is, however, usually disguised
+by the contraction, near perihelion, of their nebulous
+surroundings. The latter effect can be explained by the
+immense predominance at that conjuncture of solar over
+cometary electrical repulsion.</p>
+
+<p class='c007'>That the light-emissions of comets are largely of electrical
+origin is no longer doubtful; so that the present
+rush-ahead in this branch of knowledge cannot but
+help to elucidate many of the still mysterious circumstances
+connected with these strange visitants from the
+uttermost verge of the sun’s empire. The tie of allegiance
+hangs loosely there; but by the persevering efforts of
+the great planets it is sometimes drawn closer, with the
+<span class='pageno' id='Page_385'>385</span>result of domiciling under their control a train of dilapidated
+comets, verging towards dissolution.</p>
+
+<p class='c007'>Carbon, sodium, and iron, are the only substances directly
+known to exist in these bodies. Spectroscopic evidence also
+suggests the presence of nitrogen or hydrogen; and a
+number of chemical elements which make no show in their
+light doubtless enter into their composition. The state of
+comets when remote from the sun can only be surmised.
+Their gaseous constituents may be solidified by cold. They
+can, in any case, scarcely be other than obscure and inert
+bodies.</p>
+
+<h3 class='c004'>CHAPTER XIII.<br> <span class='c014'>METEORITES AND SHOOTING STARS.</span></h3>
+
+<p class='c003'>At Madrid, on the morning of February 10, 1896, the sunshine
+was at 9.30 overpowered by a vivid flash of bluish light,
+succeeded by a violent explosion. Much glass was broken,
+and other devastation of a minor kind wrought; above all,
+some hundreds of thousands of people were thoroughly
+frightened. The origin of the commotion was visible in a
+white cloud rushing across the sky, and leaving behind a
+dusty train. Of this débris, scattered from a height of fifteen
+miles, some fragments were picked up and analysed. They
+were composed of silicates of magnesia and iron, with very
+small quantities of aluminium, nickel, and calcium. These
+specimens were strictly “aerolites,” a term used to designate
+any solid meteoritic matter that reaches the earth.</p>
+
+<p class='c007'>Equally conspicuous apparitions of the sort are not always
+equally clamorous. There are silent, as well as detonating
+fire-balls. The cause of the difference cannot certainly be
+assigned. It resides, perhaps, in the diverse constitution of
+<span class='pageno' id='Page_386'>386</span>the exploding bodies; it is, beyond doubt, unconnected with
+their height in the atmosphere. Thus, a remarkable meteor
+was seen, but not heard, by Dr. Rambaud, the astronomer-royal
+for Ireland, at Dunsink, February 8, 1894. The
+object, he says, “suddenly burst into view with an intense
+brilliance, and shone out against the cloudless blue sky with
+a greenish metallic lustre. It fell in a vertical direction until
+it disappeared behind some trees. In shape it resembled a
+very elongated pear, like most fire-balls of the sort. It
+emitted no visible sparks, and disappeared quite noiselessly.”
+When first observed, it was at a height of about 87 miles
+above the Irish Channel; then crossing Lancashire, it descended
+so rapidly on its way, probably, to engulfment in the
+North Sea, that, when last noticed, it was scarcely, if at all,
+higher above the earth’s surface than the Madrid meteorite at
+the moment of its formidable disruption. Astonished rustic
+beholders at Kingswood and Dudley averred that it burst
+“in the next field”; but this is a common illusion. Professor
+Langley relates that some witnesses of a marvellously
+swift meteor at a presumable elevation of some fifty miles,
+sallied out of their houses next day to make sure that it had
+not struck their chimneys.</p>
+
+<p class='c007'>Such phenomena are tolerably frequent, and have been recorded
+from the remotest antiquity. Homer lends a meteoric
+aspect to Athene, when she descends from Olympus to take
+the war-path by the shore of Scamander. Chronicles abound
+with accounts substantially identical with the telegrams supplied
+by Reuter’s Agency on February 10, 1896. The fall of
+the “Crema meteorite” has a special interest as having been
+depicted by Raphael in his “Madonna di Foligno.”<a id='r100'></a><a href='#f100' class='c027'><sup>[100]</sup></a> A
+multitude of stones were discharged by it on the banks of the
+Adda, six of which weighed each one hundred pounds and
+upwards; the sulphurous smell characteristic of fresh-fallen
+aerolites is mentioned in contemporary accounts of the event,
+which occurred September 4, 1511; and it is further said that
+<span class='pageno' id='Page_387'>387</span>“sheep were killed in the fields, birds in the air, and fishes
+in the streams.” No specimen of this sky-volley is known
+to exist. In elder times, objects of this class were worshipped;
+and Professor Newton<a id='r101'></a><a href='#f101' class='c027'><sup>[101]</sup></a> has collected many
+curious facts about the meteoric cult traceable in classical
+history. To this day, indeed, the central sanctuary of
+Mahometanism—the Kaaba—owes its sacredness to the embedment
+in its masonry of a blackened aerolite.</p>
+
+<p class='c007'>Until the beginning of the present century, only the
+ignorant believed it possible that stones could come from
+heaven; philosophers regarded them as generated in the
+clouds. They were at last convinced that the popular view
+was correct by Biot’s investigation of the meteoric tempest
+which broke over L’Aigle, in the department of the Orne,
+April 26, 1803. He estimated at two thousand the number
+of fragments scattered over an area six by two and a half
+miles, one of which, weighing five pounds, is now in the
+South Kensington Museum. And at Pultulsk, January 30,
+1869, one hundred thousand stones were reported to have
+been showered upon the earth. It is not often, indeed, that
+largesse from space is so lavishly made. Yet all meteors
+(with the rarest exceptions) rendered luminous by the resistance
+of its atmosphere, become, in one way or another, incorporated
+with its mass. Their materials are no doubt often
+reduced to fine dust and gas; yet six or seven hundred solid
+masses per annum are computed to reach the surface of sea or
+land, for the most part “unrecked-of and in vain.” Of late,
+the scientific demand for them has grown keen, and their enhanced
+value has raised the legal question of their ownership.
+The decision of the American courts is that aerolites are not
+“wild game,” but “real estate,” and, as such, belong to the
+owner of the land upon which they fall.</p>
+
+<p class='c007'>No wonder they should be at a premium, those blackened
+and wasted samples of immeasurably distant globes. The
+velocities with which they entered our atmosphere alone
+suffice to prove their cosmical origin. Had it not trapped
+<span class='pageno' id='Page_388'>388</span>them, many, circuiting the sun in a hyperbolic curve, would
+have escaped for ever from our system. Their primitive disconnexion
+from it is implied by their swift motions, which
+considerably exceed, on an average, those of comets, and
+point to interstellar space as their proper habitat. The earth’s
+orbital pacing has, however, to be added or subtracted as the
+case may be; so that the actual rate of encounter varies from
+ten to forty-five miles a second. Most of this is spent before
+the earth’s surface is reached. Only considerable masses
+travelling at express speed bring any sensible proportion of
+it with them to the ground. But what is lost as motion reappears
+in other forms of energy, as light, heat, and sound.
+In front of the rushing body, the air—despite its inconceivable
+tenuity at elevations of fully one hundred miles—is suddenly
+compressed and raised to an exceedingly high temperature,
+while a corresponding vacuum behind gives rise to violent
+reactive currents. Professor Dewar calculated, by way of example,
+in 1887, that a body, three feet in diameter, moving
+eighteen miles a second at an altitude of twenty-three miles,
+where barometric pressure is reduced to one-fifth of an inch,
+would compress the air in its path 5,600 times, the resistance
+offered to its passage thus equalling that of thirty-seven
+atmospheres. The abrupt increase of heat accompanying
+compressions of this order amounts to thousands of degrees,
+and tends to rend in pieces a body arriving from frigid abysses
+where matter can only exist in a stark and, so to speak, lifeless
+state. Explosions of occluded gases ensue; vaporised
+and incandescent particles are blown behind in a luminous
+train; and, at the most, some shattered solid remnants tumble
+to our continents, or plunge into our oceans. The few that
+are rescued for examination look much the worse for their
+final adventure. The signs of the furnace and the hurricane
+(both self-created), are visible in their jetty and fused surfaces,
+“thumb-marked,” probably through the continual and irregular
+changes in the pressure exerted upon them. The crust is,
+however, a mere varnish, the interior, which is usually of a
+greyish hue, being entirely unaffected by heat. It remains, on
+<span class='pageno' id='Page_389'>389</span>the contrary, sunk in the depths of cold. Agassiz compared the
+aerolite which fell at Dhurmsala in India, in 1860, to the
+Chinese <i>chef d’œuvre</i>, a “fried ice”;<a id='r102'></a><a href='#f102' class='c027'><sup>[102]</sup></a> and a large fragment of
+it, which fell in moist earth, was found coated with ice.<a id='r103'></a><a href='#f103' class='c027'><sup>[103]</sup></a></p>
+
+<p class='c007'>Aerolites, or meteorites, as they may equally well be
+called, are roughly divided into “stones” and “irons”; the
+former being composed of various and peculiar minerals, the
+latter of iron, with a considerable percentage of nickel.<a id='r104'></a><a href='#f104' class='c027'><sup>[104]</sup></a>
+All show a more or less distinctive crystalline structure.
+Meteoric chemistry includes about thirty of the seventy or so
+terrestrial elements. The chief of them are: iron, nickel, carbon,
+oxygen, silicon, magnesium, sulphur, aluminium, phosphorus,
+with smaller quantities of chromium, cobalt, tin,
+copper, titanium, manganese, antimony, arsenic, lithium,
+hydrogen, nitrogen, argon, and helium. Argon and helium
+were expelled by heat from a piece of meteoric iron picked
+up in Augusta County, Virginia, the former coming off nearly
+a hundred times more plentifully than the latter. As the
+light of argon makes no show in the spectrum of any heavenly
+body, the proof of its cosmical diffusion thus obtained by
+Professor Ramsay is of great value. Besides argon and
+helium, hydrogen, carbonic acid, and carbonic oxide gases are
+found included in meteorites. They seem, as it were, to
+hybernate in the stony or metallic enclosures from which they
+can only be <i>boiled out</i>.</p>
+
+<p class='c007'>Although these wind-falls from space contain no strange
+elements, the manner of their composition is special to themselves.
+Their study constitutes a separate branch of mineralogy.
+They are certainly of igneous origin. They show no
+sign of water-action, and but little of oxidation. The nearest
+affinities of the minerals aggregated in them are with volcanic
+products from great depths. Thus meteorites seem broken up
+fragments of the interior parts of globes like our own.
+A few among them contain solid carbon, either amorphous,
+<span class='pageno' id='Page_390'>390</span>or in the shape of graphite, or even crystallised into minute
+diamonds. In the Cañon Diablo siderite, or meteoric iron, all
+three varieties occurred together, some of the translucent
+particles proving, when put to the test of actual combustion,
+to be indeed “gems of purest ray serene,” dwelling
+incognito in a strange environment!</p>
+
+<p class='c007'>The thin streaks of light called “shooting stars” differ
+in several respects from explosive meteorites. In the first
+place, they—probably without exception—form systems.
+Innumerable multitudes of them travel in the same paths
+round the sun. Moreover, those paths resemble cometary
+orbits; they are very elongated ellipses, inclined at all angles
+to the plane of the ecliptic, and traversed indifferently in
+either direction. Their velocities are thus sensibly parabolic,
+while fire-balls commonly attain hyperbolic speed. Finally,
+they are soundless. They slide by in ghostly silence. Most of
+them are probably not larger than a pea, yet were the shield
+of its atmosphere withdrawn, the earth would be rendered
+well-nigh uninhabitable by their pelting. Incredible numbers
+of them are encountered. They come by the million daily to
+be burnt, visibly to the naked eye, in the thin upper air.
+Kleiber’s allowance is eleven, Newton’s twenty millions; and
+these figures should be multiplied a score of times to include
+telescopic fire-specks. Now, the combined mass of all these
+particles goes to reinforce the mass of the earth; but it is
+relatively so small that ages must elapse before the contribution
+can become sensible. Our defeated meteoric assailants
+surrender to us also the heat of their arrested motion; which
+is, however, only as a spark added to the furnace of our
+supply from the sun.</p>
+
+<p class='c007'>Shooting stars, as we have seen, move in closed orbits.
+They are, then, a periodical phenomenon. Not that we ever
+see the same individual twice; its visibility implies its dissolution,
+but its companions are as the sands of the seashore.
+Their association is recognised by their agreement in direction
+and date. Unless their orbits intersected that of the earth,
+nothing could be known of them terrestrially; they come to
+<span class='pageno' id='Page_391'>391</span>our notice only through actual encounters, and encounters
+are possible only at the time of year when our planet is
+passing through the node. This is the given rendezvous,
+different, speaking generally, for each system; although,
+speaking particularly, many meteoric streams are so wide
+that the earth takes days, even weeks, to cut its way through
+them, and so may be overtaken by fresh onsets before the
+original one is exhausted. Each community is distinguished
+by the lie of its orbit—that is, by the point in the sky from
+which the flying arrows of light seem to diverge. This is
+known as the “radiant-point” of the system, and is its special
+characteristic.</p>
+
+<p class='c007'>The August meteors are a familiar example of such an
+association. Their annual recurrence is no new discovery.
+Long ago, in mediæval times, they were called the “tears of
+Saint Lawrence,” because never looked for vainly on the 10th
+of August. But they are so far from being limited to that
+particular night, that Mr. Denning has picked up skirmishers
+and stragglers from the main body all the way from July 8
+to August 22. They are distributed with tolerable evenness
+along an immensely long ellipse, traversed in 120 years; and,
+because they radiate from near the star η Persei, are known
+to science as the “Perseids.”</p>
+
+<p class='c007'>The scattering of the November meteors—or “Leonids,”
+since their point of emanation is marked by ζ Leonis—is on
+the same plan, with a difference: the Perseids might be
+compared to a plain gold ring; the Leonids, to a ring with
+a gem on it They send us some shots every year on
+the 13th and 14th of November; but three times in a century
+they open fire for a regular bombardment. An early
+Leonid display took place in 902 <span class='fss'>A.D.</span>, noted in old
+chronicles as “the year of the stars.” All night long on
+October 19—the node advances 14½ degrees in a thousand
+years—while the tyrant Ibrahim lay dying “by the judgment
+of God” before Cosenza, beholders far and near viewed with
+consternation the stars precipitating themselves from the
+sky. Recurrences of the phenomenon every thirty-three
+<span class='pageno' id='Page_392'>392</span>years received curiously little attention until Humboldt
+described, and insisted on the periodic nature of the meteoric
+tempest witnessed by him at Cumana on the morning of
+November 12, 1799. One scarcely less violent broke over
+Europe and Asia in 1832, and the American continent in
+1833. From the Gulf of Mexico to Halifax the stars were
+seen to fall as silently as snow-flakes, and almost as thickly,
+yet after a less undirected fashion. Rather they darted and
+swooped, like falcons, with a purpose; and it was noticed
+that the lines of their flight could, with essential invariability,
+be traced back to one point, or small area in the heavens.
+This remark gave the clue to their nature. They were perceived
+to be necessarily cosmical bodies. For since the
+focus of the meteors remained unaffected by the earth’s
+rotation, they showed themselves plainly extraneous to its
+domestic arrangements. “A new planetary world,” exclaimed
+Arago, “has been disclosed to us!”</p>
+
+<p class='c007'>The anticipated repetition, in 1866, of the November
+shower of 1833, came off with <i>éclat</i>. Many still remember
+the amazing spectacle presented by the heavens in the early
+morning of November 14, in that year. In 1867, when the
+earth came round again to the same point of its orbit, the
+star-rain was still falling heavily; and even in 1868 it
+amounted to a fair sprinkle. Thus the swarm was, thirty years
+ago, already so extended that it spent three years in sweeping
+past the node, at the rate of twenty-seven miles a second.
+“The meteors themselves,” according to Dr. Johnstone
+Stoney,<a id='r105'></a><a href='#f105' class='c027'><sup>[105]</sup></a> “are probably little pebbles, the larger about an
+ounce, or perhaps two ounces, in weight, and spaced in the
+densest part of the swarm at intervals of one or two miles
+asunder every way. The thickness of the stream is about
+100,000 miles, which, however, is a mere nothing compared
+with its enormous length. The width is such that the earth,
+when it passes obliquely through the stream, is exposed to
+the downpour of meteors for about five hours.” Each “pebble”
+revolves round the sun, and suffers planetary perturbation, in
+<span class='pageno' id='Page_393'>393</span>complete independence of its fellows, their orbits being only
+alike, not identical. The next full encounter with them will
+take place November 14, 1899; but avant-couriers may be
+looked for at the critical dates in 1897 and 1898, as well as a
+strong rear-guard in 1900.</p>
+
+<p class='c007'>The orbit of the November meteors is roughly bounded by
+the orbits of the earth and of Uranus. They pass perihelion
+very near our meeting-place with them; and since they run
+counter to the earth’s motion, the velocity of collision is
+nearly equal to the sum of the two orbital velocities, or forty-four
+miles a second. They are almost the swiftest shooting stars
+of our acquaintance.</p>
+
+<p class='c007'>The successful calculation of meteoric orbits by Adams,
+Schiaparelli, and Leverrier, promptly led to a discovery as
+important as it was unexpected. Late in 1866, Schiaparelli
+announced that the August meteors follow precisely the same
+track with a bright comet (1862, III.) discovered in 1862 by
+Tuttle, an American astronomer; and the reality of this
+singular relationship was, in the following year, verified by
+the detection of three similar examples. The Leonids, with
+a period of 33¼ years, proved to be close associates of Tempel’s
+comet (1866, I.); a meteoric stream flowing down upon
+the earth annually on April 20, from the direction of the
+constellation Lyra, was perceived to move in the vast ellipse
+traced out in 415 years by the comet 1861, I.; finally a star-drift,
+first noticed December 6, 1798, was rightfully claimed
+as an appurtenance of Biela’s comet.</p>
+
+<p class='c007'>Thus the fact of a close connexion between comets and
+meteors was at once rendered patent; and as to the nature of
+the connexion, the history of Biela’s comet is particularly instructive.
+Since its disappearance, the meteor-swarm sharing
+its orbit has received a notable accession. The comet seems
+to have broken up into meteors. And this, we can scarcely
+doubt, is what has really occurred. Hence, when the earth
+passes moderately, near where the comet <i>would</i> have been,
+had it survived in cometary shape (a conjuncture happening
+once in thirteen years), a vehement outburst of shooting
+<span class='pageno' id='Page_394'>394</span>stars is observed. On November 27, 1872, the “Bielids,”
+or “Andromedes,” came in tens of thousands from near γ
+Andromedæ, the very point whence the track of the disaggregated
+comet intersects the earth’s orbit at an angle of twelve
+degrees. Their movements were leisurely; for they came up
+with our globe, instead of, like the Leonids, rushing to meet
+it. They seemed to sail, rather than shoot, across the sky.
+The calculated position of the originating body was, at this
+date, two hundred millions of miles <i>in advance</i> of the node,
+and it was three hundreds of miles <i>behind</i> the same point
+when the display was renewed in 1885. It is then certain<a id='r106'></a><a href='#f106' class='c027'><sup>[106]</sup></a>
+that at least five hundred millions of miles of Biela’s route are
+densely strewn with meteoric fragments. The entire multitude,
+moreover, necessarily separated from the comet subsequently
+to an episode of disturbance by Jupiter in 1841.
+This is plainly shown by the fact that the members of the associated
+company pursue the modified track. The perturbation
+of 1841 was exerted upon them no less than upon
+the comet, with which, accordingly, they must then have
+formed one mass.</p>
+
+<p class='c007'>Biela’s comet has thus taught us that such bodies meet
+their end by getting pulverised into meteoric particles; and
+further, that the particles disperse with extraordinary rapidity
+along the length of their orbits. Solar and planetary
+<i>differential</i> action produce this kind of effect, although they
+hardly explain its amount. Subordinate swarms are also
+created by disturbance. Such an one met the earth
+November 23, 1892, when Professor Young estimated that at
+least 30,000 Andromedes furrowed the sky at Princeton.
+Heavy star-showers, however, are perishable phenomena.
+They thin out with comparative rapidity into a continuous
+drizzle. At each recurrence, diffusion is seen to have made
+progress, until at last the “gem on the ring” has vanished.
+With the Perseids this is already the case. The stream flows
+without material interruption over a bed a hundred times
+wider than that of the Leonids. These meteors, too, will no
+<span class='pageno' id='Page_395'>395</span>doubt eventually reach a similar condition. In the course of
+a couple of centuries, their thirty-three year period will be
+completely effaced. In 1799, the main body of them crossed
+the node in less than a year; at the close of the present century,
+the earth will probably make her annual round at least
+four times, before the march-past comes to an end. Obviously,
+it is about to become perennial. Leverrier concluded from his
+researches that the Leonid comet and the Leonid meteors,
+which then made part of its substance, were “captured” by
+Uranus in 126 <span class='fss'>A.D.</span>, and so introduced into the solar domain.
+The truth of the supposition may still be tested; should it be
+established, this remarkable system affords yet another example
+of the rapidity with which cometary materials become
+disintegrated and scattered.</p>
+
+<p class='c007'>The number of meteoric radiants now distinctly known is
+estimated by Mr. Denning at about three thousand; and we
+need not hesitate to ascribe to all these streams a cometary
+origin. It is true that the three thousand generating comets
+have, all but three, “gone over to the majority.” But we
+have witnessed the obsequies of Biela, and it seems only
+logical to infer that those of its 2996 congeners were, in old
+times, celebrated after the same fashion, and are still kept in
+mind by the annual blaze, in their honour, of a few representative
+sky-rockets.</p>
+
+<p class='c007'>No component of a star-burst has so far <i>undoubtedly</i> come
+to the ground. The fire-works shown are of the most
+innocuous kind. Two <i>possible</i> exceptions are, however, on
+record. On April 4, 1095, a shower of Lyraids was visible in
+Western Europe. The stars, according to the Saxon
+Chronicle,<a id='r107'></a><a href='#f107' class='c027'><sup>[107]</sup></a> crowded “so thickly that no man could count
+them.” And in France, one of the throng fell so accessibly
+that a bystander, having noted the spot, “cast water upon it,
+which was raised in steam with a great noise of boiling.”
+But, unless the aerolite came from the same radiant as the
+stars, their simultaneous arrival was an unmeaning coincidence.
+It implied no connexion, physical or dynamical,
+<span class='pageno' id='Page_396'>396</span>between them. The same coincidence was renewed during
+the Andromede shower of November 27, 1885. Just before
+it began, a “ball of fire” struck the ground at Mazapil in
+Mexico, and proved to be a substantial piece of iron containing
+nodules of graphite. It weighed eight pounds. Yet
+here again that essential circumstance, the direction of its
+fall, remained unknown. We must then, for the present,
+suspend our judgment as to whether aerolites may be regarded,
+like shooting stars, as actual cometary débris.</p>
+
+<p class='c007'>Mr. Denning’s patient watch of thirty years has led him to
+the singular discovery of “stationary radiants.” The direction
+in which meteors appear to approach the earth is
+determined by the combination of theirs with the earth’s
+movements. The effect is strictly analogous to the aberration
+of light. Meteoric radiants ought accordingly to
+shift on the sphere just as the heavenly bodies change
+their apparent places by the prescribed measure of aberration.
+And most do in this respect conform to theory, the
+Perseid radiant notably. On the other hand, certain well-known
+radiants continue fixed night after night in seeming
+independence of the earth’s orbital advance; and there are
+a good many points in the sky whence shooting stars continue
+to <i>dribble</i> without sensible interruption during many
+months of each year. The fact is undeniable, although inexplicable.</p>
+
+<p class='c007'>The future progress of meteoric astronomy depends largely
+upon the introduction of the photographic mode of observation.
+Only by its aid can the precise determination of radiant-points
+be effected; and this is the chief desideratum. Its
+realisation before the close of the century may safely be predicted.
+Dr. Elkin, director of Yale College Observatory, had
+a “meteorograph” constructed for the purpose in 1894, and
+hopes to use it for the registration of the Leonids now hastening
+to meet us. Hitherto, only casual fire-balls have printed
+their tracks on sensitive plates. Success in obtaining permanent
+records of shooting stars diverging from a radiant will
+mark a turning-point in meteoric investigations.</p>
+
+<div><span class='pageno' id='Page_397'>397</span></div>
+<div class='chapter ph1'>
+
+<div class='nf-center-c0'>
+<div class='nf-center c001'>
+    <div>ASTRONOMY</div>
+  </div>
+</div>
+
+</div>
+
+<div id='i_398'  class='figcenter id001'>
+<span class='pageno' id='Page_398'>398</span>
+<img src='images/i_398.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p>NEBULA IN ANDROMEDA. 31 MESSIER.<br>  <br>  (<i>From a Photograph, by Dr. Roberts.</i>)</p>
+</div>
+</div>
+
+<div class='chapter'>
+  <span class='pageno' id='Page_399'>399</span>
+  <h2 class='c006'>SECTION IV.—THE SIDEREAL HEAVENS.</h2>
+</div>
+
+<div class='nf-center-c0'>
+<div class='nf-center c002'>
+    <div><span class='large'><span class='sc'>By J. E. Gore, F.R.A.S.</span></span></div>
+  </div>
+</div>
+
+<h3 class='c013'>CHAPTER I.<br> <span class='c014'>THE STARS AND CONSTELLATIONS.</span></h3>
+
+<p class='c003'>The study of the sidereal heavens is one of surpassing
+interest, and tends to raise our minds above the sordid
+things of time and the petty affairs of the little planet on
+which we dwell,—a globe absolutely large, of course, when
+compared with objects around us, but relatively very small in
+comparison with the vast stellar universe which surrounds us
+on all sides, a universe so vast that even the largest telescopes
+can only partially fathom its immeasurable depths.</p>
+
+<p class='c007'>For the study of the sidereal heavens, as revealed to us by
+the giant telescopes of modern times, it will be advisable to
+begin by a consideration of the starry sky as seen by the
+naked eye, without optical assistance of any kind. On a clear
+and moonless night, when the vault of heaven is spangled
+over with shining points of light, some bright, others fainter,
+and many more barely perceptible to the unaided vision, we
+are inclined to imagine that the stars visible to the naked eye
+are innumerable, and that any attempt to count them would
+be a hopeless task. This idea, however, is quite a mistake,
+and, indeed, merely an optical illusion, due partly to the
+scintillation or twinkling of the brighter stars, and stars near
+the limit of vision, and partly to their irregular distribution
+over the surface of the heavens. As a matter of fact, the
+<span class='pageno' id='Page_400'>400</span>stars visible to the naked eye can be easily counted; and they
+have been counted and catalogued. As every book in the
+catalogue of a large library can be identified, so every star
+visible to the unaided vision—and thousands even fainter, and
+only visible in telescopes—have been mapped, and their exact
+positions are as well known to astronomers as those of every
+town and village in Great Britain are known to geographers.
+The number of stars which can be seen with ordinary eyesight
+is, in fact, very limited, and does not exceed the number of
+inhabitants in a small town. Some years ago, a German
+astronomer, Heis, who was gifted with excellent eyesight
+carefully mapped down all the stars visible to his eye without
+optical aid, and found the total number visible in the middle
+of Europe to be only 3,903. A similar work was undertaken
+for the Southern Hemisphere by Behrmann, another German
+astronomer, and the total number distinctly seen by both
+astronomers in both hemispheres of the star sphere is 7,249.
+Of course, at any given time and place only one half the star
+sphere is visible, the other half being below the horizon. It
+follows, therefore, that about 3,600 stars are visible at one
+time from any point on the earth’s surface. As, however,
+everyone does not possess the keen vision of the astronomers
+referred to above, we may safely say that not more than 3,000
+stars are, on the average, visible at a time to ordinary eyesight.
+On the other hand, persons gifted with exceptionally keen
+vision may possibly see even more than Heis and Behrmann
+did; but even to such eyes, the total number distinctly visible
+on a clear night without a moon would probably not exceed
+5,000. We may easily satisfy ourselves as to the truth of this
+statement by taking a portion of the sky, and counting the
+number of stars which can be steadily seen. Everybody
+knows the Great Bear, sometimes called the “Plough,” or
+“Charles’ Wain.” Four of the well-known stars in this remarkable
+group form a four-sided figure. Well, let the reader look
+carefully at this figure, and see how many stars can be detected
+within the space formed by imaginary lines joining the
+bright stars. Probably surprise will be felt at the small
+<span class='pageno' id='Page_401'>401</span>number which can be distinctly seen. Heis, with his keen
+vision, only shows eight on his map, and of these, four are
+very faint, and near the limit of even good eyesight. Probably
+very few eyes will see more than eight, and perhaps most persons
+will fail to see so many. As the whole hemisphere is roughly
+five hundred times larger than this spot, the number seen by
+Heis in the quadrilateral of the Plough would give a total of
+4,000 stars visible at one time. Of course, some portions of
+the sky are much richer in stars than the spot selected; but,
+on the other hand, others are much poorer, so that perhaps
+this may be taken as a spot of average richness. From this
+single example it will be seen that the idea of countless
+multitudes of stars visible to the naked eye is a mistake.
+Probably the effect of a great number is partly due to our
+catching glimpses by “averted vision” of still fainter stars,
+which cannot, however, be seen steadily when the eye is
+turned directly towards them.</p>
+
+<div id='i_401'  class='figcenter id001'>
+<img src='images/i_401.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 1.</span>—<i>Stars visible in the Northern Hemisphere.</i><br>  <br>  (From “Visible Universe.”)</p>
+</div>
+</div>
+
+<p class='c007'><span class='pageno' id='Page_402'>402</span>In speaking of stars visible to the naked eye, we do not, of
+course, include the stars in the Milky Way, that arch of
+cloudy light which spans the heavens; for although this
+wonderful zone is composed of faint stars, these stars are not
+individually visible without a telescope.</p>
+
+<p class='c007'>Notwithstanding the limited number of the visible, or lucid,
+stars, as they are called, the aspect of the starry sky still presents
+a spectacle of marvellous beauty and interest, and may
+be viewed with pleasure and profit even without a telescope.
+There are many interesting objects which may be seen without
+optical assistance of any kind. Look at the middle star
+of the three forming the “tail” of the Great Bear, or “handle”
+of the Plough. This star was called Mizar by the old Arabian
+astronomers. Close to it, good eyesight will see a small star,
+known as Alcor. This little star was called by the Arabians
+Alsuha, which means “the neglected small star.” The name
+Alcor means the “test,” and is supposed to indicate that the
+old astronomers considered it a test for keen vision; but
+the Arabians had a proverb, “I show him Alsuha, and he
+shows me the moon,” a saying which seems to imply that it
+could be easily seen by these old astronomers. The faintest
+star of the seven, the one at the root of the tail, was called
+Megrez by the Arabian astronomers. This star is supposed
+to have diminished in brightness since ancient times, as it was
+rated of the third magnitude by Ptolemy, and of the second
+by Tycho Brahé, while at present it is not much above the
+fourth magnitude. It may possibly be variable in its light,
+like many other stars in the heavens.</p>
+
+<div id='i_403'  class='figcenter id001'>
+<span class='pageno' id='Page_403'>403</span>
+<img src='images/i_403.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 2.</span>—<i>Stars visible in the Southern Hemisphere.</i><br>  <br>  (From “Visible Universe.”)</p>
+</div>
+</div>
+
+<p class='c007'>Here it may be mentioned that the stars were divided into
+magnitudes or classes according to their brightness by the
+ancient astronomers, all the brightest stars being placed in the
+first magnitude, those considerably fainter being called second
+magnitude, those fainter still third magnitude, and so on to
+the sixth magnitude, or those just visible to ordinary eyesight.
+This classification has been practically retained by modern
+astronomers, but, of course, there are stars of all degrees of
+brightness from Sirius down to the faintest stars visible in the
+largest telescopes. Sirius is the brightest star in the heavens,
+and is equal to about six average stars of the first magnitude,
+such as Altair or Aldebaran. According to the Harvard
+photometric measures, the following are the brightest stars
+in the heavens in order of magnitude:—(1) Sirius, (2) Canopus,
+<span class='pageno' id='Page_404'>404</span>(3) Arcturus, (4) Capella, (5) Vega, (6) Alpha Centauri,
+(7) Rigel, (8) Procyon, (9) Achernar, (10) Beta Centauri, (11)
+Betelgeuse (slightly variable), (12) Altair, and (13) Aldebaran.
+Of these Canopus, Alpha, and Beta Centauri, and Achernar, do
+not rise above the horizon of London. Of those brighter than
+the second magnitude, the following are north of the Equator:
+Alpha Cygni, Pollux, Castor, Eta Ursæ Majoris, Gamma
+Orionis, Beta Tauri, Epsilon Ursæ Majoris, Alpha Ursæ Majoris,
+Alpha Persei, and Beta Aurigæ; and south of the Equator:
+Alpha Crucis, Fomalhaut, Antares, Spica, Beta Crucis, Gamma
+Crucis, Epsilon Orionis, Zeta Orionis, Epsilon Canis Majoris,
+Beta Carinæ, Epsilon Carinæ, Lambda Scorpii, Alpha Triangulum
+Australis, Gamma Argûs, Alpha Gruis, Epsilon
+Sagittarii, Alpha Hydræ, Theta Scorpii, and Delta Velorum.
+Of those below the second magnitude, and brighter than the
+third, there are about 34 in the Northern Hemisphere, and
+61 in the Southern. As the brightness decreases, the numbers
+increase rapidly. Indeed, the increase is in geometrical progression,
+the number in each class of magnitude being about
+three times as many as those in the class one magnitude
+brighter. The exact magnitudes of all stars visible to the
+naked eye in both hemispheres have now been determined by
+the aid of photometers. These instruments are described in
+Section II. of the present work, Chapter XVII.</p>
+
+<p class='c007'>The stars were divided by the ancient astronomers into
+groups called constellations. Some of these were formed in
+the earliest ages of antiquity. Orion and the Pleiades are
+mentioned in Job (Chapter XXXVIII.), which is believed to
+be one of the oldest books in existence. Josephus ascribes
+the division of the stars into constellations to the family of
+Seth, the son of Adam; and according to the Book of Enoch
+the constellations were already known and named in the time
+of that patriarch. The brightest stars of each constellation
+are designated by the letters of the Greek alphabet, which
+were assigned to them by Bayer in the year 1603, Alpha
+generally denoting the brightest star, Beta the next in lustre,
+and so on. This is not, however, invariably the case, and
+<span class='pageno' id='Page_405'>405</span>Bayer seems in many cases to have followed the outline of the
+imaginary figure from which the constellation derives its
+name, rather than the relative brightness of the stars composing
+the constellation. For example, the seven stars in the
+Plough are known as Alpha, Beta, Gamma, Delta (the faint
+one), Epsilon, Zeta, and Eta, beginning with the northern of
+the two in the square farthest from the tail, thus evidently
+following the shape of the figure, and not the order of relative
+brightness. When the letters of the Greek alphabet are exhausted,
+recourse is had to numbers, those in Flamsteed’s
+catalogue being usually employed. Those only visible in
+telescopes are known by their numbers in various catalogues.
+The exact positions of the stars are fixed by determining
+their right ascensions and declinations, terms which on the
+celestial sphere correspond to longitude and latitude on the
+earth.</p>
+
+<p class='c007'>The stars Alpha and Beta of the Plough are called “the
+pointers,” because a line drawn from Beta through Alpha
+points nearly to a star of the second magnitude, called the
+Pole Star, which lies near the pole of the celestial sphere, or
+the point round which the whole star sphere seems to rotate,
+owing to the rotation of the earth on its axis, in twenty-four
+hours. The distance from Alpha to the Pole Star is about
+five times the distance between Alpha and Beta.</p>
+
+<p class='c007'>If we draw an imaginary line from the star Epsilon through
+the Pole Star, and produce it to about the same distance on
+the opposite side of the Pole, it will pass through a well-known
+group called Cassiopeia’s Chair. This consists of five fairly
+bright stars arranged in the form of an irregular W. A sixth
+star, much fainter than the others, forms with three of them a
+quadrilateral figure. It was near this faint star—known to
+astronomers as Kappa—that the famous “new,” or temporary,
+star of Tycho Brahé, sometimes called the “Pilgrim Star,” suddenly
+appeared in November, 1572, of which more hereafter.</p>
+
+<p class='c007'>If we continue the curve formed by the three stars in the
+tail of the Great Bear, it will pass near a very bright star
+of an orange colour. This is Arcturus, one of the brightest
+<span class='pageno' id='Page_406'>406</span>stars in the sky. If we can rely on the measures of distance
+which have been made of this brilliant star, it must be one of
+the largest bodies in the universe, much larger than our sun,
+which, placed at the distance assigned to Arcturus, would only
+shine as a small star, quite invisible indeed to the naked eye.</p>
+
+<p class='c007'>Returning again to the Great Bear, if we draw a line from
+Gamma to Beta and produce it, it will pass near a bright star
+of a yellow colour. This is Capella. It was called by the
+Arabian astronomers the “Guardian of the Pleiades.” It is
+the brightest star of the constellation Auriga or “the
+Charioteer,” referred to by Tennyson in the lines:</p>
+
+<div class='lg-container-b c015'>
+  <div class='linegroup'>
+    <div class='group'>
+      <div class='line'>“And the shining daffodil dies, and the Charioteer</div>
+      <div class='line'>And starry Gemini hang like glorious crowns</div>
+      <div class='line'>Over Orion’s grave low down in the West,”</div>
+    </div>
+  </div>
+</div>
+
+<p class='c016'>evidently referring to the disappearance of Orion below
+the western horizon in the evening sky of April. “Starry
+Gemini” is marked by two bright stars, Castor and Pollux,
+which may be found by drawing a line from Delta to Beta of
+the Great Bear, and producing it. Another line drawn from
+Delta to Gamma, and produced towards the south, will pass
+near a bright star called Regulus, the brightest star in the
+well-known “Sickle” in Leo or the Lion. Again, a line drawn
+from Regulus to Gamma in the Great Bear, and produced, will
+pass near another bright star, Vega in the Lyre. This is one
+of the brightest stars in the Northern Hemisphere, the three,
+Arcturus, Capella, and Vega, being nearly equal in brightness.
+The name Vega seems to be a corruption of the Arabic name
+<i>vaki</i>, or <i>al-nasr al-vaki</i>, “the falling eagle,” the wings
+of the bird being represented by the stars Epsilon and Zeta
+Lyræ, which form, with Vega, a small triangle, called by the
+Arabians <i>al-alsafi</i>, “the trivet.” But what relation exists between
+a “falling eagle” and the musical instrument known as
+the Lyre (Persian <i>al-lûra</i>) is not very obvious. Possibly,
+however, as suggested by Schjellerup, the Arabic word, <i>al-schalzâk</i>
+“a goose,”—also applied to the constellation—refers to
+the resemblance in shape between a plucked goose and a
+<span class='pageno' id='Page_407'>407</span>Greek lyre. The Greeks called the constellation χέλυς, a tortoise,
+which also somewhat resembles a lyre in shape.</p>
+
+<p class='c007'>Of the two stars which form a triangle with Vega, the
+northern, Epsilon, is a double star, which is said to have been
+seen double with the naked eye by several astronomers, but,
+probably, most people would fail to see it as anything but a
+single star, as the component stars are very close. An opera-glass
+will, however, show it distinctly. Each of the components
+is again double, so that the object forms a most
+interesting quadruple star when viewed with a good telescope.</p>
+
+<p class='c007'>To the east of Vega lies Cygnus, or the Swan, one of the
+finest of the constellations. It may be distinguished by the
+long cross formed by the principal stars which are known to
+astronomers as Alpha, Beta, Gamma, Delta, and Epsilon;
+Alpha, or Deneb, being the brightest and most northern of the
+five, and Beta the most southern and faintest. The name
+Deneb is derived from the Arabic word <i>dzanab al-dadjâdja</i>,
+or “the tail of the hen,” referring to its position in the ancient
+figure, which represents a hen or swan flying towards the
+south.</p>
+
+<p class='c007'>To the south-east of Cassiopeia’s Chair, we find the well-known
+festoon of stars which marks the constellation Perseus.
+Its brightest star is sometimes called Mirfak, a name derived
+from the Arabic word <i>marfik</i>, the elbow, referring, perhaps, to
+its position in the curved line of stars. South of Perseus, and
+the nearest bright star to Mirfak in that direction, is Algol, the
+famous variable star. Further south, we come to the constellation
+of Taurus, or the Bull, with the well-known groups of
+the Pleiades and Hyades. The Pleiades form a remarkable
+cluster, and when once recognised can never be mistaken. To
+ordinary eyesight six stars are visible, but those having
+keener vision can see more. A little south of the Pleiades is
+a V-shaped figure, the Hyades, with a bright star of a reddish
+colour. This is Aldebaran, a name derived from the Arabic
+<i>al-dabarân</i>, the attendant or follower, because it appears to
+follow the Pleiades in the diurnal motion. It was also called
+<i>aïn al-tsaur</i>, “the eye of the bull,” and by several other names
+<span class='pageno' id='Page_408'>408</span>such as <i>al-fanîk</i>, “the great camel,” the other smaller stars
+forming the Hyades being called <i>al-kilas</i>, “the young camels!”</p>
+
+<p class='c007'>South of Taurus and Gemini comes the magnificent constellation
+of Orion, perhaps the most splendid collection of
+stars in the sky. This brilliant asterism contains many fine
+objects. Looking at it when it is visible in the winter sky, we
+notice a large quadrilateral figure formed by four conspicuous
+stars. The upper one to the left is called Betelgeuse, and is
+decidedly reddish in colour—very much resembling Aldebaran
+both in tint and brightness. Its name is derived from our
+Arabic word meaning the shoulder, because it is situated on
+the right shoulder of the giant Orion on the old celestial
+globes. The upper one to the right is called Bellatrix, or the
+female warrior! The real significance of some of those old
+names is sometimes difficult to understand. Of the lower
+stars, the one on the right is a fine white star of the first
+magnitude known as Rigel. It is situated on the left foot of
+the ancient figure of Orion, and its name is derived from the
+first part of the compound Arabic name <i>ridjl-al-djauzâ</i>, “the
+leg of the giant.” The lower star on the left is known to astronomers
+by the Greek letter Kappa.</p>
+
+<p class='c007'>In the middle of the four-sided figure referred to above
+are three stars of the second magnitude, nearly in a straight
+line, forming “Orion’s Belt.” The upper one of the three is
+slightly fainter than the others, and has been suspected of
+being slightly variable in its light, but the variability is doubtful.
+South of these three conspicuous stars are three fainter
+stars, forming a nearly vertical line. This is “the Sword of
+Orion.” The middle star of the three marks the position of
+“the great nebula in Orion,” one of the finest objects in the
+heavens, of which more hereafter. To some eyes a nebulous
+glow is visible round this star. Even in a small telescope the
+nebula is an interesting object. On a very clear night the
+southern star of the three may be seen double with good eyesight.
+The stars forming Orion’s Belt were called by the
+Arabian astronomers <i>mintakat al-djauza</i>, “the Belt of the
+Giant”; and the stars forming the “sword,” <i>al-lakat</i>, the
+<span class='pageno' id='Page_409'>409</span>“gleaned ears of corn,” and also <i>saif-al-djabbâr</i>, “the Sword
+of the Giant.” Perhaps the latter word is the origin of the
+name Algebar, formerly applied to Rigel.</p>
+
+<p class='c007'>The three bright stars in Orion’s Belt nearly point (to the
+south-east) to Sirius, the brightest star in the heavens. This
+is a splendid white star, and is so much brighter than any
+other fixed star that its identity cannot be mistaken.</p>
+
+<p class='c007'>If we draw a line from the star Gamma in the Plough to
+the Pole Star, and produce it, it will pass through a somewhat
+similar four-sided figure, but of much larger size, and the stars
+rather fainter. This is known as “the Square of Pegasus.”
+The upper stars are known as Beta Pegasi (the one to the
+right) and Alpha Andromeda. To the east of Alpha Andromedæ
+is a star of the third magnitude, Delta, and to the east
+of Delta, a star of the second magnitude called Beta Andromedæ.
+A little north of Beta are two small stars, Mu and
+Nu, nearly in a line with Beta, and to the north of Nu is the
+famous “nebula in Andromeda” “the queen of the nebulæ,”
+as it has been termed. It is just visible to the naked eye as a
+hazy spot of light, and it may be well seen in a good opera-glass
+or binocular. Even in a small telescope it is a really
+splendid object. The reader should fix its exact position
+carefully, as it has been frequently mistaken for a comet
+by observers whose knowledge of the heavens is not very
+accurate.</p>
+
+<p class='c007'>The following alignments may be found useful by beginners
+in the study of the starry sky:—</p>
+
+<p class='c007'>Castor and Pollux, already mentioned, nearly point south
+to the star Alpha Hydræ, an isolated reddish star of the
+second magnitude. It is also called Alphard, from the Arabic
+<i>al-fard</i>, “the solitary one,” because there is no other bright star
+near it. It is described by Al-Sûfi, the Persian astronomer, as
+red in the tenth century. In the Chinese annals it is called
+“the Red Bird.”</p>
+
+<p class='c007'>An isosceles triangle is formed by Castor (at the vertex),
+Alphard and Sirius. Procyon is nearly in the centre of this
+triangle. Two other roughly isosceles triangles are formed,
+<span class='pageno' id='Page_410'>410</span>having Aldebaran at the vertex of each, namely: Aldebaran,
+Castor, and Procyon, and Aldebaran, Procyon, and Sirius.</p>
+
+<p class='c007'>Castor, Alpha, Delta, and Beta Orionis are nearly in a
+straight line; also Beta Pegasi, Alpha Pegasi and Fomalhaut.
+A right-angled triangle is formed by Arcturus, Spica, and
+Regulus, Spica being at the right angle.</p>
+
+<p class='c007'>In the Southern Hemisphere, the most remarkable group of
+stars is the well-known Southern Cross. It consists of four
+stars, known as Alpha, Beta, Gamma and Delta—Gamma
+being at the top of the cross, and Alpha at the bottom.
+These stars are popularly supposed to be of great brilliancy,
+but this is a mistake; their magnitudes, according to recent
+photometric measures, being Alpha, first magnitudes, Beta 1½,
+Gamma, second magnitude, and Delta, third magnitude. A
+little south of Delta is Epsilon, a star of the fourth magnitude,
+which rather spoils the symmetry of the cross-shaped figure.
+A little to the east of the Southern Cross are Alpha and Beta
+Centauri, two of the brightest stars in the sky. Another fine
+group of stars is Scorpio, or the Scorpion, of which the
+brightest star is Antares, a reddish star of about magnitude
+1½, which is visible near the southern horizon in the months
+of June and July in England.</p>
+
+<p class='c007'>When the positions of the principal stars are known, it will
+be easy to find any other required object by means of star
+maps.</p>
+
+<h3 class='c004'>CHAPTER II.<br> <span class='c014'>DOUBLE, MULTIPLE, AND COLOURED STARS.</span></h3>
+
+<p class='c003'>Many of the stars when examined with a good telescope are
+seen to be double, some triple, and a few quadruple, and even
+multiple. These when viewed with the naked eye, or even a
+<span class='pageno' id='Page_411'>411</span>powerful binocular, seem to be single, and show no sign of
+consisting of two components. These telescopic double stars
+should be carefully distinguished from those which appear
+very close together with the naked eye, and which in opera-glasses
+or telescopes of small power might be mistaken for
+wide double stars by the inexperienced observer. These
+latter stars, such as Mizar—the middle star in the tail of the
+Great Bear, and its small companion, Alcor, referred to in the
+last chapter—have been called “naked eye doubles,” but they
+are not, properly speaking, double stars at all. Telescopic
+double stars are far closer, and even the widest of them could
+not possibly be seen double without optical aid, even by those
+who are gifted with the keenest vision. Of these so-called
+“naked eye doubles,” we may mention Alpha Capricorni,
+which on a very clear night may be seen with the naked eye
+to consist of two stars. On a very fine night two stars may
+be seen in Iota Orionis, the most southern star in Orion’s
+Sword. The star Zeta Ceti has near it a fifth magnitude
+star, Chi, which may be easily seen with the unaided vision.
+The star Epsilon Lyræ (near Vega), is, as mentioned in the
+last chapter, a severe test for naked eye vision. Bessel, the
+famous German astronomer, is said to have seen it when
+thirteen years of age. Omicron Cygni (north of Alpha and
+Delta Cygni) forms another naked eye double, and other
+objects of this class may be noticed by a sharp-eyed observer.</p>
+
+<p class='c007'>The star Mizar, already referred to, is itself a wide telescopic
+double, and it seems to have been the first double star discovered
+with the telescope (by Riccioli in 1650). It consists
+of two components, of which one is considerably brighter than
+the other. It will give an idea of the closeness of even a
+“wide” telescopic double when we say that the apparent distance
+between Mizar and Alcor is nearly forty times the
+distance which separates the close components of the bright
+star. From this it will be seen that even a powerful binocular
+field-glass would fail to show Mizar as anything but a single
+star. The components may, however, be well seen with a
+3-inch telescope, or even with a good 2-inch. The colours
+<span class='pageno' id='Page_412'>412</span>of the two stars are pale green and white. Between Mizar
+and Alcor is a star of the eighth magnitude, and others
+fainter. Mizar was the first double star photographed by
+Bond.</p>
+
+<p class='c007'>The Pole Star has a small companion at a little greater
+distance than that which separates the components of Mizar,
+but owing to the faintness of this small star, the object is not
+so easy as Mizar. A telescope with a good 3-inch aperture
+should, however, show it readily. Dawes saw it with a small
+telescope of 1³⁄₁₀-inch aperture, and Ward, who has wonderful
+vision, with only 1¼-inch.</p>
+
+<p class='c007'>The star Beta Cygni is composed of a large and small star,
+of which the colours are described as “golden-yellow and
+smalt-blue.” This is a very wide double, and may be seen
+with quite a small telescope. Another fine double star is that
+known to astronomers as Gamma Andromedæ. The magnitudes
+of the components are about the same as those of Mizar,
+but a little closer. Their colours are beautiful (“gold and
+blue”). This is one of the prettiest double stars in the
+heavens. It is really a triple star, the fainter of the pair
+being a very close double star; but this is beyond the reach
+of all but the largest telescopes. The star Gamma Delphini
+is another beautiful object, the components being a little more
+unequal in magnitude, but the distance between them about
+the same as in Gamma Andromedæ. I have noted the
+colours with a 3-inch telescope as “reddish-yellow and greyish-lilac.”
+Gamma Arietis, the faintest of the three well-known
+stars in the head of Aries, is another fine double star, a little
+closer than Gamma Delphini. This is an interesting object,
+from the fact that it was one of the first double stars discovered
+with the telescope—by Hooke, in 1664, when following
+the comet of that year. He says:—“I took notice that it
+consisted of two small stars very near together, a like instance
+of which I have not else met with in all the heaven.” Eight
+years previous to this, however, in 1656, Huygens is said to
+have seen three stars in Theta Orionis, the well-known
+multiple star in the Orion nebula; and in 1650, Riccioli, at
+<span class='pageno' id='Page_413'>413</span>Bologne, saw Zeta Ursæ Majoris (Mizar) double, as already
+stated.</p>
+
+<p class='c007'>Another beautiful double star is Eta Cassiopeiæ, the components
+being about equal in brightness to those of Gamma
+Delphini, but the distance less than one half, so that a higher
+magnifying power will be required to see them well. The
+colours are, according to Webb, yellow and purple; but other
+observers have found the smaller star garnet or red. This is
+a very interesting object, the components revolving round
+each other, and forming what is called a binary star.</p>
+
+<p class='c007'>Another fine double star is Castor, which is composed of
+two nearly equal stars separated by a distance about half that
+between the components of Gamma Andromedæ. This is
+also a binary or revolving double star, but the period is long.
+Gamma Virginis is another fine double star, with components
+at about the same distance as those of Castor, and the colours
+very similar. It is also a remarkable binary star, and further
+details respecting it will be given when we come to speak of
+the binary stars.</p>
+
+<p class='c007'>Among double stars of which the components are closer
+than those mentioned above, but which are within the reach
+of a good 3-inch telescope—a common size with amateur
+observers—the following may be noticed:—Alpha Herculis,
+colours, orange or emerald green; the light of this star is
+slightly variable. Gamma Leonis, another binary star with a
+long period; colours, pale yellow and purple. Epsilon
+Boötis, a lovely double star, the colours of which Secchi
+described as “most beautiful yellow, superb blue.” This has
+been well seen with a 2¼-inch achromatic.</p>
+
+<p class='c007'>For observers in the Southern Hemisphere, the following
+fine double stars may be seen with a 3-inch telescope:—Alpha
+Centauri; this famous star, the nearest of all the fixed
+stars to the earth, is also a remarkable binary; its period, as
+recently computed by Dr. See, is 81 years, and the component
+stars are now at nearly their greatest distance apart, the distance
+being greater than that between the components of
+Mizar, so that any small telescope will show them. Theta
+<span class='pageno' id='Page_414'>414</span>Eridani is a splendid pair, but closer than Alpha Centauri. It
+is, however, an easy object with a 3-inch telescope, and with a
+telescope of this size I noted the colours in India as light
+yellow and dusky yellow. The star known as <i>f</i> Eridani is a
+very similar double to Theta, but the components are fainter.
+I noted the colours in India as yellowish-white and very light
+green. There are, of course, many other double stars in both
+hemispheres within the reach of small telescopes; but those
+described above are perhaps the finest examples.</p>
+
+<p class='c007'>In addition to these comparatively wide double stars, there
+are many of which the components are so close that they are
+quite beyond the reach of a 3-inch or even a 4-inch telescope.
+Some, indeed, are so excessively close as to tax the highest
+powers of the largest telescopes yet constructed.</p>
+
+<p class='c007'>Of triple, quadruple, and multiple stars, there are several
+which may be well seen with a small telescope. Of these may
+be mentioned Iota Orionis, the lowest star in the Sword of
+Orion, which consists of a bright star accompanied by two
+small companions. In Theta Orionis, the middle star of the
+Sword, four stars may be seen forming a quadrilateral figure,
+known to observers as the “trapezium.” I have seen these in
+India—where the star is higher in the sky than in this
+country—with a 3-inch refractor reduced by a “stop” over
+the object-glass to 1½ inch. There are two fainter stars in
+this curious object, which lie in the midst of the Orion
+nebula, but a somewhat larger telescope is required to see
+them. Within the trapezium are two very faint stars, which
+are only visible in the largest telescopes. In Sigma Orionis—a
+star closely south of Zeta, the lowest star in Orion’s
+Belt—six stars may be seen with a 3-inch telescope.
+Indeed, Ward has seen ten with a slightly smaller telescope.
+Epsilon Lyræ may be seen double with a low power, and
+each star of the pair again double with a high power; but this
+is more difficult than the other close stars mentioned above.</p>
+
+<p class='c007'>When carefully examined, many of the stars show differences
+in colour. Among the brightest stars it will be
+noticed that Sirius, Rigel, and Vega, shine with a white or
+<span class='pageno' id='Page_415'>415</span>bluish-white light; Capella is distinctly yellowish; Arcturus
+yellow or orange; and Aldebaran and Betelgeuse have a well-marked
+reddish hue. There are no stars of a decided blue
+colour visible to the naked eye, at least in the Northern
+Hemisphere. The third magnitude star, Beta Lyræ, is said to
+be greenish, but its colour is not conspicuous. Betelgeuse is
+perhaps the ruddiest of the brighter stars, and its reddish tint
+contrasts strongly with the white light of Rigel, in the same
+constellation. Aldebaran, which lies not far from Betelgeuse,
+is of nearly the same hue. But the reddest star visible to the
+naked eye in the Northern Hemisphere is the fourth magnitude
+star, Mu Cephei. It is not, however, sufficiently bright
+to enable its colour to be well seen without optical aid, but
+with an opera-glass its reddish hue is beautiful and striking
+when compared with other stars in its immediate vicinity. It
+was called by Sir William Herschel the “garnet star,” and its
+colour is certainly remarkable. Like so many of the red
+stars, it is variable in light, but numerous observations by the
+present writer seem to show that there is no regular period,
+and its light often remains for many weeks with little or no
+perceptible change.</p>
+
+<p class='c007'>Among other stars visible to the naked eye, the reddish
+colour is also conspicuous in Antares, Alphard, Eta, and Mu
+Geminorum, Mu and Nu Ursæ Majoris, Beta Ophiuchi,
+Gamma Aquilæ, and others in the Southern Hemisphere·
+Alphard was noted as red by the Persian astronomer, Al-Sûfi,
+in the tenth century, and it was called “the Red Bird,” by the
+old Chinese observers.</p>
+
+<p class='c007'>Ptolemy, in his catalogue, calls the following stars “fiery
+red”: Arcturus, Aldebaran, Pollux, Antares, Betelgeuse, and,
+curious to say, Sirius, which is now white. There is some
+little doubt as to the reality of this change of hue in Sirius,
+but Al-Sûfi distinctly describes the variable star, Algol, as
+red, whereas it is now white, or only slightly yellowish.</p>
+
+<p class='c007'>The finest examples of red stars are, however, found among
+those only visible with a telescope. Of these may be mentioned
+the star numbered 713 in Espin’s edition of Birmingham’s
+<span class='pageno' id='Page_416'>416</span>“Catalogue of Red Stars,” which Franks describes as
+“orange vermilion,” and the star Birmingham 248, which
+Espin notes as “magnificent blood-red.” Another very fine
+red star is the variable R Crateris, which Sir John Herschel
+described as “scarlet, almost blood colour,” Birmingham
+“crimson,” and Webb “very intense ruby.” Observing it in
+India with a 3-inch telescope, I noted it as “full scarlet.”
+It has near it a star of the ninth magnitude of a pale bluish
+tint. No. 4 of Birmingham’s “Catalogue” is described by
+Espin as of an “intense red colour, most wonderful.” The
+variable star U Cygni is very red, and is described by Webb
+as showing “one of the loveliest hues in the sky.” Another
+red star is the remarkable, variable R Leonis, whose fluctuations
+in light will be described in the chapter on Variable
+Stars. Hind says: “It is one of the most fiery-looking
+variables on our list—fiery in every stage from maximum to
+minimum, and is really a fine telescopic object in a dark
+sky about the time of greatest brilliancy, when its colour
+forms a striking contrast with the steady white light of the
+sixth magnitude, a little to the north.”</p>
+
+<p class='c007'>In the Southern Hemisphere there are some fine red stars.
+Epsilon Crucis, one of the stars of the Southern Cross, is said
+to be very red, and so are Mu Muscæ and Delta Gruis, the
+southern star of a naked eye double. Pi Gruis is also a wide
+double star, and Dr. Gould describes one of the pair as
+“deep crimson,” while the other is “conspicuously white.”
+The variable R Sculptoris is another fine red star, which Gould
+describes as “intense scarlet,” and Miss Clerke says it “glows
+like a live coal in the field,” a good description of these
+telescopic red stars. With reference to a small star in the
+field of view with Beta Crucis, one of the brightest stars in the
+Southern Cross, Sir John Herschel says: “The fullest and
+deepest maroon-red, the most intense blood-red of any star I
+have seen. It is like a drop of blood when contrasted with
+the whiteness of Beta Crucis.”</p>
+
+<p class='c007'>Among the double stars there are numerous examples of
+coloured suns. Of these may be mentioned Alpha Herculis,
+<span class='pageno' id='Page_417'>417</span>the components of which are orange and emerald, or bluish-green,
+and described by Smith as “a lovely object, one of
+the finest in the heavens”; Epsilon Boötis, of which the
+colours are described by Secchi as “most beautiful yellow,
+superb blue”; Beta Cygni, “golden-yellow and smalt-blue”;
+Beta Cephei, “yellow and violet”; Delta Cephei, “yellow and
+blue”; Gamma Andromedæ, “gold and blue”; and Beta
+Piscis Australis, of which the colours were noted by the
+present writer in India as white and reddish-lilac.</p>
+
+<p class='c007'>It has been found that the red stars are most numerous in
+or near the Milky Way, and one portion of the Galaxy—between
+Aquila, Lyra, and Cygnus—was called by Birmingham
+“the red region in Cygnus.” Yellow and orange stars
+seem to be most abundant in the constellations, Cetus, Pisces,
+Hydra, and Virgo, and the white stars in Orion, Cassiopeia,
+and Lyra.</p>
+
+<h3 class='c004'>CHAPTER III.<br> <span class='c014'>THE DISTANCES AND MOTIONS OF THE STARS.</span></h3>
+
+<p class='c003'>The determination of the distances of the stars from the
+earth has always formed a subject of great interest to
+astronomers. The earlier observers appear to have thought
+that the problem was an insoluble one. The famous Kepler,
+judging from what he called the “harmony of relations,”
+came to the conclusion that the distance of the fixed stars
+should be about 2,000 times the distance of Saturn from the
+sun. Saturn was then the outermost planet of the solar
+system. The distance of even the nearest star, as now known,
+is about 14 times greater than that supposed by Kepler.
+Huygens thought the determination of stellar distance by
+observation to be impossible, but made an attempt at a
+<span class='pageno' id='Page_418'>418</span>solution of the problem by a photometric comparison between
+Sirius and the sun. By this method, he found that Sirius is
+probably about 28,000 times the sun’s distance from the earth,
+but modern measures show that this estimate is far too small,
+the distance of Sirius being probably over 500,000 times the
+sun’s distance, or about 18 times greater than Huygens made
+it.</p>
+
+<p class='c007'>When the Copernican theory of the earth’s motion round
+the sun was first advanced, it was objected that, if the earth
+moved in a large orbit, its real change of place should produce
+an <i>apparent</i> change of position in the stars nearest to the
+earth, causing them to shift their relative position with reference
+to more distant stars. Copernicus replied to this
+objection—and we now know that his reply was correct—by
+saying that the distance of even the nearest stars was so great
+that the earth’s motion would have no perceptible effect in
+changing their apparent position in the heavens; in other
+words, the diameter of the earth’s orbit round the sun would
+be almost a vanishing point if viewed from the distance of the
+nearest stars. This explanation of Copernicus was at first
+ridiculed, and even the famous astronomer, Tycho Brahé,
+could not accept such a startling conclusion. This celebrated
+observer failed indeed to detect by his own observations any
+annual change of place in the stars, but he fancied that the
+brightest stars showed a perceptible disc, like the planets, a
+fact which, if true, would imply that, if the distance of the
+stars was so great as Copernicus supposed, their real diameter
+must be enormous. The invention of the telescope, however,
+dispelled this delusion of Tycho Brahé, and showed that even
+the brightest stars showed no perceptible disc. This was
+proved by Horrocks and Crabtree, who noticed that, in
+occultations of stars by the moon, the stars disappeared
+instantaneously, a fact which proved that the apparent
+diameter of the stars must be a very small fraction of a
+second of arc.</p>
+
+<p class='c007'>Galileo suggested that possibly the distance of the nearer
+stars might be determined by careful measures of double stars,
+<span class='pageno' id='Page_419'>419</span>on the assumption that the brighter star of the pair—if the
+difference in brilliancy is considerable—is nearer the earth than
+the fainter star. He says (in his “<cite>Opere di Galileo Galilei</cite>”), “I
+do not believe that all the stars are scattered over a spherical
+superficies <i>at equal distances from a common centre</i>, but I am
+of opinion that their distances from us are so various that
+some of them may be two or three times as remote as others,
+so that when some minute star is discovered by the telescope
+close to one of the larger, and yet the former is highest, it
+may be that some sensible change might take place among
+them.” Acting on this idea, Sir William Herschel, at the close
+of the eighteenth century, made a careful series of measures of
+certain double stars. He did not, however, succeed in his
+attempt, as his instruments were not sufficiently accurate for
+such an investigation, but his labours were rewarded by the
+great discovery of binary or revolving double stars, most
+interesting objects, which will be considered in the next
+chapter.</p>
+
+<p class='c007'>Numerous but unsuccessful attempts were made by Hooke,
+Flamsteed, Cassini, Molyneux, and Bradley, to find the distance
+of some of the stars. Hooke, in the year 1669, thought
+he had detected a parallax of 27 to 30 seconds arc in the
+star Gamma Draconis, but we now know that no star in the
+heavens has anything like so large a parallax. It must be
+here explained that to find the distance of any star from the
+earth, we must first measure its “parallax,” which is the
+apparent change in its place due to the earth’s motion round
+the sun. As the earth makes half a revolution in six months,
+and as the earth’s mean distance from the sun—or the radius
+of the earth’s orbit—is about 93 millions of miles, the earth is,
+at any given time, about 186 millions of miles distant from the
+point in its orbit which it occupied six months previously.
+The apparent change of position in a star’s place, known as
+parallax, is <i>one-half</i> the total displacement of the star as seen
+from opposite points of the earth’s orbit. In other words, it is
+the angle subtended at the star by the sun’s mean distance
+from the earth. The measured parallax of a star may be
+<span class='pageno' id='Page_420'>420</span>either “absolute” or “relative.” An “absolute parallax” is
+the actual parallax. A “relative parallax” is the parallax
+with reference to a faint star situated near a brighter star, the
+faint star being assumed to lie, as suggested by Galileo, at a
+much greater distance from the earth. As, however, the
+faint star may have a small parallax of its own, the “relative
+parallax” is the difference between the parallaxes of the two
+stars. Indeed, in some cases a “negative parallax” has been
+found, which, if not due to errors of observation, would imply
+that the faint star is actually the nearer of the two. From
+the observed parallax, the star’s distance in miles may be
+found by simply multiplying 93 millions of miles by 206,265
+and dividing the result by the parallax. To find the time
+that light would take to reach us from the star—the light
+journey as it is called—it is only necessary to divide the
+number 3·258 by the parallax.</p>
+
+<p class='c007'>In attempting to verify the result found by Hooke for the
+parallax of Gamma Draconis, Molyneux and Bradley found an
+apparent parallax of about 20 seconds of arc, thus apparently
+confirming Hooke’s result, but observations of other stars
+showing a similar result, Bradley came to the conclusion that the
+apparent change of position was not really due to parallax,
+but was caused by a phenomenon now known as the “aberration
+of light,” an apparent displacement in the positions of the
+stars, due to the effect of the earth’s motion in its orbit round
+the sun combined with the progressive motion of light. The
+result is that “a star is displaced by aberration along a great
+circle, joining its true place to the point on the celestial sphere
+towards which the earth is moving.” The amount of aberration
+is a maximum for stars lying in a direction at right angles to
+that of the earth’s motion. The existence of aberration is an
+absolute proof that the earth does revolve round the sun, for
+were the earth at rest—as some paradoxes contend—there
+would be no aberration of the stars. This effect of aberration
+must, of course, be carefully allowed for in all measures of
+stellar parallax. To show that “aberration” could not possibly
+be due to “parallax,” it may be stated that aberration
+<span class='pageno' id='Page_421'>421</span>shifts the apparent place of a star in one direction, while
+parallax shifts it in the opposite direction.</p>
+
+<p class='c007'>From photometric comparisons, the Rev. John Mitchell, in
+the year 1767, concluded that the parallax of Sirius is less
+than a second of arc; a result which has been fully confirmed
+by modern measures. He considered that stars of the sixth
+magnitude are probably 20 to 30 times the distance of Sirius,
+and judging from their relative brilliancy alone, this result
+would also be nearly correct. But recent measures have
+shown that some of the fainter stars are actually nearer to us
+than some of the brighter, and that the brightness of a star is
+no criterion of its distance.</p>
+
+<p class='c007'>The first stars on which observations seem to have been
+made with a view to a determination of their distance seem to
+have been Aldebaran and Sirius. From observations made in
+the years 1792 to 1804 with a vertical circle and telescope of
+3 inches aperture, Piazzi found for Aldebaran an “absolute”
+parallax of about 1½  seconds of arc. O. Struve and Shdanow,
+in 1857, using a refractor of 15 inches aperture, found a “relative”
+parallax of about half a second. This was further reduced by
+Hall with the 26-inch refractor of the Washington Observatory
+to about one-tenth of a second, and Elkin, with a heliometer
+of 6 inches aperture, finds a relative parallax of 0″·116, or
+about 30 years’ journey for light For Sirius, Piazzi found,
+in 1792–1804, an absolute parallax of four seconds, but this
+was certainly much too large. All subsequent observers find
+a much smaller parallax, recent measures giving a relative
+parallax of 0·370″ by Gill, and 0·407″ by Elkin. In the years
+1802–1804, Piazzi and Cacciatori found an absolute parallax
+of 1′·31 for the Pole Star; but this has been much reduced by
+other observers. Pritchard, by means of photography, found
+a relative parallax of only 0·073″, which agrees closely with
+some other previous results, and indicates a “light journey”
+of about 44 years!</p>
+
+<p class='c007'>For the bright star Procyon, Piazzi found a parallax of
+about three seconds, but this is also much too large, a recent
+determination by Elkin giving 0·266″, a figure in fair agreement
+<span class='pageno' id='Page_422'>422</span>with results found by Auwers and Wagner. For the
+bright star Vega, Calandrelli, in the years 1805–6, found
+an absolute parallax of nearly four seconds, but this has also
+been much reduced by modern measures; Elkin, from observations
+in the years 1887–88, finding a relative parallax of
+only 0·034″. Brinkley found a parallax of over one
+second for Arcturus, but Elkin’s result is only 0·018″. If this
+minute parallax can be relied on, Arcturus must be a sun of
+vast size.</p>
+
+<p class='c007'>Owing to the large “proper motion” of the star known as
+61 Cygni, its comparative proximity to the earth was suspected,
+and in 1812, Arago and Mathieu found, from measures
+made with a repeating circle, a parallax of over half a second.
+Various measures of its parallax have since been made,
+ranging from about 0·27″ to 0·566″. Sir Robert Ball, at
+Dunsink, Ireland, found 0·468″, and Pritchard, by means of
+photography with a 13-inch reflector, found 0·437″. We may,
+therefore, safely assume that the parallax of 61 Cygni is about
+0·45″. This implies a distance of 458,366 times the sun’s distance
+from the earth, or about 42 billions of miles, and a
+“light journey” of about 7¼ years.</p>
+
+<p class='c007'>It is usually stated that 61 Cygni is the nearest star to the
+earth in the Northern Hemisphere, but for the star known as
+Lalande 21,185, Winnecke found 0·511″, and afterwards
+0·501″. This has, however, been reduced by Kapteyn (1885–1887)
+to 0·434″; and recently a parallax of 0·465″ has been
+found by the photographic method for the binary star, Eta
+Cassiopeiæ. 61 Cygni is a wide double star, but it seems
+doubtful whether the components are physically connected,
+although several orbits have been provisionally
+computed.</p>
+
+<p class='c007'>Nearer to us than 61 Cygni is the bright southern star
+Alpha Centauri, which, so far as is known at present, is the
+nearest of all the fixed stars to the earth. The first attempt
+to find its distance was made by Henderson in the years
+1832–33, using a mural circle of 4 inches aperture and a
+transit of 5 inches. He found an “absolute” parallax of
+<span class='pageno' id='Page_423'>423</span>about one second of arc, which subsequent measures have
+shown to be rather too large. Measures in recent years range
+from 0·512″ to 0·976″, but probably the most reliable are those
+made with a heliometer of 4½ inches aperture by Dr. Gill
+(1881–82), who found a “relative” parallax of 0·76″, and
+by Dr. Elkin, using the same instrument, 0·671″. Gill’s result
+would place the star at a distance of 271,400 times the sun’s
+distance from the earth, or about 25 billions of miles, a distance
+which light, with its great velocity of 186,300 miles a
+second, would take over 4¼ years to traverse.</p>
+
+<p class='c007'>It will be understood that the parallaxes found for even the
+nearest fixed stars are so small that their exact determination
+taxes the powers of the most perfect instruments and the skill
+of the most experienced observers. One thing, however,
+seems certain, that the brightest stars are not necessarily the
+nearest, and that comparatively faint stars may be actually
+nearer to the earth than some of the brightest gems which
+deck our midnight sky. Indeed, from a discussion of the
+observed parallaxes and “proper motions” of 11 stars,
+Gylden finds a mean parallax of only 0·083″ for stars of the
+first magnitude. This agrees closely with the value 0·089″
+found by Dr. Elkin.</p>
+
+<p class='c007'>In old times the stars were supposed to be absolutely
+fixed in the celestial vault, that is to say, that their relative
+positions did not change. This was a very natural conclusion,
+for before the invention of the telescope it would have been
+impossible to detect any “proper motion”—as it is called—by
+naked eye observations. Hence the term “fixed stars,”
+used to distinguish the stars from the planets, which are always
+shifting their positions in the heavens. The existence of
+proper motion, in some at least of the stars, seems to have
+been discovered by Halley, who found from his observations
+in 1715 that the bright stars, Sirius, Arcturus, and Aldebaran,
+had apparently shifted their positions since the date of the
+earliest observations. This discovery was confirmed by James
+Cassini in 1738. He found that Arcturus had apparently
+moved through some five minutes of arc in 152 years, or about
+<span class='pageno' id='Page_424'>424</span>two seconds a year, a result which agrees fairly well with more
+exact modern measures.</p>
+
+<p class='c007'>This interesting discovery of stellar motion has been fully
+confirmed by modern observations, and we now know that,
+far from the stars being “fixed,” most of them have an
+apparent motion on the celestial vault. These motions are,
+however, very slow, and can only be detected by accurate
+measurements and a careful comparison of their positions
+after the lapse of a number of years. The largest proper
+motion hitherto detected is that of a star known as 1830 of
+Groombridge’s catalogue, a small star of about 6½ magnitude,
+which lies in the constellation Ursa Major. This star has an
+apparent motion of seven seconds per annum, which, though
+relatively large, is of course absolutely small, as the observed
+motion would only suffice to carry it through a space equal to
+the moon’s apparent diameter in about 266 years. Assuming
+a parallax of about one-sixth of a second found by Kapteyn,
+this apparent motion would indicate a real motion of about
+128 miles a second at right angles to the line of sight. As,
+however, there may be also motion <i>in</i> the line of sight, the
+above velocity would be a minimum—if the parallax can be
+relied upon—and the actual motion may be considerably
+more. From its rapidity, 1830 Groombridge has been called
+by Prof. Newcomb “the runaway star.”</p>
+
+<p class='c007'>Next in order of rapidity of motion comes the southern
+star known as Lacaille 9352, which lies in the constellation
+Piscis Australis, a little south of Fomalhaut. This seventh
+magnitude star has an apparent motion of 6·9 seconds, which,
+with a parallax of 0·285″ found by Gill, indicates a velocity
+of 71 miles per second. Next comes 61 Cygni, with a velocity
+of 30 miles, and Epsilon Indi—another southern star—with a
+velocity of nearly 68 miles a second. These velocities are,
+however, exceeded by other stars if the measured parallaxes
+are correct. Thus the star Mu Cassiopeiæ, with a proper
+motion of 3·7 seconds, has, according to Pritchard’s photographic
+measures, a parallax of only 0·036″, which would
+indicate a velocity of no less than 302 miles a second! and
+<span class='pageno' id='Page_425'>425</span>the small parallax found by Elkin for Arcturus would imply
+the startling velocity of 376 miles a second!</p>
+
+<p class='c007'>It is a remarkable fact that the eight stars with the largest
+proper motions are all below the fourth magnitude in brightness,
+and as a large proper motion probably indicates proximity
+to the earth, the conclusion seems evident that the
+brightest stars are not as a rule the nearest. Of twenty-five
+stars, with proper motions greater than two seconds of arc,
+there are only two—Arcturus and Alpha Centauri—whose
+magnitude exceeds the third. Indeed, more than half the
+stars with motions greater than one second are invisible to the
+naked eye!</p>
+
+<p class='c007'>Many stars have proper motions of less than a second of
+arc per annum. Very small proper motions have also been
+detected, which only reveal themselves after the lapse of a
+great number of years, and it seems probable that there are
+no really “fixed stars” in the heavens. For stars of the sixth
+magnitude, M. Ludwig Struve finds an average motion of only
+eight seconds in a hundred years, or about one-twelfth of a
+second per annum. If we assume that stars of the sixth
+magnitude are, on the average, of the same size and brightness
+as stars of the first magnitude, their distance from the earth
+would be ten times greater. Consequently, stars of the first
+magnitude should have an average proper motion of about
+eighty seconds in one hundred years. This, however, is not
+the case. The twenty brightest stars show an average motion
+of only sixty seconds in a hundred years. And the motion
+of stars of the second magnitude is relatively still slower.
+Instead of an average motion of fifty seconds in a hundred
+years—which they should have if the brightness were inversely
+proportional to the distance—it has been found that twenty-two
+stars of the second magnitude show an average motion
+of only seventeen seconds. This result seems to show that
+the brighter stars are not so near us as their brilliancy would
+lead us to suppose, a conclusion which has been already
+proved by actual measures of their distance.</p>
+
+<p class='c007'>From a consideration of the results found for stellar
+<span class='pageno' id='Page_426'>426</span>parallax, Mr. Thomas Lewis, F.R.A.S., of the Greenwich
+Observatory, comes to the following conclusions<a id='r108'></a><a href='#f108' class='c027'><sup>[108]</sup></a>:—</p>
+
+<p class='c003'>“(1) Leaving out a few of the brightest stars, the parallaxes
+are constant down to 2·70 magnitude.</p>
+
+<p class='c007'>“(2) After 2·70 mag. is reached, the parallaxes are doubled,
+and remain practically constant to 8·40 mag.</p>
+
+<p class='c007'>“(3) Up to the 3rd mag. the velocities are very small,
+averaging about 9 miles per second, while after the 3rd mag.
+the velocity is 38 miles per second.</p>
+
+<p class='c007'>“Hence we may fairly deduce—</p>
+
+<p class='c007'>“(1) That there are a few stars (about 8) of exceptional
+brilliancy in our immediate neighbourhood, and scattered
+about amongst these a number of small stars (at present about
+40 are known).</p>
+
+<p class='c007'>“(2) Stars of mag. 1·0 to 3·0 are, as a class, far outside this
+inner space, and have very small velocities.</p>
+
+<p class='c007'>“(3) The small stars here dealt with have apparently large
+velocities across the line of sight.</p>
+
+<p class='c007'>“These results show that the generally received idea that
+parallaxes are to be sought for in stars with large proper
+motion is correct, and we may add that this holds good, no
+matter what may be the star’s magnitude.”</p>
+
+<p class='c003'>The “proper motion” of a star only indicates its motion at
+right angles to the line of sight—that is, its motion on the surface
+of the celestial vault—and gives us no information as to
+whether the star is approaching to or receding from the earth.
+This motion “in the line of sight” cannot be detected by
+micrometrical measures with an ordinary telescope, and
+would probably have remained for ever unknown had the
+spectroscope not been invented. Dr. Huggins was the first to
+show that motions in the line of sight could be determined by
+measuring the displacement of the spectral lines caused by
+the approach or recession of the source of light, the lines
+being slightly shifted towards the blue end of the spectrum
+<span class='pageno' id='Page_427'>427</span>when the star is approaching the earth, and towards the red
+end when it is receding from us. The effect would, of course,
+be exactly the same if the star were at rest and the earth in
+motion. By carefully measuring this observed displacement
+of the spectral lines, the velocity in the line of sight can be
+easily computed. Dr. Huggins’ observations were fully confirmed
+by Dr. Vogel.</p>
+
+<p class='c007'>The earlier determinations of motion in the line of sight
+were made by eye measurements with a micrometer, and
+owing to the difficulty and delicacy of these measures, the
+results were very discordant. The method has recently been
+much improved by photographing the spectra and measuring
+the positions of the lines on the photograph. Both methods
+agree in showing that the following stars, among others, are
+certainly <i>approaching</i> the earth: Arcturus, Vega, Procyon,
+Pollux, Altair, Spica, Alpha Cephei, Alpha Persei, Alpha
+Arietis, 61 Cygni, and the Pole Star; and the following are
+certainly <i>receding</i>: Capella, Rigel, Betelgeuse, Aldebaran,
+and Regulus.</p>
+
+<p class='c007'>Measures of photographic stellar spectra have yielded much
+more accurate results than the old method. Some of the
+velocities found in this way by Dr. Vogel—who has given
+especial attention to this subject—are very considerable. For
+the bright star Rigel he finds a velocity of recession of about
+39 miles a second, for Aldebaran 30 miles, and for Capella 15
+miles. He finds that the Pole Star is approaching the earth
+at the rate of 16 miles a second, and Procyon about 7 miles.</p>
+
+<p class='c007'>Dr. Bélopolsky has recently investigated the <i>absolute</i>
+velocity in space of the brighter component of 61 Cygni—that
+is, the motion across the line of sight combined with the
+motion <i>in</i> the line of sight. Assuming a parallax of half a
+second and a proper motion of 5·2 seconds, he finds that the
+motion across the line of sight, corrected for the sun’s motion
+in space, is about 22½ miles per second. The motion <i>in</i> the
+line of sight, also corrected for the sun’s motion, he finds, from
+photographs taken at Pulkova, to be about 27 miles a second
+towards the earth. Combining these motions, he finds the
+<span class='pageno' id='Page_428'>428</span>absolute velocity of the star in space to be about 35 miles a
+second, or nearly double the velocity of the earth in its orbit</p>
+
+<p class='c007'>This method of measuring velocities in the line of sight has
+also been applied to the nebulæ. Mr. Keeler has observed and
+measured a displacement of the line known as the chief
+nebular line in several planetary nebulæ, and finds considerable
+motion in the line of sight. For example, in the nebula
+numbered 6790 in the “New General Catalogue,” he finds a
+motion of recession of about 38 miles a second. Some of
+these motions may possibly be due, in part at least, to the
+sun’s motion in space, carrying the earth with it, a motion
+which will now be considered. The method has also led to
+the discovery of the so-called “spectroscopic binary stars,” a
+most interesting class of objects, which will be considered in
+the next chapter.</p>
+
+<p class='c007'>The proper motions of the stars long since suggested the
+idea that possibly the observed motion may be—to some
+extent, at least—merely apparent, and due to the real motion
+of the sun and solar system through space. The first investigation
+of this interesting question was made by Sir William
+Herschel in 1783, and he came to the conclusion that the sun
+is moving towards a point near Lambda Herculis, a result not
+differing widely from modern determinations. The reality of
+Herschel’s result has been fully confirmed by subsequent
+investigations, and Argelander placed it beyond doubt by a
+comparison of the positions of a large number of stars determined
+at Abo with those found by Bradley in 1752. The
+accuracy of Argelander’s result was confirmed by Otto Struve.
+According to the elder Struve, the results arrived at by
+Argelander, O. Struve, and Peters, is to place the point
+towards which the sun is moving, between the stars Pi and
+Mu Herculis, “at a quarter of the apparent distance of these
+stars from Pi Herculis,” and they estimated the annual motion
+at about 33½ million miles geographical. The general accuracy
+of this conclusion has been verified by modern researches,
+although the results found by different astronomers vary to
+some extent. The accompanying diagram shows some of
+<span class='pageno' id='Page_429'>429</span>the different positions found by various computers. The
+later determinations seem to place the “apex of the solar
+motion,” as it is termed, not far from the bright star Vega, or
+further to the east than Herschel placed it. The velocity of
+the sun’s motion in space has not been so well determined
+as its direction. L. Struve’s computations would indicate a
+velocity of about 14 miles a second; but other results give
+a much smaller velocity.</p>
+
+<div id='i_429'  class='figcenter id003'>
+<img src='images/i_429.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 3.</span>—<i>Diagram showing “Solar Apex,” and the different Positions found by various Computers.</i><br>  <br>  (From “Visible Universe.”)</p>
+</div>
+</div>
+
+<p class='c007'>From a recent investigation of the nature of the sun’s
+motion in space by Mr. G. C. Bompas,<a id='r109'></a><a href='#f109' class='c027'><sup>[109]</sup></a> he considers that the
+various positions of the sun’s “apex” show a tendency to a
+drift along the edge of the Milky Way, and that this drift
+“seems to point to a plane of motion of the sun nearly coinciding
+with the plane of the Milky Way, or, perhaps, more
+nearly with the plane of that great circle of bright stars first
+described by Sir Wm. Herschel as inclined about 20° to the
+galaxy, and which passes through Lyra, in or near which
+constellation the solar apex lies,” and he concludes, from the
+motion of the nearer stars, “that the sun moves in a retrograde
+orbit from east to west, and in a plane inclined a few degrees
+to that of the Milky Way.” With reference to this very interesting
+conclusion, which may, perhaps, be confirmed by
+further observations, Mr. Bompas quotes the following
+passage from “The Visible Universe,” p. 197, by the present
+writer:—“With reference to a possible motion of the stars in
+some general system, M. Rancken has found, from an examination
+of 106 stars, a tendency to drift along the course of the
+Milky Way from Aquila towards Cygnus and Cassiopeia, and
+past Capella through Orion to Argo. The <i>larger</i> motions,
+shown in Proctor’s map of ‘proper motions,’ exhibit this
+tendency in a marked degree between Cygnus and Capella, and
+less clearly on the Sirius, but the smaller motions not so well,”
+and Mr. Bompas points out that this apparent drift of the
+stars in the Milky Way, from west to east, “is just such as
+would be occasioned by a real motion of the sun in that plane,
+in a contrary direction from east to west.”</p>
+
+<div>
+  <span class='pageno' id='Page_430'>430</span>
+  <h3 class='c004'>CHAPTER IV.<br> <span class='c014'>BINARY STARS.</span></h3>
+</div>
+
+<p class='c003'>Double and multiple stars may be either optical or real.
+Optical double stars are those in which the component stars
+are merely apparently close together, owing to their being seen
+in nearly the same direction in space. Two stars may <i>seem</i>
+to be close together, while, in reality, one of them may
+be placed at an immense distance behind the other. Just as
+two lighthouses at sea may, on a dark night, appear close together
+when viewed from a certain point, whereas they may be
+really miles apart. In the case of double stars it is, of course,
+always difficult to determine whether the apparent closeness
+of the stars is real or merely optical. But when, from a long
+series of observations of their relative position, we find that one
+is apparently moving round the other, we know that the stars
+must be comparatively close, and linked together by some
+physical bond of union. These most interesting objects are
+known to astronomers as binary or revolving double stars.
+The probable existence of such objects was predicted from
+abstract reasoning by Mitchell in the eighteenth century; but
+the discovery of their actual existence was made by Sir
+William Herschel, while engaged on an attempt to determine
+the distance of some of the double stars from the earth.
+“Instead of finding, as he expected, that annual fluctuation to
+and fro of one component of a double star with respect to the
+other—that alternate increase and decrease of their distance
+and angle of position, which the parallax of the earth’s
+annual motion would produce—he observed, in many cases, a
+regular progressive change; in some cases bearing chiefly on
+their distance, in others on their position, and advancing
+steadily in one direction, so as clearly to indicate a real
+motion of the stars themselves,” and measurements made
+during the subsequent 25 years fully proved the truth of
+<span class='pageno' id='Page_431'>431</span>the illustrious astronomer’s discovery. It was found that
+in many double stars an orbital motion round each other
+was evident after a number of years of careful observation of
+their relative positions. Unlike the planetary orbits, which are
+nearly circular, at least those of the larger planets of the
+solar system, it was found that the orbits of these double
+stars differ, in many cases, widely from the circular form, in
+some cases, indeed, approaching in shape more the orbit of a
+comet than a planet.</p>
+
+<p class='c007'>The binary stars are among the most interesting objects in
+the heavens. The number now known probably amounts to
+nearly one thousand. In most of them, however, the motion
+is very slow, and in only about seventy cases has the change
+of position, since their discovery, been sufficient to enable an
+orbit to be computed. In most cases the plane of the real
+orbit, or ellipse, described by the companion round the principal
+star, is inclined to the line of sight. We therefore see the
+orbit foreshortened into a more elongated ellipse.</p>
+
+<p class='c007'>The relation of the apparent ellipse—or the ellipse we see
+described by one star round the other—to the real ellipse will
+be easily understood by the following illustration. Suppose a
+cylinder or rod of an elliptical, not circular, section to be cut
+across obliquely to its axis. This oblique section will represent
+the <i>real</i> orbit of a binary star, and the section at right angles
+to the axis, the <i>apparent</i> orbit. The angle between these two
+sections will represent the inclination of the real orbit to the
+plane of projection, or background of the sky. In the apparent
+orbit, the primary star, which is assumed to be situated in one
+of the foci of the real ellipse, does not lie in the focus of the
+apparent ellipse, and from its observed position in this latter
+ellipse we can deduce, mathematically, the particular angle
+at which the oblique section must be made to agree with the
+observed place of the primary star, and other details respecting
+the real ellipse.</p>
+
+<p class='c007'>Savary, in 1830, was the first astronomer who attempted
+to compute the orbit of a binary star, namely, the star Xi
+Ursæ Majoris. This remarkable pair was discovered by Sir
+<span class='pageno' id='Page_432'>432</span>William Herschel in 1780, and as the period of revolution is
+about 61 years, a considerable portion of the ellipse had been
+described in 1830, when it was attacked by Savary. Since
+that year, orbits have been computed for a number of binary
+stars by several computers, among whom may be mentioned
+Sir R. Ball, Behrmann, Casey, Celoria, Doberck, Dunér, Elkin,
+Fritsche, Glasenapp, Sir J. Herschel, Hind, Jacob, Mädler,
+Mann, Schur, See, Thiele, Villarceau, and the present writer.
+The computation of a double star orbit is a matter of considerable
+trouble and difficulty, and cannot be described here. An
+account of the principal results arrived at by astronomers in
+this interesting branch of sidereal astronomy may, however,
+prove of interest to the general reader.</p>
+
+<p class='c007'>We will first consider the binary stars with short periods of
+revolution, which are, of course, the most interesting, and
+those whose orbits can be computed with greater accuracy
+than binaries having periods of considerable length. The
+binary star with the shortest period known at present seems
+to be the fourth magnitude star Kappa Pegasi. It was discovered
+as a wide double star by Sir William Herschel in
+1786, the companion star being of the ninth magnitude. In
+August, 1880, Mr. Burnham, the famous American double star
+observer, examining the star with the 18½ inch refractor of the
+Dearborn Observatory, found the brighter star to be a very
+close double, with a distance between the components of only
+a quarter of a second of arc. A few years’ observations showed
+that this pair were in rapid motion round each other, and from
+measures up to the year 1892, Burnham finds a period of
+11·37 years. A later determination by Dr. See makes the
+period 11·42 years, so that we may conclude that the orbit is
+now pretty accurately determined. The plane of the orbit is
+highly inclined to the line of sight. Dr. See makes the inclination
+81°.</p>
+
+<p class='c007'>Another binary star, with a period of about the same length,
+is Delta Equulei, which was discovered to be a close double
+by Otto Struve in 1851. As in the case of Kappa Pegasi, the
+orbit is highly inclined to the line of sight. In the year 1887,
+<span class='pageno' id='Page_433'>433</span>Wrublewsky, the Russian computer, found a period of about
+11½ years, with an orbit nearly circular. A new orbit was
+published in 1895 by Dr. See, who finds a period of 11·45
+years, and an orbit agreeing fairly well with that of Wrublewsky,
+the orbit differing little from the circular form, and
+inclined to the line of sight at the high angle of 79 degrees.
+Burnham found only a “slight elongation” in the star with
+the great 36-inch telescope of the Lick Observatory in July,
+1889. The distance between the components does not at any
+time exceed half a second of arc, so that it is always beyond
+the reach of all but the largest telescopes.</p>
+
+<p class='c007'>Next in order of shortness of period comes the southern
+binary star Zeta Sagittarii, for which an orbit was first computed
+in the year 1886 by the present writer, who found a
+period of 18·69 years. The orbit was re-computed in 1893,
+with the aid of recent measures by Mr. J. W. Froley, who finds
+a period of 17·71 years. The orbit of this star will, I think,
+require still further revision, but the period of about 18 years
+is probably not far from the truth.</p>
+
+<p class='c007'>Another remarkably rapid binary star is 85 Pegasi, for
+which Schaeberle computed a period of 22·3 years, but a later
+orbit by Prof. Glasenapp makes the period 17½ years, and
+Burnham thinks it will certainly be less than 20 years. Dr.
+See, however, finds a period of 24 years. The primary star
+is about the sixth magnitude, and the companion only the
+eleventh, a difference of five magnitudes, which implies
+that the larger star is one hundred times brighter than the
+companion.</p>
+
+<p class='c007'>Next in order of rapidity of motion we have the southern
+binary star 9 Argûs. For this pair, Burnham finds a period
+of 23·3 years, and Dr. See 22 years, the other elements of the
+orbit being also in close agreement. In this case also the
+orbit plane is highly inclined to the line of sight.</p>
+
+<p class='c007'>The star 42 Comæ Berenices has a period of about 25¾ years,
+according to Otto Struve. The orbit is remarkable from
+the fact that its plane passes through or nearly through the
+earth, and is, therefore, projected into a straight line, the companion
+<span class='pageno' id='Page_434'>434</span>star oscillating backwards and forwards on each side
+of its primary. I find that the plane of the orbit is at right
+angles to the general plane of the Milky Way.</p>
+
+<p class='c007'>The star Beta Delphini—the most southern of the four stars
+in the “Dolphin’s Rhomb”—is also a fast-moving binary,
+discovered by Burnham in 1873, for which periods have been
+computed of 22·97 years by Glasenapp, 26·07 years by
+Dubjago, 27·66 years by Dr. See, and 30·91 years by the
+present writer. Burnham thinks the period will prove to be
+about 28 years. The spectrum of the light of Beta Delphini
+is similar to that of our sun, so that the two bodies should be
+comparable in intrinsic brilliancy. From my orbit of the
+pair, the “hypothetical parallax” is 0·052″—that is, this is the
+parallax the star would have on the supposition that the combined
+mass of its components is equal to the mass of the
+sun. Now, assuming the value of the sun’s stellar magnitude
+which I have recently computed (<cite>Knowledge</cite>, June, 1895)—namely,
+27·15—I find that the sun, if placed at the distance
+indicated for Beta Delphini, would be reduced to a star of
+5·84 magnitude. As the star was measured 3·74 at Harvard,
+we have a difference of 2·1 magnitude, denoting that the
+binary—if of the same mass as the sun—must be about seven
+times brighter. As the spectrum is of the same type, this
+seems improbable, and we must conclude that the star’s
+parallax is more than 0·052″.</p>
+
+<p class='c007'>Another remarkable binary star with a comparatively short
+period is Zeta Herculis. This pair have now performed three
+complete revolutions since their discovery in 1782 by Sir
+William Herschel. Several orbits have been computed, but
+Dr. See’s period of 35 years is probably the best The companion
+is now not far from its maximum distance (1½ seconds)
+from the primary star, and is within the reach of moderate-sized
+telescopes. The companion is, however, rather faint,
+being only 6½ magnitude, while the primary star is of the
+third. When at their nearest, some observers have spoken of
+an “occultation” of one star by the other, but no real occultation
+ever takes place, the components never approaching
+<span class='pageno' id='Page_435'>435</span>within half a second of arc. The companion merely disappears
+owing to its faintness in telescopes of moderate power.
+An occultation of one component of a binary star by the other
+cannot take place except—as in the case of 42 Comæ—when
+the plane of the orbit passes through the earth.</p>
+
+<div id='i_436'  class='figcenter id001'>
+<img src='images/i_436.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 4.</span>—<i>Apparent Orbit of Zeta Herculis.</i> (From “Worlds of Space.”)</p>
+</div>
+</div>
+
+<p class='c007'>In the case of the binary star, Eta Coronæ Borealis, it was,
+some forty years ago, uncertain whether its period was 43 or
+66 years, but now that two complete revolutions have been
+performed since its discovery by Sir William Herschel in
+1781, the question has been finally decided in favour of the
+shorter period. Numerous orbits have been computed, but
+these by Dr. Doberck and Dr. Dunér are probably the best.
+Those give a period of about 41½ years. The components
+are nearly equal in brightness, but at their present distance
+are not within the reach of small telescopes.</p>
+
+<p class='c007'>The brilliant star Sirius is also an interesting binary star.
+The companion, which is relatively very faint—about tenth
+magnitude,—was discovered by Alvan Clark in 1862. The
+existence of some such disturbing body was previously
+suspected by astronomers, owing to observed irregularities in
+the proper motion of Sirius. Several orbits, giving periods of
+about 50 years, have been computed. Some measures in
+recent years, however, seemed to show that this period was
+somewhat too short, but a period of about 58½ years, computed
+by the present writer in 1889, will probably prove too
+long. Some few years ago, Burnham found the companion
+an easy object with the 36-inch refractor of the Lick Observatory,
+but towards the end of the year 1890 it passed beyond
+the power of even this giant telescope. It will probably,
+however, emerge very soon now from the rays of its brilliant
+primary.<a id='r110'></a><a href='#f110' class='c027'><sup>[110]</sup></a> Burnham finds a period of about 52 years, but the
+German astronomer, Auwers, who has carefully investigated
+the observed irregularities in the proper motion of Sirius,
+adheres to a period of about 49½ years. The great brilliancy
+of Sirius, the brightest star in the heavens, naturally suggests
+a sun of great size. Recent investigations, however, do not
+<span class='pageno' id='Page_436'>436</span>favour this idea. Assuming a parallax of 0″·39 (about a mean
+of the results found by Elkin and Gill), Auwers finds the mass
+of the system to be about three times the mass of the sun, the
+mass of the companion being about equal to the sun’s mass.
+Placed at the distance of Sirius, the sun would, I find, be
+reduced to a star of about 1½ magnitude. As Sirius is about
+1 magnitude brighter than the zero magnitude—that is,
+about 2 magnitudes brighter than a standard star of the
+first magnitude—it follows that it is about 2½ magnitudes, or
+about ten times brighter than the sun would be in the same
+position. Its spectrum is, however, of the first type, and the
+star is therefore not comparable with the sun in brilliancy.
+The above result would indicate that stars of the first or
+Sirian type are intrinsically brighter than our sun.</p>
+
+<p class='c007'>Sirius is about 11 magnitudes brighter than its faint
+companion. This makes the light of Sirius about 25,000
+times the light of the small star. If, therefore, the two bodies
+were of the same intrinsic brilliancy, their diameters would be
+in the ratio of 158·5 to 1, and if of the same density, the mass
+of Sirius would be nearly five million times the mass of the
+companion! But, according to Auwers’ calculations, the companion’s
+mass is about one-half that of its primary. The two
+bodies must, therefore, be differently constituted, and, indeed,
+the companion must be nearly a dark body. It has been
+suggested that the companion may possibly shine by reflected
+light from Sirius; but this I have shown elsewhere to be quite
+impossible.<a id='r111'></a><a href='#f111' class='c027'><sup>[111]</sup></a> Even with a diameter equal to that of the sun,
+I find that with reflected light only it would be quite invisible
+in all parts of its orbit, even with the great Lick telescope. It
+must, therefore, shine with inherent light of its own, and it
+seems probable that it is a large body, cooling down and
+approaching the complete extinction of its light. If Sirius has
+any planets revolving round it—like those of our solar system—they
+must for ever remain invisible in our largest telescopes.
+This remark, of course, applies to all the fixed stars, single
+and double. They may possibly have attendant families of
+<span class='pageno' id='Page_437'>437</span>planets, like our sun, but if so, the fact can never be ascertained
+by direct observation. I find that the plane of the
+orbit of Sirius is at right angles to the general plane of the
+Milky Way.</p>
+
+<div id='i_439'  class='figcenter id001'>
+<img src='images/i_439.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 5.</span>—<i>Apparent Orbit of the Companion of Sirius.</i><br>  <br>  (From “Old and New Astronomy.”)</p>
+</div>
+</div>
+
+<p class='c007'>The star Zeta Cancri is a well-known triple star, the close
+pair revolving in a period of about 60 years. Nearly two
+revolutions have now been completed since its discovery by
+<span class='pageno' id='Page_438'>438</span>Sir William Herschel in 1781. All three stars probably form
+a connected system, but the motion of the third star round
+the binary pair is very slow and irregular. The motion of
+this interesting system has recently been investigated by
+Professor Seeliger, and he comes to the conclusion that,
+to make the observations agree with calculation, it is
+necessary to assume that the third star is in reality a very
+close double, the components of which revolve round their
+centre of gravity in about 17½ years, and both round the
+known binary pair. If this be so, we have here a remarkable
+quadruple pair; but it must be added that all efforts with
+large telescopes to see the companion star double have failed,
+and that the existence of the fourth star rests only on theory.
+Burnham, in 1889, using a power of 1500, failed to see any
+other component.</p>
+
+<p class='c007'>Another interesting binary star is Xi Ursæ Majoris. As
+already stated, this was the first pair for which an orbit was
+computed. More than a complete revolution has now been
+performed since its discovery by Sir William Herschel in
+1780. The period has, therefore, been well determined, and
+seems to be about 60 years. Although the components are
+not near their maximum distance at present, they are still
+within the reach of moderate telescopes, the distance being
+about 1¾ seconds, and the magnitudes of the components, not
+very unequal, about 4 and 5.</p>
+
+<p class='c007'>The bright southern star, Alpha Centauri, the nearest of
+all the fixed stars to the earth, so far as is known at present,
+is also a remarkable binary star. It seems to have been first
+noticed as a double star by Richaud in 1690. Several orbits
+have been computed, ranging from about 75 to 88½ years, but
+recent calculations by Mr. A. W. Roberts and Dr. See make
+the period about 81 years, which agrees closely with Dr.
+Elkin’s period of 80⅓ years. Combining Dr. Gill’s parallax
+of 0″·76 with Elkin’s elements, I find the sum of the masses
+nearly twice the mass of our sun, and the mean distance between
+the components about 23 times the earth’s distance from
+the sun, or somewhat greater than the distance between the sun
+<span class='pageno' id='Page_439'>439</span>and Uranus. Dr. Doberck finds a period of about 79 years,
+and assuming a parallax of 0″·75, he finds the mean distance
+between the components 24·6 times the earth’s distance from
+the sun; and he points out that if we suppose that their
+diameter does not differ much from that of our sun, each component
+“would appear from the other as a mere star to unaided
+vision, the distance being too great to show a disc.”<a id='r112'></a><a href='#f112' class='c027'><sup>[112]</sup></a>
+From a recent investigation of the proper motion and position
+of Alpha Centauri, Mr. A. W. Roberts finds that the
+masses of the components are nearly equal, and the combined
+mass equal to twice the mass of our sun, a conclusion in close
+agreement with the result found above from the orbit. According
+to Dr. Gill, the difference in brightness of the two
+components is 1·25 magnitude, and Professor Bailey makes
+their photometric magnitudes 0·50 and 1·75. As this difference
+would make the brighter component over three times
+brighter than the companion, it follows that its surface must
+be much brighter, and Mr. Roberts concludes that the companion
+has proceeded “some distance on the down track from a
+sun to an ordinary planet.” Assuming my value of the sun’s
+stellar magnitude (about 27), I find that the sun, if placed at
+the distance of Alpha Centauri, would appear of about the
+same brightness as the star does to us. As, according to
+Professor Pickering, the spectrum of Alpha Centauri is of the
+second or solar type, it would seem that in mass, brightness,
+and physical condition, the star closely resembles our sun.</p>
+
+<p class='c007'>We next come to another very interesting binary star,
+known to astronomers as 70 Ophiuchi. It is a very fine double
+star, the magnitudes of the components being about 4 and 6,
+and the colours yellow and orange. More than a complete
+revolution has now been described by the components since
+its discovery by Sir William Herschel in 1779. Numerous
+orbits have been computed with periods ranging from 73¾ to
+98 years. An orbit computed by the present writer, in 1888,
+gave a period of 87·84 years, and this was confirmed in 1894
+by Burnham, who found a period of 87·85 years. A subsequent
+<span class='pageno' id='Page_440'>440</span>investigation by Schur gives a period of 88·356 years.
+My orbit, combined with Krüger’s parallax of 0″·162, give for
+the combined mass of the components 2·777 times the mass
+of the sun, and the distance between them 27·777 times the
+earth’s distance from the sun, or somewhat less than the distance
+of Neptune from the sun. Schur has, however, recently
+found a parallax of 0″·286, which would reduce the mass of
+the system, and also the distance between the components.
+Recent observations show that the companion is now in
+advance of the theoretical position indicated by Schur’s orbit,
+and Dr. See thinks that the observed irregularities in the orbital
+motion of the pair indicate the existence of a third body, and
+that either the primary star or the companion, probably the
+latter, is a very close binary star. Careful search, however,
+for a third body, made with large telescopes, have failed to reveal
+its existence, and so the matter remains in suspense.
+Placed at the distance indicated by Krüger’s parallax, I find
+that our sun would be reduced to a star of about magnitude
+3½, which shows that the sun and star are of about equal
+brightness. The spectrum is of the solar type, according to
+Vogel. I find that the plane of the orbit is at right angles to
+the plane of the Milky Way.</p>
+
+<p class='c007'>The star Gamma, in Corona Borealis, is a close and difficult
+binary star. Dr. Doberck finds a period of 95½ years, and
+Celoria about 85¼. As in the case of 42 Comæ, the plane of
+the orbit nearly passes through the earth, and the apparent
+orbit is, consequently, nearly a straight line. I find that the
+plane of the orbit is at right angles to the plane of the Milky
+Way.</p>
+
+<p class='c007'>The star Xi Scorpii is a remarkable triple star, like Zeta
+Cancri, the magnitudes of the components being about 4½, 5,
+and 7½. The components of the close pair have described a
+complete revolution since their discovery by Sir William
+Herschel in 1780. Dr. Doberck finds a period of about 96
+years, and Schorr 105 years. The real orbit is nearly circular,
+but owing to its high inclination, about 70°, the apparent
+orbit is a very elongated ellipse. All three stars have
+<span class='pageno' id='Page_441'>441</span>a common proper motion through space, and, probably,
+form one system, but the motion of the third star is very
+slow, and its period of revolution must be several hundred
+years.</p>
+
+<div id='i_443'  class='figcenter id003'>
+<img src='images/i_443.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p>APPARENT ORBIT OF 70 OPHIUCHI, COMPUTED BY J. E. GORE (1888).<br>  <br>  (<i>Showing positions of companion star in<br>  different years.</i>)<br>  <br>  (From “The Scenery of the Heavens.”)</p>
+</div>
+</div>
+
+<p class='c007'><span class='pageno' id='Page_442'>442</span>The star ο<sup>2</sup>, or 40 Eridani, is another interesting object. It
+is a star of about 4½ magnitude, with a distant ninth magnitude
+companion, which is a double and binary star. It is sometimes
+stated that the bright star is the binary, but this is quite incorrect;
+the large star is single—at least, as far as is known at
+present. An orbit for the binary pair was computed, in 1886,
+by the present writer, who found a period of 139 years; but
+Burnham, using later observations, finds a period of 180 years.
+A physical connexion may possibly exist between the binary
+pair and the bright star, as both have the same common
+motion through space, but the angular motion, if any, is very
+slow. Professor Asaph Hall found a parallax of about one-fifth
+of a second of arc, and this, combined with Burnham’s
+orbit, gives the combined mass of the binary pair about two-thirds
+of the sun’s mass, a result which seems remarkable, for
+the sun, placed at the distance indicated by Hall’s parallax
+would, I find, shine as a star of about the third magnitude, or
+considerably brighter than the principal star of 40 Eridani.
+Owing to the faintness of the binary pair, the nature of its
+spectrum has not been determined. Computed by a well-known
+formula, its “relative brightness”—that is, its brightness
+compared with that of other binaries—is very small.</p>
+
+<p class='c007'>A very famous binary star is that known to astronomers as
+Gamma Virginis. Its history is a very interesting one. It
+lies close to the celestial equator, about one degree to the
+south and about fifteen degrees to the north-west of the bright
+star Spica (Alpha of the same constellation), with which it
+forms the stem of a Y-shaped figure, formed by the brightest
+stars of the constellation Virgo, or the Virgin, Gamma being
+at the junction of the two upper branches. The brightness of
+Gamma Virginis is a little greater than an average star of the
+third magnitude. Photometric measures made at Oxford and
+Harvard Observatories agree closely, and make its brightness
+about 2·7 magnitude—that is to say, rather nearer the third
+than the second magnitude. Variation of light has, however,
+been suspected in one or both components, and this question
+of light variation will be considered further on. The Persian
+<span class='pageno' id='Page_443'>443</span>astronomer, Al-Sûfi, in his description of the heavens, written
+in the tenth century, rates it of the third magnitude, and
+describes it as “the third of the stars of <i>al-auvâ</i>, which is a
+mansion of the moon,” the first and second stars of this
+“mansion” being Beta and Eta Virginis, the fourth star
+Delta, and the fifth Epsilon, these five stars forming the two
+upper branches of the Y-shaped figure above referred to.
+Gamma was called <i>Zawiyah-al-auvâ</i>, “the corner of the
+barkers!” perhaps from its position in the figure, which
+formed the thirteenth Lunar Mansion of the old astrologers.
+It was also called <i>Porrima</i> and <i>Postvarta</i> in the old calendars.
+These ancient names of the stars are curious, and their origin
+doubtful.</p>
+
+<p class='c007'>The fact that Gamma Virginis really consists of two stars
+very close together seems to have been discovered by the
+famous astronomer, Bradley, in 1718. He recorded the position
+of the components by stating that the line joining them
+was then exactly parallel to a line joining Alpha and Delta of
+the same constellation. This was, of course, only a rough
+method of measurement, and the position thus found by
+Bradley being probably more or less erroneous, has given
+much trouble to computers of the orbit described by the component
+stars round each other, or, rather, round their common
+centre of gravity. Bradley does not give the apparent distance
+between the component stars; but we may conclude
+from the orbit, which is now well determined, that they were
+then at nearly their greatest possible distance apart. It is curious
+that between Bradley’s time and 1794, the star was on several
+occasions occulted by the moon; but none of the observers
+refer to its duplicity. It was again measured by Cassini in
+1720, by Tobias Mayer in 1756, and by Sir William Herschel
+in 1780. These measures showed that the distance between
+the components was steadily diminishing, and that the position
+angle of the two stars was also decreasing. This decrease
+in the position angle—measured from the north round by
+the east, south, and west, from 0 to 360°—shows that
+the apparent orbital motion is what is called retrograde, or in
+<span class='pageno' id='Page_444'>444</span>the direction of the hands of a clock, direct or “planetary
+motion” being in the opposite direction. The star was again
+measured by Sir John Herschel and South in the years
+1822–38, by Struve in the same years, and by Dawes and
+other observers from 1831 to the present time. The recorded
+measures are very numerous, and have enabled computers to
+determine the orbit with considerable accuracy. The rapid
+decrease in the apparent distance from 1780–1834 indicated
+that the apparent orbit is very elongated, and that possibly
+the two stars might “close up” altogether, and appear as a
+single star even in telescopes of considerable power. This
+actually occurred in the year 1836, or, at least, the stars were
+then so close together that the most powerful telescopes of
+that day failed to show Gamma Virginis as anything but a
+single star. Of course, it would not have been beyond the
+reach of the giant telescopes of our day. From the year 1836
+the pair began to open out again, and at present the distance
+is again approaching a maximum. It is now within the reach
+of small telescopes, and forms a fine telescopic object with a
+moderate-sized instrument.</p>
+
+<p class='c007'>The general character of the orbital motion may be described
+as follows:—In 1718, at the time of Bradley’s observation,
+the companion star was to the north-west of the primary
+star; it then gradually moved towards the west and south,
+and in 1836, when at its minimum distance, it was to the
+south-east. From that date it again turned towards the
+north, and at present it is north-west of the primary star, and
+not far from the position found by Bradley in 1718.</p>
+
+<p class='c007'>The first to attempt a calculation of the orbit described by
+this remarkable pair of suns was Sir John Herschel, who in
+the year 1831 found a period of about 513 years. In 1833, he
+re-calculated the orbit, and found nearly 629 years. We now
+know that both these periods are much too long; but the
+data then available were insufficient for the calculation of an
+accurate orbit. From these results Herschel predicted that
+“the latter end of the year 1833, or the beginning of the year
+1834, will witness one of the most striking phenomena which
+<span class='pageno' id='Page_445'>445</span>sidereal astronomy has yet afforded, <i>viz.</i>, the perihelion passage
+of one star round another, with the immense angular velocity
+of between 60° and 70° per annum, that is to say, of a degree
+in five days. As the two stars will then, however, be within
+little more than half a second of each other, and as they are
+both large and nearly equal, none but the very finest telescopes
+will have any chance of showing this magnificent phenomenon.
+The prospect, however, of witnessing a visible and measurable
+change in the state of an object so remote, in a time so short,
+may reasonably be expected to call into action the most
+powerful instrumental means which can be brought to bear on
+it.” This prediction was not verified until the year 1836,
+when the pair “closed up out of all telescopic reach,” except
+at the Dorpat Observatory, where a magnifying power of 848
+still showed an elongation in the telescopic disc of the star.
+The orbit found by Sir John Herschel was a tolerably elongated
+ellipse, with its longer axis lying north-east and south-west.
+This was not quite correct, for we now know that this
+axis lies north-west and south-east, and that the apparent
+orbit is much more elongated than Sir John Herschel at first
+supposed. This was soon recognised by Herschel himself,
+and he came to the conclusion that he and other computers
+had been misled by Bradley’s observation in 1718. He then
+rejected this early, and evidently faulty, observation, and
+using the measures up to 1845, he found a period of about
+182 years, which we now know to be near the truth. The
+orbit was also computed by the famous German astronomer,
+Mädler, who found periods of 145, 157, and 169 years; by
+Hind, 141 years; by Henderson, 143 years; by Jacob, 133½,
+157½ and 171 years; by Adams, 174 years; by Flammarion,
+175 years; and by Admiral Smyth, 148 and 178 years. All
+these periods, we now know, are too small. Fletcher found
+184½ years, and Thiele 185 years. Two orbits were computed
+by Dr. Doberck, in recent years, with periods of 180½ and 179½
+years; but very recently (1895) the orbit has been re-computed
+by Dr. See, and he finds a period of 194 years. A comparison
+of the observed and computed positions shows, he thinks,
+<span class='pageno' id='Page_446'>446</span>that his elements are the most exact yet determined for any
+binary star.</p>
+
+<p class='c007'>The apparent orbit of the pair is a very elongated ellipse,
+and as Admiral Smyth said, “more like a comet’s than a
+planet’s.” The real ellipse has a very high eccentricity, nearly
+0·9—indeed, the greatest of all the known binary stars, and
+not much less than that of Halley’s comet</p>
+
+<p class='c007'>As I said above, the variability of the light of one or both
+components of Gamma Virginis has been strongly suspected.
+So far back as 1851 and 1852, O. Struve paid particular attention
+to this point. His observations in these years show that
+sometimes the component stars were exactly equal in brilliancy,
+and sometimes the southern star—the one generally
+taken as the primary—was from 0·2 to 0·7 magnitude brighter
+than the other. There seems to be little doubt that some
+variation really takes place in the relative brightness of the
+pair. This is clearly indicated by the measures of position
+angle. For example, in the year 1886, Professor Hall recorded
+the position as 154·9, evidently measuring from the northern
+star as the brightest of the two; while, in 1887, Schiaparelli
+gives 334°·2—or about 180° more—thus indicating that he
+considered the <i>southern</i> star as the primary, or brighter, of the
+pair. Burnham found 153°·4 in 1889, and Dr. See 332°·50 in
+1891. This is also shown by earlier measures, for Otto Struve
+found the southern star half a magnitude brighter than the
+other on April 3, 1852, while on April 29 of the same year he
+found them “perfectly equal.” He thought the variation was
+about 0·7 of a magnitude, but that the climate of Poulkova,
+where he observed, was not suitable for such observations.
+This variation is very interesting, and the question should be
+thoroughly investigated with a good telescope.</p>
+
+<p class='c007'>As the distance of Gamma Virginis from the earth has not
+been determined, it is not possible to calculate the actual
+dimensions of the orbit and the mass of the system. If we
+assume that the combined mass of the components is equal
+to the sun’s mass, I find from Dr. See’s orbit that the “hypothetical
+parallax” would be 0·119″, implying a distance of
+<span class='pageno' id='Page_447'>447</span>1,733,319 times the sun’s distance from the earth. If, however,
+we suppose that the mass of each of the components is
+equal to the sun’s mass, or the mass of the system double that
+of the sun—perhaps a more probable supposition—I find that
+the parallax would be about one-tenth of a second, denoting
+a distance of 2,062,650 times the sun’s distance from the earth.
+Placed at this last distance, the sun would, I find, be reduced
+to a star of about 4½ magnitude, or about 1¾ magnitudes
+fainter than Gamma Virginis appears to us. This difference
+implies that, supposing each of the component stars of the
+binary to have a mass equal to the sun’s mass, their combined
+light is about five times greater than the sun would emit if
+placed at the same distance, and as the components are nearly
+equal in brightness, each of them would be 2½ times brighter
+than the sun. According to Vogel, the star’s light gives a
+spectrum of the first or Sirian type, but according to the
+Draper “Catalogue of Stellar Spectra,” the spectrum is of the
+solar type. If the spectrum is of the first type, its brilliancy
+is easily explained; for, as I have shown elsewhere, the Sirian
+stars, are intrinsically much brighter in proportion to their mass
+than those of the solar type. But if its spectrum is of the
+solar type, it is not so easy to explain its brilliancy. Computing
+by a well-known formula, I find its relative brightness
+is nearly five times greater than that of Xi Ursæ Majoris, the
+spectrum of which is of the solar type. If, to account for its
+brilliancy, we assume that the star is nearer to the earth than
+the parallax assumed above would imply, then the mass of
+the system must be less than the mass of our sun. As we
+have seen above, doubling the supposed mass increased the
+distance; so, on the other hand, if we diminish the distance,
+we must diminish the mass also. Thus, if we reduce the
+distance to one-half, we must reduce the mass to one-eighth of
+the sun’s mass. A distance of one-third would give a mass
+of ¹⁄₂₇th, and a distance of one-fourth would imply a mass only
+¹⁄₆₄th of the sun’s mass. To reduce the sun to the same brightness
+as Gamma Virginis, it should be removed to a distance
+indicated by a parallax of one-tenth of a second multiplied
+<span class='pageno' id='Page_448'>448</span>by the square root of five, or 0·223″. If, however, the
+star’s parallax were so much as this, it is probable that it
+would have been detected and measured long ago. In the
+case of the binary star Castor, I find from the orbit and a small
+parallax found by Johnson (about one-fifth of a second) that
+its mass is only ¹⁄₁₉th of the sun’s mass, but in this case the
+spectrum is of the Sirian type, and stars of this type are very
+bright in proportion to their mass. The colours of the components
+of Gamma Virginis, which are very similar to those of
+Castor—white or pale yellow—would suggest that they may
+belong to the same type.</p>
+
+<p class='c007'>Another interesting binary star is Eta Cassiopeiæ. The
+components are about 4 and 7½ magnitude, and the pair have
+described a considerable portion of their orbit since its discovery
+in 1779 by Sir William Herschel, the distance diminishing
+from about 11 seconds to 4¾. Periods ranging from 149
+to 222½ years have been found by different computers. The
+most recent computation makes it about 196 years. Assuming
+a parallax of 0·154″ found by Struve, the mass of the
+system will be from 5¾ to 10¾ times the mass of the sun,
+according to the length of the period we assume. A much
+larger parallax of 0″·3743 was, however, found by Schweizer
+and Socoloff, which would considerably reduce the mass, and
+recently a still larger parallax of 0″·465 has been found by
+photography, which, with Grüber’s elements of the orbit,
+would reduce the mass of the system to ⅙th of that of the sun.</p>
+
+<p class='c007'>The bright star Gamma Leonis, situated in the well-known
+“Sickle in Leo,” is also a binary star, but only a small portion
+of the orbit has been described since its discovery by Sir
+William Herschel in 1782. Dr. Doberck finds a period of 407
+years. It is remarkable for its very high “relative brightness,”
+which is curious, as its spectrum is of the solar type. This
+pair forms a fine object for a small telescope.</p>
+
+<p class='c007'>The star known as 12 Lyncis is a triple star, the components
+being 5, 6, and 7½ magnitude. The close pair form a
+binary system, for which an orbit has been computed by the
+present writer, who finds a period of about 486 years. Sir
+<span class='pageno' id='Page_449'>449</span>John Herschel predicted in 1823 that the angular motion of
+the pair would “bring the three stars into a straight line in 57
+years.” This prediction was fulfilled in 1887, when measures
+by Tarrant showed that the stars were then exactly in a
+straight line.</p>
+
+<div id='i_451'  class='figcenter id003'>
+<img src='images/i_451.jpg' alt='12 Lyncis (1887). ζ Cancri (1886).' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 7.</span>—<i>Triple Stars.</i><br>  <br>  (From “Scenery of the Heavens.”)</p>
+</div>
+</div>
+
+<p class='c007'>The bright star Castor is a famous double star, and has been
+known since the year 1718, when it was observed by Bradley
+and Pond. It was also observed by Maskelyne in 1759, and
+frequently by Sir William Herschel from 1799 to 1803. Numerous
+orbits have been computed, with periods ranging from
+199 years by Mädler, and 1,001 years by Doberck. Wilson
+found a period of about 983 years, and Thiele about 997
+years, so that the longest period would seem to be nearest the
+truth. According to a somewhat doubtful parallax found by
+Johnson, the distance of Castor from the earth is about double
+that of Sirius. With this distance, and Doberck’s elements of
+the orbit, I find that the mass of the system of Castor is only
+¹⁄₁₉th of the sun’s mass, a result which would imply that the
+components are masses of glowing gas! The spectrum of
+Sirius is of the first, or Sirian, type, another example of the
+great brilliancy of stars of this type. Quite recently (1896),
+Dr. Bélopolsky has found, with the spectroscope, that the
+brighter component is a close binary star with a dark companion,
+like Algol. The period of revolution is about 3 days,
+and the relative orbital velocity about 20¾ miles a second.
+Dr. Bélopolsky’s observations show that the system is receding
+<span class='pageno' id='Page_450'>450</span>from the earth at the rate of about 4½ miles per second.
+Assuming the bright and dark companion to be of equal
+mass, and hence the absolute orbital velocity of each one half
+the relative velocity found by Bélopolsky, I find that, if the
+orbit is circular, the distance between the components is about
+85,400 miles, or slightly less than the sun’s diameter, and their
+combined mass about ¹⁄₈₇th of the sun’s mass. This result
+would imply a still smaller mass for the whole system of
+Castor than that found from the orbit of the two bright components,
+but tends strongly to confirm the opinion already
+expressed, that the components of this remarkable system are
+merely masses of glowing gas. Assuming that all three components
+are of equal mass, the combined mass of the system
+would be ¹⁄₅₈th of the sun’s mass. From this result we can
+easily compute the stars’ parallax, which, from Dr. Doberck’s
+orbit, I find to be 0″·2873, a quantity which might be measured
+by the photographic method.</p>
+
+<p class='c007'>With reference to the colours of the components of binary
+stars, the following relation between colour and relative
+brightness has been established<a id='r113'></a><a href='#f113' class='c027'><sup>[113]</sup></a>:—</p>
+
+<p class='c007'>(1.) When the magnitudes of the components are equal, or
+approaching equality, the colours are generally the same, or
+similar.</p>
+
+<p class='c007'>(2.) When the magnitudes of the components differ considerably,
+there is also a considerable difference in colour.</p>
+
+<p class='c007'>A new class of binary stars has been discovered within the
+last few years by means of the spectroscope. These have
+been called “spectroscopic binaries,” and the brighter component
+of Castor, referred to above, is an example of the class.
+They are supposed to consist of two component stars, so close
+together that the highest powers of the largest telescopes fail
+to show them as anything but single stars. Indeed, the
+velocities indicated by the spectroscope show that they must
+be so close that the components must for ever remain invisible
+by the most powerful telescopes which could ever be constructed
+by man. In some of these remarkable objects, the
+<span class='pageno' id='Page_451'>451</span>doubling of the spectral lines indicates that the components
+are both bright bodies, but in others, as in Algol, the lines are
+merely shifted from their normal position, not doubled, thus
+denoting that one of the components is a dark body. In
+either case, the motion in the line of sight can be measured by
+the spectroscope, and we can, therefore, calculate the actual
+dimensions of the system in miles, and thence its mass in
+terms of the sun’s mass, although the star’s distance from the
+earth remains unknown. Judging, however, from the brightness
+of the star, and the character of its spectrum, we can
+make an estimate of its probable distance from the earth.</p>
+
+<p class='c007'>Let us first take the case of Algol. This famous variable
+star has, according to the Draper catalogue, a spectrum of the
+Sirian type. It may, therefore, be comparable with that
+brilliant star in intrinsic brightness and density. Assuming
+the mass of Sirius at 2·20 times the mass of the sun, as found
+by Auwers, and that of the brighter component of Algol at
+four-ninths of the sun’s mass, as given by Vogel,<a id='r114'></a><a href='#f114' class='c027'><sup>[114]</sup></a> I find that
+for the <i>same distance</i> Sirius would be about 2·8 times brighter
+than Algol. But photometric measures show that Sirius is
+about 22 times brighter than Algol, from which it follows—since
+light varies inversely as the square of the distance—that
+Algol is 2·77 times further from the earth. Assuming the
+parallax of Sirius at 0·39″, this would give for the parallax of
+Algol O·14″, or a journey for light of about 23 years. From
+the dimensions of the system, as given by Vogel—about
+3,230,000 miles from centre to centre of the components—this
+parallax would give an apparent distance between the components
+of less than ¹⁄₂₀₀th of a second, a quantity much too
+small to be visible in our largest telescopes, or probably in
+any telescope which man can ever construct From a consideration
+of irregularities in the proper motion of Algol and in
+the period of its light changes, Dr. Chandler infers the existence
+of a third dark body and a parallax of 0·07″. As this
+is exactly one-half the parallax found above, it implies a distance
+just double of what I have found, and would, of course,
+<span class='pageno' id='Page_452'>452</span>indicate that Algol is intrinsically four times brighter than
+Sirius. This greater brilliancy would suggest greater heat, and
+would agree with its small density, which, from its diameter,
+as given by Vogel—1,061,000 miles—I find to be only one-third
+of that of water.</p>
+
+<p class='c007'>Let us now consider the case of Beta Aurigæ, which spectroscopic
+observations show to be a close binary star with a period
+of about four days, and a distance between the components of
+about eight millions of miles. This period and distance imply
+that the mass of the system is about five times that of the
+sun. As in this case the spectral lines are doubled at regular
+intervals of two days, and not merely shifted, as in the case of
+Algol, we may conclude that both the components are bright
+bodies, and we may not be far wrong in supposing that they
+are of equal mass, each having 2½ times the mass of the sun.
+As the spectrum of Beta Aurigæ is of the same type as Sirius,
+we may compare it with that star, as we did in the case of
+Algol. Assuming the same density and intrinsic brightness
+for both Beta Aurigæ and Sirius, I find that Beta Aurigæ
+should be about twice as bright as Sirius. Now, according to
+the Oxford photometric measures, Sirius is 2·89 magnitudes,
+or 14·32 times brighter than Beta Aurigæ. Hence it follows
+that the distance of Beta Aurigæ should be about 5½ times
+greater than the distance of Sirius. Hence, assuming the
+parallax of Sirius at 0″·39, that of Beta Aurigæ should be
+about 0″·061. From actual measures of the parallax of Beta
+Aurigæ, made by the late Prof. Pritchard at Oxford, he found,
+from two companion stars, a mean parallax of 0″·062, a result
+in remarkably close agreement with that computed above from
+a consideration of the star’s mass and light, compared with
+that of Sirius. As the actual distance between the components
+of Beta Aurigæ is equal to the sun’s diameter
+divided by 11·625, we have the maximum angular separation
+between the components equal to 0″·062 divided by 11·625, or
+about ¹⁄₂₀₀th of a second, or nearly the same as in the case of
+Algol.</p>
+
+<p class='c007'>The bright star Spica has also been found by the spectroscope
+<span class='pageno' id='Page_453'>453</span>to be a close binary star. Vogel finds a period of four
+days with a distance between the components of about 6¼
+millions of miles, and assuming that the components have
+equal mass and are moving in a circular orbit, he finds the
+mass of the system about 2·6 times the mass of our sun. This
+would give each of the components 1·3 times the mass of the
+sun, and it follows that the light of Spica—which gives a
+spectrum of the Sirian type—should, for equal distances,
+exceed that of Sirius about 1·4 times. Now, the photometric
+measures at Oxford show that Sirius is 1·91 magnitude, or
+5·8 times brighter than Spica. Hence it follows that the distance
+of Spica should be 2·85 times the distance of Sirius.
+This would make the parallax of Spica about 0″·137. So far
+as I know, a measurable parallax has not yet been found for
+this star. Brioschi, in 1819–20, observing with a vertical circle
+of four inches aperture, found a negative parallax, which would
+imply that its parallax is too small to be measurable. Still,
+the above result would seem to indicate that its parallax
+might be measurable by the photographic method. The
+parallax found above would imply that the maximum distance
+between the components of Spica would not exceed ⅒th
+of a second, a quantity much too small to be detected
+by the most powerful telescopes. In addition to its orbital
+motion, Vogel finds that Spica is approaching the sun at the
+rate of over 9 miles per second.</p>
+
+<p class='c007'>We now come to Zeta Ursæ Majoris (Mizar), which has
+also a spectrum of the Sirian type, and which the spectroscopic
+measures indicate is a close binary star with a period
+of about 104 days, and a combined mass equal to forty times
+the mass of the sun. Proceeding as before, we find that the
+light of Mizar should be about 8·7 times that of Sirius. But
+the photometric measures show that Sirius is about three
+magnitudes, or about sixteen times brighter than Mizar.
+Hence the distance of Mizar should be nearly twelve times
+the distance of Sirius. This gives for the parallax of Mizar
+about 0″·033. Klinkerfues found a parallax of 0″·0429 to
+0″·0477, which does not differ widely from the above result.
+<span class='pageno' id='Page_454'>454</span>As the velocity of the orbital motion shown by the spectroscope
+indicates a distance between the components of about
+143 millions of miles, or about the distance of Mars from the
+sun, it follows that the maximum distance between the components
+would be 0″032, multiplied by 1½ or 0″·048, a quantity
+beyond the reach of our present telescopes.</p>
+
+<p class='c007'>The well-known variable star, Delta Cephei, has recently
+been added to the list of “spectroscopic binaries.” From observations
+with the great 30-inch refractor of the Pulkowa
+Observatory in the summer of 1894, M. Bélopolsky finds that
+the star is probably a very close double, the companion being
+a nearly, or wholly, dark body, as in the case of Algol, and
+the orbit a very eccentric one. The observed variation of
+light indicates, however, that there is no eclipse, as occurs in
+Algol, so that the fluctuations in the light of Delta Cephei are
+probably due to some other cause. The spectrum of the star
+is of the solar type, so that in this respect it differs from the
+other spectroscopic binaries referred to above. The observations
+show that the system is approaching the sun at the
+rate of about 15 miles a second. Spectroscopic observations
+also suggest that the well-known variable star Beta Lyræ may
+also consist of two close companions. Further details respecting
+these observations will be given in the next chapter.</p>
+
+<p class='c007'>From a recent investigation of the proper motion of the
+star Tau Virginis, Dr. Fritz Cohen thinks it is probably a close
+binary, the companion star of which has not yet been detected.</p>
+
+<p class='c007'>It should be mentioned that in the case of Beta Aurigæ,
+Spica, Zeta Ursæ Majoris, and Castor, as there is no variation
+of light, as in Algol, the plane of the orbit is probably inclined
+to the line of sight. This would have the effect of increasing
+the computed mass of the system, and thus diminishing the
+calculated parallax. As the above calculations have been
+made on the assumption that the plane of the orbit passes
+through the earth, it follows that the computed parallax is a
+maximum, and that these remarkable objects may be really
+further from the earth than even the minute parallaxes found
+<span class='pageno' id='Page_455'>455</span>above would indicate. As the parallaxes of the nearest stars,
+such as Alpha Centauri, 61 Cygni, Sirius, and some other
+stars, are considerably greater than those found above, it
+would seem that our solar system is not situated in a region
+of binary stars, and that these wonderful objects lie beyond
+our immediate neighbourhood. It is also remarkable that, with
+the exception of Delta Cephei, they have all spectra of the
+Sirian type, including those Algol variables whose spectra have
+been examined.</p>
+
+<p class='c007'>By the aid of the parallaxes computed above, we can easily
+calculate the relative brightness of the sun compared with that
+of the spectroscopic binaries. Assuming that the sun is 27
+magnitudes brighter than the Zero magnitude, or 28 magnitudes
+brighter than a standard star of the first magnitude, and
+taking the parallax of Algol as 0″·07, I find that the sun,
+placed at the distance indicated by this parallax, would be reduced
+to a star of 5·35 magnitude, or about three magnitudes
+fainter than Algol, which implies that Algol is about 15½ times
+brighter than our sun. In the case of Beta Aurigæ, if the sun
+were placed at the distance indicated by the parallax of
+0″·061, it would be reduced to a star of 5·65 magnitude, or
+about 3·7 magnitudes fainter than Beta Aurigæ, which would
+imply that Beta Aurigæ is about thirty times brighter than
+the sun. In the case of Spica we have the sun reduced to a
+star of about the fourth magnitude, or about three magnitudes
+fainter than Spica, indicating that Spica is, like Algol, about
+15½ times brighter than the sun, although the mass of Spica
+is only 2·6 times the mass of the sun. Finally, in the case of
+Mizar, we have the sun reduced to a star of about the seventh, or
+about five magnitudes fainter than Mizar, indicating that
+Mizar is no less than one hundred times brighter than our sun.
+These results show the great relative brilliancy of stars with a
+Sirian spectrum, when compared with that of the sun, a consideration
+which has already been arrived at from other
+considerations.</p>
+
+<div>
+  <span class='pageno' id='Page_456'>456</span>
+  <h3 class='c004'>CHAPTER V.<br> <span class='c014'>VARIABLE AND TEMPORARY STARS.</span></h3>
+</div>
+
+<p class='c003'>To ordinary observers, the light of the stars seems to be constant.
+Even to those who are familiar with the constellations,
+the stars appear to maintain their relative brilliancy unchanged.
+To a great extent this is, of course, true; the great
+majority of the stars remaining of the same brightness from
+day to day, and from year to year. There are, however,
+numerous exceptions to this rule. Many of the stars, when
+carefully watched, are found to fluctuate in their light, being
+sometimes brighter, and sometimes fainter. These are known
+as “variable stars”—one of the most interesting class of
+objects in the heavens. Some of these have been known for
+a great number of years, and their variations having been
+carefully watched, the laws governing their light changes
+have been well determined.</p>
+
+<p class='c007'>We will first consider the variable stars with long periods
+of variation, as these generally show the largest fluctuations of
+light. Among these, the first star in which variation of light
+seems to have been noticed is the extraordinary object,
+Omicron Ceti, popularly known as Mira, or the “wonderful”
+star. It appears to have been first noticed by David Fabricius
+in the year 1596. He observed that the star now called
+Omicron, in the constellation Cetus, was of the third magnitude
+on April 13 of that year, and that in the following year
+it had disappeared. Bayer saw it again in 1603, when forming
+his maps of the constellations, and assigned to it the
+Greek letter Omicron, but does not seem to have noticed the
+fact that it was the same star which had been observed by
+Fabricius seven years previously. No further attention seems
+to have been paid to it until 1638 and 1639, when it was observed
+at Francker by Professor Phocylides Holwarda to be of
+the third magnitude in December, 1638, invisible in the following
+<span class='pageno' id='Page_457'>457</span>summer, and again visible in October, 1639. From 1648 to
+1662 it was carefully observed by Hevelius, and in subsequent
+years by several observers. Its variations are now regularly
+followed from year to year, and it forms one of the most
+interesting objects of its kind in the heavens. Its light varies
+from about the second magnitude to the ninth, but its brightness
+at maximum is variable to a considerable extent. Heis
+found its <i>average</i> brightness at maximum in the years 1840–58
+to be about the third magnitude, but on November 6, 1799,
+Sir William Herschel found it but little inferior to Aldebaran.
+On the other hand, at the maximum of 1868, November 7,
+Heis found it only of the fifth magnitude, and fainter than he
+had seen it for twenty-seven years. Sawyer also observed a
+maximum of about the fifth magnitude (4·9) on November 10,
+1887. M. Dumenel finds (1896) that in the last twelve periods
+the magnitude at maximum varied from 2·5 to 4·7.<a id='r115'></a><a href='#f115' class='c027'><sup>[115]</sup></a></p>
+
+<p class='c007'>It is stated in several books on astronomy, on the authority
+of Hevelius, that in the years 1672–76 Mira was invisible at
+the epoch of maximum. This is, however, quite a mistake,
+for it was long since (1837) pointed out by Bianchi that the
+supposed non-appearance of Mira in those years can be
+simply accounted for by the fact that the star was near the
+sun at the time of maxima, and could not be observed.
+If the star happens to be at a maximum in April or May, it
+will be too near the sun to be seen, and as the mean period is
+about 331 days, this occurs every ten years. For this reason
+the maxima seems to have passed unobserved in the years
+1852, 1853, and 1854, and again in 1883. The star will be
+very favourably placed for observation in the year 1897, and
+some following years. It has also been stated that Mira
+wholly disappears at the maximum, but this is another error,
+for the star never becomes fainter than 9½ magnitude at any
+time, and always remains visible in a 3-inch telescope. The
+colour of the star is decidedly reddish, but this hue seems to
+be more marked at minimum than at maximum. The
+spectrum is a remarkable one of the third type, in which
+<span class='pageno' id='Page_458'>458</span>bright lines have been seen by Espin, Maunder, and Secchi.
+At the minimum of February, 1896, the spectrum was photographed
+by Professor Wilsing, and he found it very similar to
+a photograph taken by Professor Pickering some years previously.
+The recent photograph shows the lines of hydrogen
+broad and bright. There seems to be no other bright lines
+except those of hydrogen. The blue end of the spectrum is
+very similar to that of our sun, but towards the red end
+there are “dark flutings, fading towards the red.” The
+bright hydrogen lines have only been seen at maximum, but
+the instruments used by Professor Wilsing were not sufficiently
+powerful to show whether they are also visible at
+minimum.<a id='r116'></a><a href='#f116' class='c027'><sup>[116]</sup></a> Professor Pickering thinks that “probably most
+of the stars of long period give a spectrum resembling that of
+ο Ceti, and having the hydrogen lines G, <i>h</i>, α, β, γ, and δ,
+bright about the time of maximum. When the photographic
+spectrum is faint, only the brighter lines, G and <i>h</i>, are visible.”
+Within the last few years, Mrs. Fleming, while examining the
+photographs of stellar spectra taken for the Henry Draper
+Memorial, has detected a number of variable stars of long
+period by the presence of bright lines in their spectra. These
+are mostly telescopic stars.</p>
+
+<p class='c007'>Although the average period of Mira is about 331 days, it
+is subject to marked irregularities, which Argelander has
+attempted to represent by an elaborate formula. In recent
+years, however, the epochs of maxima have deviated considerably
+from the dates computed from this formula, and at the
+maximum of February, 1896, the star did not reach its
+maximum light until nearly two months after the predicted
+time.</p>
+
+<p class='c007'>Perhaps the long period variable star next in order of
+interest—at least to observers in the Northern Hemisphere—is
+that known as Chi Cygni. It was discovered by Kirch in
+1686. A mistake is often made about the identity of this
+remarkable object It is sometimes confused with the neighbouring
+star, 17 Cygni of Flamsteed’s catalogue. At the
+<span class='pageno' id='Page_459'>459</span>time of Flamsteed’s observation, the variable star—which is the
+true Chi Cygni of Bayer’s map (made in 1603)—happened to
+be faint, and Flamsteed, not being able to find Bayer’s star,
+affixed the Greek letter χ to his No. 17. It was proposed by
+Struve to call Flamsteed’s star χ<sup>1</sup>, and the variable χ<sup>2</sup>; but
+there seems to be no necessity to perpetuate Flamsteed’s error,
+which has been frequently pointed out. All authorities on the
+variable stars now give this variable its proper designation—χ
+Cygni. The star varies at maximum from 4 to 6½ magnitude,
+and at the minimum it sinks to below the thirteenth magnitude.
+At some maxima, therefore, it is easily visible to the naked
+eye, and at others it is just below the limit of ordinary vision.
+At the maximum of 1847, it was visible to the naked eye for
+a period of 97 days. The average period is about 406 days;
+but, according to Schönfeld—a well-known authority on the
+variables—observations indicate a small lengthening of the
+period. Observations in recent years show that the minimum
+occurs about 185 days before the maximum. This gives 221
+days for the fall from maximum to minimum, and illustrates
+a feature common to many of the variable stars, namely, that
+the increase of light is more rapid than the decrease. This
+peculiarity is especially marked in the short period variables,
+which will be considered further on. Chi Cygni is said to be
+“strikingly variable in colour.” Espin’s observations in different
+years show it “sometimes quite red, at others only pale
+orange-red.” In the spectroscope, its light shows a splendid
+spectrum of the third type (or banded spectrum, very characteristic
+of these long period variables), in which bright lines
+were observed by Espin in May, 1889. One of these bright
+lines seems to be identical with the coronal line D<sub>3</sub>, the
+characteristic line of helium.</p>
+
+<p class='c007'>R Leonis is another remarkable variable star, which is
+sometimes visible to the naked eye at maximum. It lies
+closely south of the star known as 19 Leonis. It was discovered
+by Koch in 1782. At the maximum, its brightness
+varies from 5·2 to 7 magnitude, and at minimum it fades to
+about the tenth magnitude. The mean period is about 313
+<span class='pageno' id='Page_460'>460</span>days; but this is subject to some irregularities, and Chandler
+finds “good evidence of cyclical variation of period, with a
+long term.” The star is red in all phases of its light, and
+forms a fine telescopic object. Close to it are two small stars,
+which form, with the variable, an isosceles triangle. The
+spectrum is a fine one of the third type, a type very characteristic
+of these long period variables. Espin finds that the
+bright bands of the spectrum are brighter when the star is
+increasing in light, and fainter when decreasing. At the
+maximum of 1889, he found bright lines in its spectrum.</p>
+
+<p class='c007'>Another long period variable star which is visible to the
+naked eye at maximum is R Hydræ—the Upsilon Hydræ of
+Bayer—but it is rather too far south to be well observed in
+this country. Its variability was discerned by Maraldi in
+1704; but the star was also observed by Hevelius in 1672.
+Its light at maximum varies from 3½ to 5½ magnitude, and at
+minimum it fades to nearly the tenth magnitude. The period
+has diminished considerably since the year 1708, when it was
+about 500 days. This had decreased to about 487 days in
+1785, to 461 days in 1825, and to 437 days in 1870, and it
+seems to be still diminishing. Formulæ have been computed
+by Gould and Chandler, but do not agree. Schmidt found
+that the minimum occurs about 200 days before the maximum.
+The star is very reddish, and the spectrum is a fine one of the
+third type, which Dunér describes as of “extraordinary
+beauty,” the typical bands of this type of spectrum being very
+large, and perfectly black. At the maximum of 1889, Espin
+observed a bright line in its spectrum, and finds—as in R
+Leonis—that the bright bands are brighter when the star is
+increasing in light, and fainter as it decreases.</p>
+
+<p class='c007'>There is a very remarkable variable star in the Southern
+Hemisphere known as Eta Argûs. It lies in the midst of the
+great nebula in Argo, and the history of its fluctuations in
+light is very interesting. Observed by Halley in 1677 as a
+star of the fourth magnitude, it was seen of the second magnitude
+by Lacaille in 1751. After this, it must have again
+faded, for Burchell found it of only the fourth magnitude from
+<span class='pageno' id='Page_461'>461</span>1811 to 1815. From 1822 to 1826, it was again of the second
+magnitude, as observed by Fallows and Brisbane; but on
+Feb. 1, 1827, it was estimated of the first magnitude by
+Burchell. It then faded again, for on Feb. 29, 1828, Burchell
+found it of the second magnitude. From 1829 to 1833,
+Johnson and Taylor rated it of the second magnitude; and it
+was still of this magnitude, or a little brighter, when Sir John
+Herschel commenced his observations at the Cape of Good
+Hope in 1834. It does not seem to have varied much in
+brightness from that time until December, 1837, when
+Herschel was astonished to find its light “nearly tripled.”
+He says:<a id='r117'></a><a href='#f117' class='c027'><sup>[117]</sup></a> “It very decidedly surpassed Procyon, which was
+about the same altitude, and was far superior to Aldebaran.
+It exceeded α Orionis, and the only star (Sirius and Canopus
+excepted) which could at all be compared with it was Rigel,
+which, as I have already stated, it somewhat surpassed.”</p>
+
+<p class='c007'>From this time its light continued to increase. On the
+28th December it was far superior to Rigel, and could only be
+compared with α Centauri, which it equalled, having the
+advantage of altitude, but fell somewhat short of it as the
+altitudes approached equality. The maximum of brightness
+seems to have been obtained about the 2nd January, 1838, on
+which night, both stars being high and the sky clear and pure,
+it was judged to be very nearly matched, indeed, with α Centauri,
+sometimes the one, sometimes the other, being judged
+brighter; but, on the whole, a was considered to have some
+little superiority. After this, the light began to fade. Already
+on the 7th and 15th January, α Centauri was unhesitatingly
+placed above, and Rigel as unhesitatingly below, it. On the
+20th, it was “visibly diminished—now much less than
+α Centauri, and not <i>much</i> greater than Rigel. The change
+is palpable.” And on the 22nd, Arcturus (the nearest star in
+light and colour to α Centauri which the heavens afford),
+when only 10° high, surpassed η, the latter being on the
+meridian; η was still, however, superior to β Centauri,
+α Crucis, and Spica, and continued so (and even superior to
+<span class='pageno' id='Page_462'>462</span>Rigel) during the whole of February, nor was it until the
+14th April, 1838, that it had so far faded as to bear comparison
+with Aldebaran, though still somewhat brighter than that
+star. In 1843, it again increased in brightness, and in April
+of that year it was observed by Maclear to be brighter than
+Canopus, and nearly equal to Sirius! It then faded slightly,
+but seems to have remained nearly as bright as Canopus until
+February, 1850, since which time its brilliancy gradually decreased.
+It was still of the first magnitude in 1856, according
+to Abbott, but was rated a little below the second magnitude
+by Powell in 1858. Tebbutt found it of the third magnitude
+in 1860; Abbott a little below the fourth in 1861. Ellery
+rated it fifth magnitude in 1863, and Tebbutt sixth magnitude
+in 1867. In 1874 it was estimated 6·8 magnitude at Cordoba,
+and only 7·4 in November, 1878. Tebbutt’s observations from
+1877–86 show that it did not rise above the seventh magnitude
+in those years, and in March, 1886, it was rated 7·6 magnitude
+by Finlay at the Cape of Good Hope. This seems to have
+been the minimum of light, for in May, 1888, Tebbutt found
+that it “had increased fully half a magnitude” since April,
+1887, and might “be rated as a star of 7·0 magnitude.” From
+photometric measures made with the meridian photometer in
+Peru in the years 1889–91, Professor Bailey found its mean
+magnitude to be 6·32, so that probably the star is now slowly
+rising to another maximum. Bailey found the hydrogen lines
+Hβ, Hγ, and Hδ, bright in the spectrum of its light. Wolf
+suggested a period of 46 years, and Loomis, 67; but Schönfeld
+thought that a regular period is very improbable. The star
+is very reddish in colour.</p>
+
+<p class='c007'>There are many other variables of long period, but they are
+too numerous to be described in detail in a work of this character.
+Particulars respecting some of them will be found in
+“The Scenery of the Heavens,” by the present writer.</p>
+
+<p class='c007'>We will now consider the variables of short period, which are
+particularly interesting objects, owing to the comparative
+rapidity of their light changes. The periods vary in length
+from about 17¼ days down to a few hours. Perhaps the most
+<span class='pageno' id='Page_463'>463</span>interesting of these short period variables, at least to the amateur
+observer, is the star Beta Lyræ, which is easily visible to
+the naked eye in all phases of its light. It can be readily
+identified, as it is the nearest bright star to the south of the
+brilliant Vega, and one of two stars of nearly the same magnitude,
+the second being Gamma Lyræ. The variability of
+Beta Lyræ was discovered by Goodricke in the year 1784.
+The period is about 12 days, 21 hours, 46 minutes, 58 seconds.
+At maximum the star is about 3·4 magnitude, and there are
+two minima, one of magnitude 3·9, and the other—the chief
+minima—of 4·5 magnitude. That is, the star has at maximum
+2¾ times the light of the chief minimum, and 1·6 times the
+light of the secondary minimum. In other words, if we represent
+the light of the star at maximum by 27 candles, placed
+at a suitable distance from the eye, the secondary minimum
+will be represented by 17 candles, and the chief minimum by
+10 candles. These fluctuations, although not very great, can
+be easily recognised with the naked eye by comparison with
+the neighbouring star Gamma Lyræ. Professor Pickering
+thought that this variation in the light of Beta might be explained
+by supposing that the star rotated on its axis in the
+period indicated by the variation, that the ratio of the axis of
+the rotating spheroid is as 5 to 3, and that there is a darker
+portion at one of the ends, which is “symmetrically situated as
+regards the longer axis.” Recent observations with the spectroscope,
+however, render this explanation doubtful, and
+indicate rather that the star is a very close double or “spectroscopic
+binary,” although it does not seem certain that an
+actual eclipse of one component by the other takes place, as
+in the case of Algol. Bright lines were detected in the star’s
+spectrum by Secchi so far back as 1866. In 1883, M. Von
+Gothard noticed that the appearance of these bright lines
+varied in appearance, and from an examination of photographs
+taken at Harvard Observatory in 1891, Mrs. Fleming found
+displacements of bright and dark lines in a double spectrum,
+the period of which agreed fairly well with that of the star’s
+light changes. Professor Pickering thence concluded that the
+<span class='pageno' id='Page_464'>464</span>star consists of two components, one stellar and the other
+gaseous, but this conclusion has been somewhat modified by
+subsequent investigations. M. Bélopolsky, from photographs
+taken with the great 30-inch telescope at the Pulkowa Observatory,
+confirms the periodical displacement in the bright spectral
+lines “in a period identical with that of the star’s usual double
+fluctuation,” but Keeler and Vogel agree that the observed displacements
+are incompatible with the supposed occurrence of
+eclipses. Vogel, however, is “convinced that Beta Lyræ represents
+a binary or multiple system, the fundamental revolutions
+of which, in 12 days 22 hours, in some way control the light
+change, while the spectral variations, although intimately associated
+with the star’s phases, are subject, besides, to complicated
+disturbances running through a cycle perhaps measured by
+years.”<a id='r118'></a><a href='#f118' class='c027'><sup>[118]</sup></a> The helium line, D<sub>3</sub>, is visible in the spectrum.</p>
+
+<p class='c007'>Another interesting star of short period is Delta Cephei,
+which is one of three stars forming an isosceles triangle a little
+to the west of Cassiopeia’s Chair, the variable being at the
+vertex of the triangle, and the nearest of the three to Cassiopeia.
+Its variability was also discovered by Goodricke in
+1784. It varies from 3·7 to 4·9 magnitude, with a period of 5
+days, 8 hours, 47 minutes, 40 seconds. The amount of the
+variation is, therefore, the same as in the case of Algol, the
+star’s light at maximum being about three times its light at
+minimum. The period and light curve, however, show, according
+to Schönfeld, some irregularities, the computed times
+of maxima and minima being sometimes in error to the extent
+of over an hour. These are, however, small, and, on the
+whole, the star seems to be very uniform in its fluctuations.
+From seven years’ observations, Argelander found no deviation
+from perfect uniformity. The curve representing the
+light variations is not, however, very smooth, particularly
+during the decrease of light, when a nearly stationary period
+seems to occur from 16 to 24 hours after the maximum. The
+rise from minimum to maximum occupies about one-third of
+the period, another example of the feature so characteristic of
+<span class='pageno' id='Page_465'>465</span>variable stars, namely, that the increase of light is quicker than
+the decrease. As already stated (Chapter IV.), observations
+of the spectrum recently made by M. Bélopolsky, with the
+great Pulkowa telescope, show that, like Beta Lyræ, the star
+is probably a close binary, the period of the observed fluctuations
+in the positions of the spectral lines agreeing with that
+of the star’s light changes. In this case, however, the lines are
+not doubled, as in Beta Lyræ, but merely displaced from their
+normal position, indicating that, as in the case of Algol, one of
+the components is a dark body. There are, however, no indications
+that any eclipse of the bright star by its dark companion
+takes place. Indeed, the nature of the light changes, which are
+continuous and not confined to a few hours, as in Algol, are
+inconsistent with the occurrence of an eclipse. We must,
+therefore, conclude that the fluctuations of light are caused in
+some way by physical disturbances produced by the approach
+and recession of the two component bodies in an elliptic orbit
+round their centre of gravity. The observations indicate that
+the component stars, when furthest apart in their orbital revolution,
+are separated by a distance three times as great as
+when at their point of nearest approach. The observations
+also show that Delta Cephei is approaching the earth at the
+rate of about 8¾ miles a second. Its spectrum is of the second
+or solar type, differing in this respect from the other spectroscopic
+binaries, which show a spectrum of the first or Sirian
+type. The colour of the star is yellow, and it has a distant
+bluish companion of about the fifth magnitude, which may
+possibly have some physical connexion with the brighter
+star, as both stars have a common proper motion through
+space.</p>
+
+<p class='c007'>Another remarkable star of short period is Eta Aquilæ, the
+variability of which was discovered by Pigott in 1784. It
+varies from magnitude 3·5 to 4·7, with a period of 7 days, 4
+hours, 14 minutes, but Schönfeld found marked deviations
+from a uniform period. It will be seen that the amount of the
+light change, 1·2 magnitude, is the same as that of Delta
+Cephei. Its colour is yellow, and its spectrum, like that of
+<span class='pageno' id='Page_466'>466</span>Delta Cephei, of the second or solar type. The minimum
+takes place about three days before the maximum.</p>
+
+<p class='c007'>Zeta Geminorum is another variable star with a comparatively
+short period. It varies from about 3·7 to 4·5 magnitude,
+with a period of 10 days, 3 hours, 41½ minutes. Here the
+variation of light is only 0·8 of a magnitude, or, in other words,
+the light at maximum is about double the light of minimum,
+as in the case of the Algol type variable, Lambda Tauri. Its
+light curve, unlike that of Delta Cephei and Eta Aquilæ, is
+nearly symmetrical; that is, the period occupied in the increase
+of light is about the same as that of the decrease. Prof.
+Pickering thinks that Zeta Geminorum is possibly a “surface
+of revolution,” one side of the rotating star being about four-fifths
+of the brightness of the other; but Prof. Lockyer finds it
+to be a “spectroscopic binary,” like Beta Lyræ and Delta
+Cephei.</p>
+
+<p class='c007'>Among variables with very short periods may be mentioned
+the southern star R Muscæ, which is close to Alpha Muscæ.
+It varies from 6·6 to 7·4, and goes through all its changes in
+the short period of 21 hours 20 minutes. The minimum takes
+place about nine hours before the maximum. It was discovered
+at the Cordoba Observatory, and Dr. Gould remarks that “its
+average brightness is so near the limit of ordinary visibility
+in a clear sky at Cordoba, that the small regular fluctuations
+of light place it every few hours alternately within or beyond
+this limit.”</p>
+
+<p class='c007'>A remarkable variable star of short period was discovered
+in 1888 by Mr. Paul in the southern constellation Antlia. It
+varies from magnitude 6·7 to 7·3, with the wonderfully short
+period of 7 hours, 46 minutes, 48 seconds, all the light changes
+being gone through no less than three times in twenty-four
+hours! It was for some years believed that the variation was
+of the Algol type, but recent measures made at the Harvard
+College Observatory show that it belongs to the same class
+as Delta Cephei and Eta Aquilæ.</p>
+
+<p class='c007'>A telescopic variable with a wonderfully short period was
+discovered by Chandler in 1894. It lies a little to the west of
+<span class='pageno' id='Page_467'>467</span>the star Gamma Pegasi, and has been designated U Pegasi.
+It varies from magnitude 8·9 to 9·7, and was first supposed to
+be of the Algol type with a period of about two days, but
+further observations showed that the period was much shorter,
+and only 5 hours, 31 minutes, 9 seconds. The light curve is
+quite different from the Algol type, and also from that of Delta
+Cephei and other short period variables, the times of increase
+and decrease of light being about equal, as in the case of Zeta
+Geminorum. This fact, combined with the remarkable rapidity
+of its light changes, which are gone through four times in less
+than twenty-four hours, makes this remarkable star a most
+interesting object. Possibly there may be other stars in the
+heavens with a similar rapidity of variation which have
+hitherto escaped detection.</p>
+
+<p class='c007'>Several southern variables of short period have been discovered
+in recent years by Mr. A. W. Roberts at Lovedale in
+South Africa.</p>
+
+<p class='c007'>Unlike the variable stars of long period which seem
+scattered indifferently over the surface of the heavens, the
+great majority of the short period variables are found in a
+zone which nearly coincides with the course of the Milky
+Way. The most notable exceptions to this rule are W
+Virginis with the comparatively long period of 17¼ days, and
+U Pegasi, above described, which has the shortest known
+period of all the variable stars. Another peculiarity is that
+most of them are situated in what may be called the following
+hemisphere, that is between 12 hours and 24 hours of
+right ascension. The most remarkable exception to this rule
+is Zeta Geminorum. The above rules do not apply to
+variables of the Algol type, which we will now proceed to
+consider.</p>
+
+<p class='c007'>Algol, or Beta Persei, is a famous variable star, and the
+typical star of the class to which it belongs. Its name, Algol,
+is derived from a Persian word, meaning the “demon,” which
+suggests that the ancient astronomers may have detected
+some peculiarity in its behaviour. The real discovery of
+its variation was, however, made by Montanari in 1667, and
+<span class='pageno' id='Page_468'>468</span>his observations were confirmed by Maraldi in 1692. Its
+fluctuations of light were also noticed by Kirch and Palitzsch,
+but the true character of its variations was first determined by
+the English astronomer, Goodricke, in 1782. Its fluctuations
+of light are very curious and interesting. Shining with a constant,
+or nearly constant, brightness for a period of about 59
+hours as a star of a little less than the second magnitude, it
+suddenly begins to diminish in brightness, and in about 4½
+hours it is reduced to a star of about magnitude 3½. In other
+words, its light is reduced to about one-third of its normal
+brightness. If we suppose three candles placed side by side
+at such a distance that their combined light is merged into
+one, and equal to the usual brightness of Algol, then if two of
+these candles are extinguished, the remaining candle will
+represent the light of Algol at its minimum brilliancy. It is
+stated in several books on astronomy that Algol varies to the
+extent of two magnitudes, but this is quite incorrect, as a
+change of two magnitudes would imply that the light at
+maximum is over six times the light at minimum, which is
+more than double the star’s real variation. The star remains
+at its minimum, or faintest, for only about 15 minutes. It
+then begins to increase, and in about 5 hours recovers its
+normal brightness, all the light changes being gone through
+in a period of about 10 hours out of nearly 69 hours, which
+elapse between successive minima. These curious changes
+take place with great regularity, and the exact hour at which
+a minimum of light may be expected can be predicted with as
+much certainty as an eclipse of the sun.</p>
+
+<p class='c007'>Goodricke, comparing his own observations with one made
+by Flamsteed in the year 1696, found the period from minimum
+to minimum to be 2 days, 20 hours, 48 minutes, 59½
+seconds, and he came to the conclusion that the diminution
+in the light of the star is probably due to a partial eclipse by
+“a large body revolving round Algol.” This hypothesis was
+fully confirmed in the years 1888–89 by Professor Vogel with
+the spectroscope. As no close companion to Algol is visible
+in the largest telescopes, we must conclude that either the
+<span class='pageno' id='Page_469'>469</span>satellite is a dark body, or else so close to the primary that no
+telescope could show it. As has been stated in Chapter III.,
+the motion of a star in the line of sight can be ascertained by
+measuring displacements in the positions of the spectral lines.
+Now, if the diminution in Algol’s light is due to a dark body
+revolving round it, and periodically coming between us and
+the bright star, it follows that both components will be in
+motion, and both will revolve round the common centre of
+gravity of the pair. A little before a minimum of light takes
+place, the dark companion should therefore be approaching
+the eye, and, consequently, the bright companion will be
+receding. During the minimum there will be no apparent
+motion in the line of sight, as the motion of both bodies will
+be at right angles to the visual ray. After the minimum is
+over, the motion of the two bodies will be reversed, the bright
+one approaching the eye, and the dark one receding. Now,
+this is exactly what Vogel found. Before the diminution in
+the light of Algol begins, the spectroscope showed that the
+star is receding from the earth, and after the minimum, that it
+is approaching the eye. That the companion is dark and not
+bright, like the primary, is evident from the fact that the
+spectral lines are merely shifted from their normal position
+and not doubled, as would be the case were both components
+bright, as in the case of some of the “spectroscopic binaries”—for
+example, Beta Aurigæ—which has been considered in
+the chapter on binary stars (Chapter IV.). Vogel found that
+before the minimum of light, Algol is receding from the earth
+with the velocity of 24½ miles a second, and after the minimum
+it is approaching at the rate of 28½ miles a second. The
+difference between the observed velocities indicates that the
+system is approaching the earth with a velocity of about
+2 miles a second. Knowing, then, the orbital velocity, which
+is evidently about 26½ miles a second, and assuming the orbit
+to be circular, it is easy, with the observed period of revolution,
+or the period of light variation, to calculate the diameter of
+the orbit in miles, although the star’s distance from the earth
+remains unknown. Further, comparing its period of revolution
+<span class='pageno' id='Page_470'>470</span>and the dimensions of the orbit with that of the earth
+round the sun, it is easy to calculate, by Kepler’s third law of
+motion, the mass of the system in terms of the sun’s mass, and
+the probable size of the component bodies. Calculating in
+this way, Vogel computes that the diameter of Algol is about
+1,061,000 miles, and that of the dark companion 830,300 miles,
+with a distance between their centres of 3,230,000 miles, and
+a combined mass equal to two-thirds of the sun’s mass, the
+mass of Algol being four-ninths, and that of the companion
+two-ninths, of the mass of the sun. Taking the diameter of
+the sun as 866,000 miles, and its density as 1·44 (water being
+unity), I find that the above dimensions give a mean density
+for the components of Algol of about one-third that of water,
+so that the components are probably gaseous bodies, as
+Hall has already concluded.</p>
+
+<p class='c007'>From the recorded observations of minima in past years, it
+has been found that the period of variation of Algol’s light
+has been slowly diminishing since Goodricke’s time, and Dr.
+Chandler finds the present period is about 2 days, 20 hours,
+48 minutes, 51 seconds, or about 8½ seconds less than
+Goodricke made it. Chandler thinks that this variation in
+the length of the period is cyclical, and that it has now about
+reached its smallest value, and will soon begin to increase
+again. He believes that this variation is probably due to the
+orbital revolution of the pair round a third body in a period
+of about 130 years. M. Tisserand, however, explains the
+irregularities by supposing an elliptical orbit, and a slight
+flattening or polar compression in the primary star. Professor
+Boss is inclined to favour Chandler’s hypothesis.</p>
+
+<p class='c007'>It is a curious fact that Al-Sûfi, the Persian astronomer, in
+his “Description of the Heavens,” written in the tenth
+century, speaks distinctly of Algol as a red star (<i>étoile,
+brillant; d’un éclat, rouge</i>), while at present it is white, or at
+the most, of a yellow colour. A similar change of colour is
+supposed to have taken place in the case of Sirius, but the
+change in Algol seems more certain, as Al-Sûfi’s descriptions
+are generally most accurate and reliable.</p>
+
+<p class='c007'><span class='pageno' id='Page_471'>471</span>Stars of the Algol type of variable are very rare objects,
+only a dozen or so having been hitherto discovered in the
+whole heavens. Those visible to the naked eye, when at
+their normal brightness, are: Algol, Lambda Tauri, Delta
+Libræ, R Canis Majoris, and U Ophiuchi. The variation of
+Lambda Tauri was discovered by Baxendell in 1848. It
+varies from magnitude 3·4 to 4·2, and its period from minimum
+to minimum of light is about 3 days, 22 hours, 52
+minutes, 12 seconds. Its fluctuations have not been so
+well studied as those of Algol, but it is known that the
+“period is subject to marked inequalities,” sometimes amounting
+to 3 hours. The variation of light is less than that of
+Algol, the light at maximum being only twice the light at
+minimum. Two candles at a suitable distance would therefore
+represent the maximum light, and one candle the minimum
+brightness. All the light changes take place in a
+period of about 10 hours. The star is white like Algol.</p>
+
+<p class='c007'>The variability of Delta Libræ was discovered by Schmidt
+in 1859. It varies from magnitude 4·9 to 6·1, with a period
+of 2 days, 7 hours, 51 minutes, 22·8 seconds. The period is,
+however, according to Schönfeld, subject to some irregularities.
+The variation of light is about the same as that of
+Algol, the light at maximum being about three times the
+light at minimum. The variation takes about 12 hours, of
+which the decrease occupies 5½ hours. The star is white like
+Algol.</p>
+
+<p class='c007'>The variability of R Canis Majoris was detected by Sawyer
+in 1887. The variation is from 5·9 to 6·7 magnitude, or
+about equal in amount to that of Lambda Tauri, and the
+period 1 day, 3 hours, 15 minutes, 55 seconds.</p>
+
+<p class='c007'>U Ophiuchi was also discovered by Sawyer in 1881. Its
+variation is from magnitude 6·0 to 6·7, or slightly less than
+that of Lambda Tauri, and the period 20 hours, 7 minutes,
+41·6 seconds, but subject to an apparent diminution. The
+maximum brightness lasts for about 16 hours, and all the
+fluctuations of light take place in the short period of 4 hours.
+Its colour is white, like most stars of the Algol type.</p>
+
+<p class='c007'><span class='pageno' id='Page_472'>472</span>U Cephei is a very interesting variable of the Algol type,
+discovered by Ceraski in 1880. It varies from 7·1 to 9·5, with
+a period of 2 days, 11 hours, 49 minutes, 45 seconds. Here
+the variation of light is greater than that of Algol, the light
+at maximum being nearly seven times the light at minimum.
+Its rapidity of variation is very great, sometimes exceeding a
+magnitude in an hour. The light variations occupy about 6
+hours, and the minimum lasts for about an hour and a half,
+Professor Pickering thinks that the variation of light is, as in
+the case of Algol, caused by an eclipsing satellite, but that in
+this case the eclipse may possibly be total, the light at minimum
+being that due to the satellite, which may have some
+inherent light of its own. Lord Crawford examined the star
+with the spectroscope, and found that at the minimum the
+blue end of the spectroscope faded, and the red was intensified,
+which seems to suggest that the light of the star in that
+phase shines through a gaseous medium, and that the
+eclipsing body may be surrounded with an atmosphere.</p>
+
+<p class='c007'>Another interesting Algol variable is that known as Y
+Cygni, which was discovered by Chandler in 1886, while
+using it as a comparison star for the short period variable X
+Cygni. It varies from 7·1 to 7·9 magnitude, or about the
+same amount as Lambda Tauri, with a period of 1 day, 11
+hours, 56 minutes, 48 seconds. It has alternate bright and
+faint minima, which suggest, according to Dunér, that the star
+consists of two <i>bright</i> components, one of them being brighter
+than the other, and both revolving round their common centre
+of gravity in an elliptic orbit, with a period double that of the
+light variation. Yendell, who has carefully observed the star’s
+fluctuations, fully concurs in Dunér’s views, and says “the
+substantial corrections of his fundamental assumption appears
+to be proved beyond the possibility of a cavil.”</p>
+
+<p class='c007'>The variability of the star known as S Cancri was discovered
+by Hind in 1848. It varies from 8·2 to 9·8, or it is
+said, at some minima, to 11·7, with the comparatively long
+period of 9 days, 11 hours, 37 minutes, 45 seconds. The
+variations of light occupy about 21½ hours. If the minimum
+<span class='pageno' id='Page_473'>473</span>of 11·7 is correct, we have a variation of no less than 3½
+magnitudes, which implies that the normal light of the star is
+25 times its light at a faint minimum. If this be so, the
+eclipse must be nearly total. Argelander found that after the
+minimum the light increases very rapidly, and he thinks that
+the descent from the maximum is even more rapid.</p>
+
+<p class='c007'>Some interesting examples of the Algol type of variable
+have been discovered in recent years. One detected by
+Chandler, in 1894, and now known as Z Herculis, varies from
+about the seventh to the eighth magnitude, and has a period
+of 3 days, 23 hours, 48½ minutes. Faint and very bright
+minima alternate in periods of 47 and 49 hours, the ratios of
+the light at maximum and minima being 3, 2, and 1. These
+Professor Dunér considers, indicate that the star consists of
+two revolving components of equal size, one of which is twice
+as bright as the other, and he computes that the components
+revolve round their common centre of gravity in an elliptic
+orbit, the plane of which is in the line of sight, and the semi-axis
+major about six times the diameter of the stars. If
+we assume that the diameter of each component is equal to
+the diameter of our sun, I find, from the above <a id='t475'></a>data, that the
+combined mass of the system is about 1½ times the mass of
+the sun.</p>
+
+<p class='c007'>Another remarkable example of the Algol type was discovered
+by Miss Wells in 1895. The star lies a little north of the
+“Dolphin’s rhomb,” and at its normal brightness is about
+magnitude 9½. The period of variation is about four days.
+The variation somewhat resembles that of U Cephei. Professor
+Pickering says: “For nearly two hours before and after
+the minimum it is fainter than the twelfth magnitude. It is
+impossible at present to say how much fainter it becomes, or
+whether it disappears entirely. It increases at first very
+rapidly, and then more slowly, attaining its full brightness,
+magnitude 9·5, about five hours after the minimum. One
+hundred and thirty photographs indicate that, during the four
+days between the successive minima, it does not vary more
+than a few hundredths of a magnitude. The variation may
+<span class='pageno' id='Page_474'>474</span>be explained by assuming that the star revolves round a
+comparatively dark body, and is totally eclipsed by it for two
+or three hours, the light at minimum, if any, being entirely
+that of the dark body.”<a id='r119'></a><a href='#f119' class='c027'><sup>[119]</sup></a> This seems to be an unique object,
+and it should be carefully followed through its minimum with
+a large telescope.<a id='r120'></a><a href='#f120' class='c027'><sup>[120]</sup></a></p>
+
+<p class='c007'>With reference to the Algol type of variable stars, Chandler
+finds that “the shorter the period of the star, the higher the
+ratio which the time of oscillation bears to the entire period.”
+Thus, in U Ophiuchi, with a period of about 20 hours, the
+light changes occupy five hours, or one-fourth of the period,
+while in S Cancri, which has a period of 227½ hours, the
+fluctuations of light take up 21½ hours, or only about one-tenth
+of the period. In all cases in which the Algol type
+variables have been examined with the spectroscope, the
+spectrum has been found to be of the first or Sirian type, and
+they seem to be the only stars with spectra of the Sirian type
+whose light is variable. It should be noted, however, that, on
+the eclipse theory, the variation of light in these stars is due
+merely to an occultation of one star by another, and not to
+any physical change in the star itself. The bright star Spica,
+although shown by the spectroscope to be a close binary star,
+like Algol, is not variable, because, in this case, the plane of
+its orbit is inclined to the line of sight, and hence the comparison
+star does not transit the disc of its primary. Seen from
+some other point in space, it would probably be an Algol
+variable.</p>
+
+<p class='c007'>A remarkable peculiarity about the variable stars in general
+is that none of them have any considerable proper motion.
+As a large proper motion is generally considered to indicate
+proximity to the earth, we may conclude, with great probability,
+that the variable stars, as a rule, lie at a great distance
+from our system. In other words, it appears that the sun
+does not lie in a region of variable stars, and, with the exception
+<span class='pageno' id='Page_475'>475</span>of Alpha Cassiopeiæ and Alpha Herculis, a measurable
+parallax has not yet been found, so far as I know, for any
+known variable star.</p>
+
+<p class='c007'>Plotting the known variables on star charts, I find a marked
+tendency to cluster into groups. Thus, in and near the constellation,
+Corona Borealis, there are five; near Cassiopeia’s
+Chair, five. In Cancer there are four in a limited area. Near
+Eta Argûs there are several, and in a comparatively small
+region in the northern portion of Scorpio there are no less than
+fifteen variable stars.</p>
+
+<p class='c007'>We now come to the interesting and mysterious class of
+objects known as “new” or “temporary” stars. These
+phenomena are of very rare occurrence, and but few undoubted
+examples of the class are recorded in the annals of astronomy.
+Possibly in some cases they have been merely variable stars,
+of irregular period and fitful variability; but others may have
+been due to a real catastrophe, such as the collision of two
+dark bodies in space, or, possibly, the passage of a bright
+or dark body through a gaseous nebula.</p>
+
+<p class='c007'>The earliest temporary star of which we have any reliable
+information seems to be one which is recorded in the Chinese
+annals of Ma-tuan-lin, as having appeared in the year 134 <span class='fss'>B.C.</span>
+in the constellation Scorpio. Its position seems to have been
+somewhere between the stars Beta and Rho of Scorpio. Pliny
+informs us that it was the sudden appearance of a new star
+which induced the famous astronomer Hipparchus to form
+his catalogue of stars, the first ever constructed. As the date
+of Hipparchus’ catalogue is 125 <span class='fss'>B.C.</span>, it seems highly probable
+that the new star referred to by Pliny was the same as that
+recorded by the Chinese astronomer as having appeared nine
+years previously.</p>
+
+<p class='c007'>A new star is said to have appeared in the year 76 <span class='fss'>B.C.</span>
+between the stars Alpha and Delta in the Plough, but the
+accounts are vague.</p>
+
+<p class='c007'>In 101 <span class='fss'>A.D.</span>, a small “yellowish-blue” star is said to have
+appeared in the “sickle” in Leo, but its exact position is not
+known. In 107 <span class='fss'>A.D.</span>, a new star is mentioned near Delta,
+<span class='pageno' id='Page_476'>476</span>Epsilon and Eta in Canis Major, three bright stars south-east
+of Sirius. In 123 <span class='fss'>A.D.</span>, another new star is recorded by Ma-tuan-lin
+to have appeared between Alpha Herculis and Alpha
+Ophiuchi.</p>
+
+<p class='c007'>The Chinese annals record that on Dec. 10, 173 <span class='fss'>A.D.</span>, a
+brilliant star appeared between Alpha and Beta Centauri in
+the Southern Hemisphere. It remained visible for eight
+months, and is described as resembling “a large bamboo
+mat!”—a curious description. There is at present close to
+the spot indicated, a known variable star—R Centauri—of
+which the period seems to be long and the variation of light
+irregular. Possibly an unusually bright maximum of this
+variable star formed the star of the Chinese annals, or perhaps
+the variable star is the remnant of the outburst which took
+place in the first century. The variable is a very reddish star,
+and at present varies from about the sixth to the tenth
+magnitude</p>
+
+<p class='c007'>A new star is recorded in the year 386 <span class='fss'>A.D.</span> as having appeared
+between Lambda and Phi Sagittarii. Near the position
+indicated, Flamsteed observed a star, No. 65 of his catalogue,
+which is now missing; and it has been conjectured that the
+star seen by Flamsteed may possibly have been a return of
+the star mentioned in the Chinese annals.</p>
+
+<p class='c007'>Cuspianus relates that a star as bright as Venus appeared
+near Altair in 389 <span class='fss'>A.D.</span>, during the reign of the Emperor
+Honorius, and that he had himself seen it. There is some
+doubt, however, about the exact date, as other accounts give
+the year 388 or 398. The star seems to have disappeared in
+about three weeks.</p>
+
+<p class='c007'>In the year 393 <span class='fss'>A.D.</span>, another strange star is recorded in the
+tail of Scorpio. An extraordinary star is said to have been
+seen near Alpha Crateris in 561 <span class='fss'>A.D.</span> Here again a known
+variable and red star—R Crateris—is close to the position indicated
+by the ancient records.</p>
+
+<p class='c007'>The Chinese annals record a new star in 829 <span class='fss'>A.D.</span>, somewhere
+in the vicinity of the bright star Procyon, and in this
+locality there are several known variable stars.</p>
+
+<p class='c007'><span class='pageno' id='Page_477'>477</span>The Bohemian astronomer, Cyprianus Leoviticus, mentions
+the appearance of new stars in Cassiopeia in the years 945 <span class='fss'>A.D.</span>
+and 1264, and it has been conjectured that perhaps these were
+apparitions of Tycho Brahé’s famous star of 1572 (to be presently
+described), forming a variable star with a period of over
+300 years. Lynn and Sadler, however, have shown that the
+supposed stars of 945 and 1264 were, in all probability,
+comets.</p>
+
+<p class='c007'>Extraordinary stars are recorded near Zeta Sagittarii in 1011
+<span class='fss'>A.D.</span>, near Mu Scorpii in 1203, and near Pi Scorpii on July 1,
+1584. It is remarkable how many of these objects seem to
+have appeared in this portion of the heavens.</p>
+
+<p class='c007'>A very brilliant star is mentioned by Hepidannus as having
+appeared in Aries in May, 1012. He describes it as “dazzling
+the eye.” Other temporary stars are mentioned in 1054 <span class='fss'>A.D.</span>,
+near Zeta Tauri, and in 1139, near Kappa Virginis; but the
+accounts of these are very vague, and it seems by no means
+certain that they were really new stars.</p>
+
+<p class='c007'>No possible doubt, however, can be entertained with reference
+to the appearance of the object which suddenly
+blazed out in Cassiopeia’s Chair in November, 1572. It was
+called the “Pilgrim Star,” and was observed by the famous
+astronomer, Tycho Brahé, who has left us a very elaborate
+account of its appearance, position, etc. Although usually
+spoken of as Tycho Brahé’s star, it seems to have been really
+discovered by Cornelius Gemma on the evening of November
+9. That its appearance was very sudden may be inferred
+from Cornelius Gemma’s statement, that it was not visible on
+the preceding night in a clear sky. Tycho Brahé’s attention
+was first attracted to it on November 11. His description
+of the new star is as follows—as quoted by Humboldt:<a id='r121'></a><a href='#f121' class='c027'><sup>[121]</sup></a>—“On
+my return to the Danish islands from my travels in
+Germany, I resided for some time with my uncle, Steno Bille,
+in the old and pleasantly situated monastery of Herritzwadt,
+and here I made it a practice not to leave my chemical
+laboratory until the evening. Raising my eyes, as usual,
+<span class='pageno' id='Page_478'>478</span>during one of my walks, to the well-known vault of heaven, I
+observed with indescribable astonishment, near the zenith in
+Cassiopeia, a radiant fixed star of a magnitude never before
+seen. In my amazement, I doubted the evidence of my
+senses. However, to convince myself that it was no illusion,
+and to have the testimony of others, I summoned my assistants
+from the laboratory, and inquired of them, and of all the
+country people that passed by, if they also observed the
+star that had thus suddenly burst forth. I subsequently
+heard that in Germany, waggoners and other common people
+first called the attention of astronomers to this great
+phenomenon in the heavens—a circumstance which, as in the
+case of non-predicted comets, furnished fresh occasion for the
+usual raillery at the expense of the learned. This new star I
+found to be without a tail, not surrounded by any nebula,
+and perfectly like all other fixed stars, with the exception
+that it scintillated more strongly than stars of the first
+magnitude. Its brightness was greater than that of Sirius,
+α Lyræ, or Jupiter. For splendour, it was only comparable to
+Venus when nearest to the earth (that is, when only a quarter
+of her disc is illuminated). Those gifted with keen sight
+could, when the air was clear, discern the new star in the day-time,
+and even at noon. At night, when the sky was overcast,
+so that all other stars were hidden, it was often visible
+through the clouds, if they were not very dense (<i>nubes non
+admodum densas</i>). Its distances from the nearest stars of
+Cassiopeia, which throughout the whole of the following year
+I measured with great care, convinced me of its perfect
+immobility. Already, in December, 1572, its brilliancy began
+to diminish, and the star gradually resembled Jupiter, but by
+January, 1573, it had become less bright than that planet.
+Successive photometric estimates gave the following results:
+for February and March, equality with stars of the first
+magnitude (<i>stellarum affixarum primi honoris</i>—for Tycho
+Brahé seems to have disliked Manilius’ expression of <i>stellæ
+fixæ</i>); for April and May, with stars of the second magnitude;
+for July and August, with those of the third; for October
+<span class='pageno' id='Page_479'>479</span>and November, those of the fourth magnitude. Towards the
+month of November, the new star was not brighter than the
+eleventh in the lower part of Cassiopeia’s Chair. The transition
+to the fifth and sixth magnitude took place between
+December, 1573, and February, 1574. In the following
+month the new star disappeared, and, after having shone
+seventeen months, was no longer discernible to the naked
+eye.” (The telescope was not invented until thirty-seven
+years afterwards.) Humboldt adds:—“At its first appearance,
+as long as it had the brilliancy of Venus and Jupiter, it
+was for two months white, and then passed through yellow
+into red. In the spring of 1573, Tycho Brahé compared
+it to Mars; afterwards he thought it nearly resembled
+Betelgeuse, the star in the right shoulder of Orion. The
+colour for the most part was like the red tint of
+Aldebaran. In the spring of 1573, and especially in May, its
+white colour returned (<i>albedinam quandam sublividam induebat,
+qualis Saturni stellæ subesse videtur</i>). So it remained
+in January, 1574; being, up to the time of its entire disappearance
+in the month of March, 1574, of the fifth magnitude,
+and white, but of a duller whiteness, and exhibiting a
+remarkably strong scintillation in proportion to its faintness.”</p>
+
+<div id='i_481'  class='figcenter id001'>
+<img src='images/i_481.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 8.</span>—<i>The Temporary Star of 1572.</i><br>  <br>  (From “Planetary and Stellar Studies.”)</p>
+</div>
+</div>
+
+<p class='c007'><span class='pageno' id='Page_480'>480</span>According to a sketch of the position given in Tycho
+Brahé’s work, referred to above, the star was situated a little
+to the north of Kappa Cassiopeiæ, the faintest star in the
+Chair. This position is confirmed by Argelander’s examination
+of Tycho Brahé’s observations: The spot is a
+rather blank one to the naked eye, and even with an opera-glass,
+only a few faint stars are visible. Quite close to the
+place fixed by Argelander, d’Arrest observed in 1865 a star of
+the eleventh magnitude, which seems to have escaped Argelander’s
+notice. Hind and Plummer observed this small star
+in 1873, and thought they could detect fluctuations in its light
+to the extent of about one magnitude. Espin has also
+observed it, and the region has been photographed by Dr.
+Roberts. Some have thought that Tycho Brahé’s star might
+possibly be identical with the Star of Bethlehem, and this
+idea has been supported by Cardanus, Chladni, and Klinkerfues,
+but Lynn and Sadler have shown that the theory is quite
+untenable, and it has now been rejected by all astronomers.</p>
+
+<p class='c007'>Ma-tuan-lin speaks of a star in 1578 “as large as the sun”(!)
+but does not state its position.</p>
+
+<p class='c007'>The star known as P (34) Cygni is sometimes spoken of as a
+“Nova,” or new star; but it is still visible to the naked eye as
+a star of the fifth magnitude. It was observed of the third
+magnitude by Jansen in 1600 and by Kepler in 1602. After
+the year 1619, it appears to have diminished in brightness, and
+is said to have vanished in 1621; but it may merely have
+become too faint to be seen with the naked eye. It was again
+observed of the third magnitude by Dominique Cassini in
+1655, and it afterwards disappeared. It was again seen by
+Hevelius in November, 1665. In 1667, 1682, and 1715, it is
+recorded as of the sixth magnitude, and there is no further
+<span class='pageno' id='Page_481'>481</span>record of any marked increase in its light. A period of
+about 18 years was assumed by Pigott; but this is now disproved,
+and it seems probable that the star is a variable of
+irregular period and fitful variability, and not, properly speaking,
+a temporary star. Its present colour is yellow, and
+bright lines have been seen in its spectrum.</p>
+
+<p class='c007'>Another remarkable object of the temporary class was
+observed by Kepler in 1604 in Ophiuchus, and is described by
+him in his work, “De Stella Nova in pede Serpentarii.” He
+and his assistants were observing the planets Mars, Jupiter,
+and Saturn, which were then near each other in this region of
+the heavens, a few degrees to the south-east of the star Eta
+Ophiuchi, and on the evening of October 10, Brunowski, a
+pupil of Kepler’s, noticed that a new and very brilliant star
+was added to the group<a id='r122'></a><a href='#f122' class='c027'><sup>[122]</sup></a>. When first seen, it was white, and
+exceeded in brightness Mars and Jupiter, but seems not to
+have quite equalled Venus in brilliancy. It slowly diminished,
+and in January, 1605, it was brighter than Antares but less than
+Arcturus. At the end of March, 1605, it had faded to the third
+magnitude. Its proximity to the sun then prevented further
+observations for several months. In March, 1605, it had disappeared
+to the naked eye. It was also observed by Galileo
+and by David Fabricius, whose observations place it about midway
+between the fifth magnitude star Xi and 58 Ophiuchi. Its
+exact position, however, does not seem to be known with such
+accuracy as that of Tycho Brahé’s star, nor is there any known
+star very close to the spot indicated by Schönfeld from an
+examination of Fabricius’ observations. It seems possible
+that Kepler’s star may have been seen previously by Ptolemy,
+for in his catalogue he gives a star of the fourth magnitude
+close to the position of Kepler’s star; but there is some doubt
+about the exact position indicated by Ptolemy. The Chinese
+annals mention a “ball-like” star as having appeared near
+Pi Scorpii on September 30, 1604, and remaining visible until
+March, 1606, which may possibly be identical with Kepler’s
+star.</p>
+
+<p class='c007'><span class='pageno' id='Page_482'>482</span>A new star of the third magnitude was observed near Beta
+Cygni by the Carthusian monk Anthelmus in 1670. It remained
+visible for about two years, and is said to have
+increased and diminished several times before its final disappearance.
+Schönfeld computed its exact position from
+observations made by Hevelius and Picard. Quite close to
+the spot indicated, a star of the eleventh magnitude has been
+observed at the Greenwich Observatory, and fluctuations of
+light were suspected in this small star by Hind and others.
+Hind says that, to his eye, “there is a hazy, ill-defined
+appearance about it which is not perceptible in other stars in
+the same field of view. Mr. Talmage received the same
+impression; and I may add that Mr. Baxendell, who has
+examined it with Mr. Worthington’s reflector, observed that
+no adjustment of focus would bring the star up to a sharp
+focus.” This hazy appearance is very suggestive, as it indicates
+that the “Nova” may possibly have faded into a small
+planetary nebula, as in the case of the new star in Cygnus,
+observed by Schmidt in 1876, and the new star in Auriga,
+found by Dr. Anderson in 1892. Near the position of
+Anthelm’s new star is a known variable star, S Vulpeculæ,
+discovered by Hind in 1861, which might be suspected to be
+identical with Anthelm’s star; but Hind has shown that the
+variable has no proper motion which would account for the
+difference of position since 1670, and he concludes that, “from
+the fixity of its position during eight years, it may be inferred
+that the variable is distinct from Anthelm’s.” It has been
+supposed that the star 11 Vulpeculæ in Flamsteed’s catalogue
+is identical with Anthelm’s star; but Baily could not find any
+evidence to show that Flamsteed’s star ever really existed, and
+he says: “Under the presumption, however, that it may be a
+variable and not a <i>lost</i> star, I have preserved its recorded
+position with a view of inducing astronomers to look out for
+it from time to time.”</p>
+
+<p class='c007'>On the evening of April 28, 1848, Hind, observing at Mr.
+Bishop’s private observatory, in Regent’s Park, London,
+noticed a new star of about the fifth magnitude, between
+<span class='pageno' id='Page_483'>483</span>Zeta and Eta Ophiuchi. Its colour was reddish-yellow, and
+it seems to have subsequently increased in brightness to
+nearly the fourth magnitude, but it soon faded to the tenth
+or eleventh magnitude. This curious object has become very
+faint in recent years. In 1866, it was of the twelfth magnitude,
+and in 1874 and 1875, not above the thirteenth.</p>
+
+<p class='c007'>On May 28, 1860, Pogson discovered a new star in the
+globular cluster, 80 Messier, which lies between Antares and
+Beta Scorpii. When first noticed, it was about the seventh
+magnitude, and its brightness was sufficient to obscure the
+cluster. In other words, the cluster was apparently replaced
+by a star. On June 10, the star had nearly disappeared, and
+the cluster again shone with great brilliancy, and with a
+condensed centre. The observations of Auwers and Luther
+confirm those of Pogson. Pogson states that he examined
+the cluster on May 9, but noticed nothing peculiar; and,
+according to Schönfeld, the cluster presented its usual
+appearance on May 18, when examined at the Königsberg
+Observatory. The apparition of the temporary star was,
+therefore, probably sudden, as in the case of other “new”
+stars. The phenomenon was possibly caused by a collision
+between two of the stars composing the cluster, which is, at
+least, apparently very condensed.</p>
+
+<p class='c007'>A very remarkable star, sometimes called the “Blaze
+Star,” suddenly appeared in Corona Borealis, in May, 1866.
+It was first seen by the late Mr. Birmingham, at Tuam,
+Ireland, about midnight, on the evening of May 12, when it
+was of the second magnitude, and equal to Alphecca, “the gem
+of the coronet.” Its appearance must have been very sudden, for
+Schmidt, the Director of the Athens Observatory, stated that
+he was observing the constellation on the same evening, about
+2½ hours previous to Birmingham’s discovery, and observed
+nothing unusual. He was certain that no star, of even the
+fifth magnitude, could possibly have escaped his notice. On
+the following night it was seen by several observers in
+different parts of the world. M. Faye, the French astronomer,
+in his work—“L’Origine du Monde”—attributes the discovery
+<span class='pageno' id='Page_484'>484</span>to M. Courbebaisse, a French engineer, and does not mention
+Mr. Birmingham! He says M. Courbebaisse first saw it on
+the evening of May 13. This may be true; he was not the
+only observer who saw it on that evening; but it was,
+undoubtedly, <i>first</i> seen by Mr. Birmingham on the <i>preceding</i>
+night, and to Mr. Birmingham alone is certainly due the
+credit of the discovery. The star rapidly diminished in
+brightness, and on May 24 of the same year, had faded to
+8½ magnitude. It afterwards increased to about 7·8 magnitude,
+but soon diminished again. Soon after its discovery it was
+found that the star was not really a new one, as it had been
+previously observed at Bonn by Schönfeld, in May, 1855, and
+March, 1856, while making the observations for Argelander’s
+<cite>Durchmusterung</cite>, in which it appears as No. 2765, in degree
+26. On both occasions it was rated as 9½ magnitude, and no
+suspicion of variable light seems to have arisen. When viewed
+with the naked eye at the time of its greatest brilliancy, it
+was remarked by some observers that it twinkled decidedly
+more than other stars in the vicinity, and that this peculiarity
+made it very difficult to form a correct estimation of its
+relative brilliancy During the years 1866 to 1876, fluctuations
+in its light were observed by Schmidt, and he deduced
+a probable period of about 94 days, with a variation from the
+seventh to the ninth magnitude. This conclusion was
+confirmed by Schönfeld, and the star would therefore seem to
+be an irregular variable, and not a true temporary star.</p>
+
+<p class='c007'>A very remarkable and interesting variable star was discovered
+by Schmidt at Athens, near Rho Cygni, on the
+evening of November 24, 1876, when it was about the third
+magnitude, and somewhat brighter than Eta Pegasi. Schmidt
+stated that he had observed the vicinity on several occasions
+between November 1 and 20, and was certain that no star of
+even the fifth magnitude could possibly have escaped his
+notice, so that the star probably blazed out very suddenly, as
+most of these extraordinary objects have done. Between
+November 20 and 24, the sky was overcast, so the exact
+time of its appearance is unknown. The star would seem to
+<span class='pageno' id='Page_485'>485</span>be quite new, as there is no star in any of the catalogues in
+the position of the “Nova,” the nearest being one of the ninth
+magnitude, which occurs in the Bonn observations. The new
+star rapidly faded, and on November 30 had descended to the
+fifth magnitude. On the night of its discovery it was remarked
+that its brightness was such as to render its near neighbour,
+75 Cygni (a sixth magnitude star), invisible; while on December
+14 and 15, 75 Cygni, in its turn, nearly obliterated the
+light of the stranger. In the 48 hours following the night of
+November 27, the star diminished in light to the extent of nearly
+1½ magnitude! It afterwards faded very regularly to August,
+1877, and showed no oscillations of brightness as have been
+observed in other temporary stars. On the evening of its discovery,
+Schmidt considered the star to be of a strong golden-yellow,
+and that it afterwards remained of a deep golden-yellow,
+but at no time was it as ruddy as 75 Cygni. I could see
+no trace of colour in the star with a 3-inch telescope in the
+Punjab on January 12, 1877, but it had then faded to the
+eighth magnitude. On February 7, 1877, I estimated it ninth
+magnitude. A few days after its discovery, it was examined
+with the spectroscope, and its spectrum showed bright lines
+similar to the “Blaze Star” in Corona, which appeared in
+May, 1866. One of the bright lines was thought to be
+identical with the line numbered 1474 by Kirchoff, visible in
+the spectrum of the solar Corona during total eclipses of the
+sun. The other bright lines were identified by M. Cornu of
+the Paris Observatory with some of the lines of hydrogen,
+sodium, and magnesium. In September, 1877, the star was
+examined with a 15-inch refractor by Lord Lindsay (now
+Lord Crawford), who found “the light coming from it almost
+entirely monochromatic, that is, of only one colour, the star
+appearing exactly the same as when looked at without the
+spectroscope, the direct prism having no effect on it,” and he
+considers that “there is little doubt that the star has changed
+into a planetary nebula of small angular diameter!” On
+September 3, the star’s magnitude was 10½; “faint blue, near
+another star of same size rather red.” Lord Crawford remarks
+<span class='pageno' id='Page_486'>486</span>that no observer, discovering the object in its present state,
+would, after viewing it through a prism, hesitate to pronounce
+as to its nebulous character,<a id='r123'></a><a href='#f123' class='c027'><sup>[123]</sup></a> but no disc was detected with
+powers ranging up to 1000 diameters. Ward found the star only
+sixteenth magnitude in October, 1881, and it was estimated
+fifteenth magnitude at Mr. Wigglesworth’s Observatory in
+September, 1885. At Lord Crawford’s Observatory the exact
+position of the star, with reference to above fifty closely
+adjacent stars, was carefully determined with the micrometer.
+The vicinity was photographed by Dr. Roberts on September
+27, 1891, with an exposure of two hours, and “the <i>Nova</i> appears
+as a star of about the thirteenth magnitude.” Observations
+in 1894 and 1895, made its magnitude about 14·8, with
+an apparently continuous spectrum.<a id='r124'></a><a href='#f124' class='c027'><sup>[124]</sup></a></p>
+
+<p class='c007'>In August, 1885, a star of about the seventh magnitude
+made its appearance close to the nucleus of the Great Nebula
+in Andromeda (Messier 31), a remarkable nebula, which will be
+described in the next chapter. The new star was independently
+discovered by several observers towards the end of
+August. It was not visible to Tempel at the Florence
+Observatory on August 15 and 16, but is said to have been
+seen by M. Ludovic Gully on August 17. It was, however,
+certainly seen by Mr. I. W. Ward at Belfast on August 19, at
+11 P.M., when he estimated it 9½ magnitude, and it was independently
+detected by the Baroness Podmaniczky on August
+22, by M. Lajoye on August 30, by Dr. Hartwig, at Dorpat,
+on August 31, and by Mr. G. T. Davis, at Theale, near
+Reading, on September 1. On September 3, the star was
+estimated 7½ magnitude by Lord Crawford and Dr. Copeland,
+and its spectrum was found to be “fairly continuous.” On
+September 4, Mr. Maunder, at the Greenwich Observatory,
+found the spectrum “of precisely the same character as that of
+the nebula, <i>i.e.</i>, it was perfectly continuous, no lines, either
+bright or dark, being visible, and the red end was wanting.”
+Dr. Huggins, however, on September 9, thought he could see
+<span class='pageno' id='Page_487'>487</span>a few bright lines in its spectrum, a continuous spectrum
+being visible from the line D to F. The star gradually
+faded away. On December 10, 1885, it was estimated of the
+fourteenth magnitude at the Radcliffe Observatory, Oxford,
+and on February 7, 1886, it was rated only sixteenth magnitude
+with the 26-inch refractor of the Washington Observatory.
+A series of measures by Professor Hall, from September 29,
+1885, to February 9, 1886, showed “no certain indications of
+any parallax,” so that the star and the nebula, in which it probably
+lies, are evidently situated at a vast distance from the
+earth. Seeliger has investigated the decrease in the light
+of the star on the hypothesis that it was a cooling body,
+which had been suddenly raised to an intense heat by the
+shock of a collision, and finds a fair agreement between
+theory and observation. Auwers points out the similarity
+between this outburst and the new star of 1860, in the
+cluster 80 Messier (already described), and thinks it probable
+that both phenomena were caused by physical changes
+in the nebulæ in which they occurred. Proctor considered
+that the evidence of the spectroscope shows that the new
+star was situated <i>in</i> the nebula, and in this opinion I fully
+concur.</p>
+
+<p class='c007'>Several temporary stars have been detected in recent years
+by Mrs. Fleming, from an examination of photographs of
+stellar spectra, taken at the Harvard Observatory, for the
+Draper Memorial. Plates of the constellation Perseus show
+the existence of a star in 1887, the spectrum of which shows
+the bright lines of hydrogen, and it was on this account assumed
+to be a long period variable. During the following
+eight years, however, 81 photographs of the same region show
+no trace of the star, and it has been frequently looked for with
+a telescope, but without success. It would, therefore, seem
+probable that the star was a temporary one. Its magnitude
+was about the ninth.</p>
+
+<p class='c007'>A remarkable and very interesting temporary star was discovered
+in 1892 in the constellation Auriga. On February 1,
+of that year, an anonymous post-card was received by Dr.
+<span class='pageno' id='Page_488'>488</span>Copeland at the Royal Observatory, Edinburgh, with the
+following announcement:</p>
+
+<p class='c007'>“Nova in Auriga. In Milky Way, about two degrees south
+of χ Aurigæ, preceding 26 Aurigæ. Fifth magnitude, slightly
+brighter than χ.”</p>
+
+<p class='c007'>Such an announcement evidently required immediate attention,
+and on that evening, Dr. Copeland and his assistants
+looked for the new star, and easily found it with an opera-glass
+at 6 hours 8 minutes. They estimated it of the sixth magnitude,
+and equal to 26 Aurigæ. It was of a yellow colour. When
+examined with a prism placed before the eye-piece of a 24-inch
+reflector, its spectrum was seen to resemble the “Blaze
+Star” of 1866 in Corona. “The C line was intensely bright, a
+yellow line about D fairly visible; four bright lines, or bands,
+were conspicuous in the green; and, lastly, a bright line in the
+violet (probably Hγ) was easily seen.” Notice of the discovery
+was at once telegraphed to Greenwich and Keil Observatories,
+and the star was photographed at Greenwich on the same
+night. It is not in the Bonn star charts, which show stars to
+nearly the tenth magnitude. In <cite>Nature</cite> of February 18,
+1892, a letter appeared, signed Thomas D. Anderson, in which
+the writer stated that the post-card was sent by him, and he
+gives the following details respecting the discovery:</p>
+
+<p class='c007'>“Prof. Copeland has suggested to me that as I am the
+writer of the anonymous post-card mentioned by you a fortnight
+ago (p. 325), I should tell your readers what I know
+about the Nova.</p>
+
+<p class='c007'>“It was visible as a star of the fifth magnitude certainly
+for two or three days, very probably even for a week, before
+Prof. Copeland received my post-card. I am almost certain
+that at two o’clock on the morning of Sunday, the 24th ult.,
+I saw a fifth magnitude star making a very large obtuse angle
+with β Tauri and χ Aurigæ, and I am positive that I saw it,
+at least, twice subsequently during that week. Unfortunately,
+I mistook it on each occasion for 26 Aurigæ, merely remarking
+to myself that 26 was a much brighter star than I used to
+think it. It was only on the morning of Sunday, the 31st
+<span class='pageno' id='Page_489'>489</span>ult., that I satisfied myself that it was a strange body. On
+each occasion of my seeing it, it was slightly brighter than χ.
+How long before the 24th ult. it was visible to the naked eye
+I cannot tell, as it was many months since I had looked
+minutely at that region of the heavens.</p>
+
+<p class='c007'>“You might also allow me to state, for the benefit of your
+readers, that my case is one that can afford encouragement to
+even the humblest of amateurs. My knowledge of the technicalities
+of astronomy is, unfortunately, of the most meagre
+description; and all the means at my disposal on the morning
+of the 31st ult., when I made sure that a strange body was
+present in the sky, were Klein’s ‘Star Atlas’ and a small
+pocket-telescope, which magnifies ten times.”</p>
+
+<p class='c007'>Soon after the discovery of the new star, an examination
+was made by Professor Pickering of photographs taken of the
+region at Harvard Observatory, previous to Dr. Anderson’s
+discovery. It was found that on eighteen photographs taken
+between the dates November 3, 1885, and November 2, 1891,
+there is no trace of the new star; but in those taken from
+December 16, 1891, to January 31, 1892, a star of the fifth
+magnitude is shown in the position of the new star. “In
+another series of plates taken with the transit photometer, no
+record of the new star up to December 1, 1891, was obtained,
+although χ Aurigæ (magnitude 5·0) was always visible, but
+the plates taken on the nights of December 10, 1891, and
+ending January 20, 1892, indicated clearly the position of the
+new star.” Professor Pickering says: “It appears that the
+star was fainter than the eleventh magnitude on November 2,
+1891, than the sixth magnitude on December 1, and that it
+was increasing rapidly on December 10. A graphical construction
+indicates that it had probably attained the seventh
+magnitude within a day or two of December 2, and the sixth
+magnitude on December 7. The brightness increased rapidly
+until December 18, attaining its maximum about December
+20, when its magnitude was 4·4. It then began to decrease
+slowly, with slight fluctuations, until January 20, when it was
+slightly below the fifth magnitude. All these changes took
+<span class='pageno' id='Page_490'>490</span>place before its discovery, so that it escaped observation nearly
+two months. During half of this time it was probably brighter
+than the fifth magnitude.”</p>
+
+<p class='c007'>It would seem from the above remarks that the star did not—like
+some other temporary stars—attain its full brilliancy at
+once, but increased gradually in brightness. After the decrease
+of light in January, 1892, it seems to have again risen
+to another maximum, for photographs taken at the Greenwich
+Observatory after its discovery show that the star rose to a
+magnitude of 3·5 (photographic) on February 3, and then
+began to fade again slowly during February, but rapidly
+during the month of March. Owing to cloudy weather in the
+west of Ireland, I could not observe the new star until February
+14. The following are my observations, made with a binocular
+field-glass, the comparison stars being Chi Aurigæ, 26 Aurigæ,
+and D M + 30°, 898:—February 14, 4·55 magnitude; February
+15, 5·56; February 16, 5·84; February 18, 5·51; February 21,
+5·56; February 24, 5·66; February 28, 5·44; March 1, 5·68;
+March 5, 5·66; March 10, 7·3; March 11, 7¾; March 16, 8½,
+or fainter; March 18, 9 magnitude, or less, “only <i>very</i> faint
+stars seem near the place of the Nova; clear sky, no moon.”
+The general accuracy of the above observations were confirmed
+by the photographic estimates of the star’s light made at
+Greenwich,<a id='r125'></a><a href='#f125' class='c027'><sup>[125]</sup></a> and also by Schaeberle’s observations of its
+brightness.</p>
+
+<p class='c007'>After March 18, the light of the star steadily and
+rapidly decreased, and on April 1, it had faded to nearly the
+fifteenth magnitude, and afterwards to about the sixteenth.
+In August, 1892, it brightened again, as it was found by Corder
+of about the ninth magnitude on August 21. Dr. J. Holetschek
+of the Vienna Observatory observed it from August 24
+to September 2, 1892, and estimated it about 9½ magnitude.
+In October, 1892, most observers rated it between 10 and
+10½ magnitude. Observations by Mr. C. E. Peck, “from
+October 3, 1893, to May 4, 1894, only vary from 10·1 to 11·0
+magnitude, and observations up to the end of 1894 give the
+<span class='pageno' id='Page_491'>491</span>same results.”<a id='r126'></a><a href='#f126' class='c027'><sup>[126]</sup></a> In 1895 Professor Barnard found that it “is
+still visible as a small star, and has not changed in physical
+appearance since the autumn of 1892. It remains perfectly
+fixed with reference to the comparison stars.”<a id='r127'></a><a href='#f127' class='c027'><sup>[127]</sup></a></p>
+
+<p class='c007'>Examined with the spectroscope soon after its discovery,
+many bright lines were seen in its spectrum, and it was found
+that “the bright lines in the spectrum of the new star were
+accompanied by dark ones on their more refrangible sides,”
+that is, the dark lines were on the blue side of the bright ones.
+This suggested the idea that the outburst was probably due to
+a collision between two bodies, one of which, having a spectrum
+of dark lines, was rushing towards the earth, and the other,
+with a bright-line spectrum, was receding. Lockyer supposed
+the outburst to be due to a collision between two swarms of
+meteorites. Dr. Huggins advanced the view that the phenomenon
+was due to the near approach of two gaseous bodies.
+“But,” he says, “a casual near approach of two bodies of great
+size would be a greatly less improbable event than an actual
+collision. The phenomena of the new star scarcely permits
+us to suppose even a partial collision, though, if the bodies
+were diffused enough, or the approach close enough, there may
+have been, possibly, some interpenetration and mingling, of
+the rare gases near the boundaries.” But Maunder and
+Seeliger consider this hypothesis to be untenable. Mr.
+Monck suggested that a star or swarm of meteorites rushing
+through a gaseous nebula might explain the phenomena.
+Seeliger advocates a similar theory. Maunder also favours a
+collision theory.</p>
+
+<p class='c007'>A photograph of the spectrum taken by Maunder on February
+22, 1892 (when the photographic magnitude was 4·78, and
+visual magnitude about 5·7), showed a displacement of the
+dark lines, which implied a relative motion of the two supposed
+colliding bodies of about 820 miles a second! Vogel
+found that the bright lines showed a double maxima, and he
+thought that these were due to “two different bodies moving
+<span class='pageno' id='Page_492'>492</span>with different velocities, so that the spectrum of the Nova
+consists of, at least, three spectra superposed. The measurement
+of the photograph gives the body showing the dark line
+spectrum as approaching the earth with a speed of nearly 420
+miles per second, one of the two bright line bodies as approaching
+with a speed of 22 miles, whilst the other is receding
+with a speed of 300 miles a second.”<a id='r128'></a><a href='#f128' class='c027'><sup>[128]</sup></a></p>
+
+<p class='c007'>At the time of its increase of brightness, in August, 1892,
+Professor Barnard, observing it with the great 36-inch Lick
+telescope, says, the “Nova appeared as a small, bright nebula,
+with a star-like nucleus of the tenth magnitude. The nebulosity
+was pretty bright and dense, and was 3″ in diameter.
+Surrounding this was a fainter glow, perhaps half a minute in
+diameter.” At this time, Professor Campbell of the Lick
+Observatory found that its spectrum showed the characteristic
+nebular lines. This observation was confirmed by Dr.
+Copeland on August 25 and 26, and by Herr Gothard, who
+photographed the spectra of a number of nebulæ, and compared
+them with his photograph of the spectrum of the new star.
+He says, “Each new photograph increased the probability,
+which may be considered as a proved fact, that the <i>spectrum
+not only resembles, but that the aspect and position of the lines
+show it to be identical with the spectra of the planetary nebula</i>.
+In other words, the new star has changed into a planetary
+nebula.”</p>
+
+<p class='c007'>A nebulous spectrum was also found by Espin. From observations
+of the spectrum in November, 1894, Professor
+Campbell finds that “the spectrum is not only nebular, but it
+is approaching the average type of nebular spectrum,” and he
+adds, “We may say that only five ‘new stars’ have been discovered
+since the application of the spectroscope to astronomical
+investigations, and that three of these had substantially
+identical spectroscopic histories.” Espin found the star
+distinctly nebulous on December 9, 1895, and its magnitude
+about 10½.</p>
+
+<p class='c007'>Another new star was discovered by Mrs. Fleming by the
+<span class='pageno' id='Page_493'>493</span>photographic method in the southern constellation, Norma, in
+the year 1893. When at its brightest, it seems to have been
+about the seventh magnitude. It was situated in the Milky
+Way, a little to the east of the pair of stars known as Gamma
+one and Gamma two Normæ. Its spectrum was similar to
+that of the new star in Auriga, when it first appeared, and,
+like that object, the spectrum has now, according to Professor
+Campbell, “become distinctly nebular.”</p>
+
+<p class='c007'>Another temporary star of about the eighth magnitude was
+also discovered by Mrs. Fleming in 1895, in that portion of
+the southern constellation Argo, known as Carina. It was in
+or close to the Milky Way—like so many of these new stars—between
+the variable star Eta Argûs and the star Lambda
+Centauri, near the Southern Cross, and close to a star of
+magnitude 5½. The photographic plates on which the discovery
+was made were taken at the Arequipa Station, in
+Peru. An examination of 62 photographs of the region
+showed no trace of the star on May 17, 1889, and March 5,
+1895, although stars so faint as the fourteenth magnitude are
+visible on some of the plates. On nine plates, however, taken
+between April 8, 1895, and July 1, 1895, the star is visible,
+and during this interval the brightness diminished from the
+eighth to the eleventh magnitude. The spectrum showed the
+bright lines of hydrogen “accompanied by dark lines of
+slightly shorter wave-length,” and in all its “essential features”
+was “apparently identical” with the spectra of the temporary
+stars in Auriga and Norma.</p>
+
+<p class='c007'>With reference to this outburst, and the similarity of the
+star’s spectrum to that of the new star in Auriga, Professor
+William H. Pickering points out “the improbability of two
+successive collisions between stars, occurring nearly in the line
+of sight, in both cases a bright and a dark line star being involved,
+and in each case the bright-line star being the one to
+recede from us. The same remark applies to the theory of a
+collision of a star and a nebula. As a substitute I offered an
+explosion hypothesis, in which a dark sun suddenly gave out
+in all directions large quantities of hydrogen in an incandescent
+<span class='pageno' id='Page_494'>494</span>state. This would, of course, merely produce a spectrum
+with bright lines. But if the expulsion of hydrogen continued,
+the outer layers of gas would cool, producing absorption
+lines in the spectrum of the approaching hydrogen, but still
+leaving the spectrum lines of the receding hydrogen bright.
+Finally, when the expulsion ceased, we should find a heated
+spherical mass of gas, similar to a planetary nebula. It was
+shown that the velocities which were observed in the cases of
+these two <i>novæ</i> were less than fifty per cent. greater than had
+been observed in our own sun. The discovery of this third
+<i>nova</i>, with a spectrum identical with that of the two others, increases
+many times the improbability of the collision theories,
+and thereby strengthens the explosion hypothesis. If this
+latter is correct, we must look upon the phenomena presented
+by a <i>nova</i> not as indicating the birth of a new star, but rather
+as a cataclysm testifying to the death and final disrupture of
+an old one.”<a id='r129'></a><a href='#f129' class='c027'><sup>[129]</sup></a></p>
+
+<p class='c007'>Another apparently new star was detected by Mrs. Fleming
+in 1895, in the constellation Centaurus. It was situated about
+three degrees north-west of the double star 3 Centauri, and
+when at its brightest, seems to have been about the seventh
+magnitude. Mrs. Fleming’s attention was first directed to it
+by its peculiar spectrum, as shown on a photographic plate
+taken at Arequipa in July, 1895. No trace of the star is
+visible on 55 plates taken from May 21, 1889, to June 14,
+1895, but on plates taken on July 8 and 10, 1895, it appears
+of about the seventh magnitude. A photograph taken on
+December 16, 1895, shows it as a star of about the eleventh
+magnitude. On that date, and on December 19, it was seen
+about the same magnitude by Mr. O. C. Wendell, with a
+15-inch telescope. The spectrum at first resembled that of
+the nebula 30 Doradus, and was unlike the spectra of the
+temporary stars in Auriga, Norma, and Carina. When it had
+faded to the eleventh magnitude, its spectrum seemed to be
+monochromatic, and very similar to that of a neighbouring
+nebula, N G C 5253, so that, like the new stars in Cygnus,
+<span class='pageno' id='Page_495'>495</span>Auriga, and Norma, “it appears to have changed into a
+gaseous nebula.”</p>
+
+<p class='c007'>It is a remarkable fact that the great majority of the temporary
+stars appeared in or near the Milky Way. The chief
+exceptions to this rule are:—the star of 76 <span class='fss'>B. C.</span>, in the
+Plough, the star recorded by Hepidannus in Aries, 1012, <span class='fss'>A.D.</span>,
+and the “Blaze Star” of 1866 in Corona Borealis.</p>
+
+<h3 class='c004'>CHAPTER VI.<br> <span class='c014'>CLUSTERS AND NEBULÆ.</span></h3>
+
+<p class='c003'>Clusters of stars and nebulæ are frequently classed together
+in one group. But this is incorrect. The term nebulæ
+should be restricted to those objects which the spectroscope
+shows to consist of gaseous matter, while the term cluster
+should be applied to those groups of stars in which the components
+are individually visible as distinct star-like points.
+There may be, of course, intermediate forms, like the Great
+Nebula in Andromeda, which, although not resolvable into
+stars with powerful telescopes, the spectroscope shows to be
+not gaseous. We will begin with clusters of stars, many of
+which can be seen with telescopes of moderate power, and
+some, like the Pleiades, even with the naked eye.</p>
+
+<p class='c007'>The Pleiades form perhaps the most remarkable group of
+stars in the heavens, and are probably familiar to most people,
+even to those whose knowledge of the constellations is limited
+to a few of the brighter stars. The cluster is a very remarkable
+and brilliant one, and forms a striking object in a clear
+sky. There is no other group visible to the naked eye in
+either hemisphere similar to it in the brightness and closeness
+of the component stars. It seems to have attracted
+the attention of observers since the earliest ages. Job says:
+<span class='pageno' id='Page_496'>496</span>“Can’st thou bind the sweet influences of Pleiades, or loose
+the bands of Orion?”</p>
+
+<p class='c007'>Hesiod, writing nearly 1,000 years <span class='fss'>B.C.</span>, speaks of the
+Pleiades in words thus translated by Cooke:—</p>
+
+<div class='lg-container-b c015'>
+  <div class='linegroup'>
+    <div class='group'>
+      <div class='line'>“There is a time when forty days they lie,</div>
+      <div class='line'>And forty nights conceal’d from human eye;</div>
+      <div class='line'>But in the course of the revolving year,</div>
+      <div class='line'>When the swain sharps the scythe, again appear.”</div>
+    </div>
+  </div>
+</div>
+
+<p class='c007'>This passage refers to the disappearance of the group in the
+sun’s rays in summer, and their reappearance in the evening
+sky in the east at harvest time. Hesiod also speaks of them
+as the seven sisters, and in Cicero’s “Aratus,” they are represented
+as female heads, bearing the names Merope, Alcyone,
+Celæno, Electra, Taygeta, Asterope, and Maia, names by
+which they are still known to astronomers. The origin of the
+name Pleiades is somewhat doubtful. Some think that it is
+derived from the Greek word <i>pleia</i>, to sail. Others from the
+words <i>pleios</i>, full, a name perhaps suggested by the appearance
+of the cluster. Although seven stars are mentioned by
+Hipparchus and Aratus, Homer only speaks of six, and this
+is the number now visible to average eyesight. A larger
+number has, however, been seen with the naked eye by those
+gifted with exceptionally keen eyesight. Möstlin, Kepler’s
+tutor, is said to have seen fourteen, and he actually measured
+and recorded the position of eleven, with wonderful accuracy,
+without the aid of a telescope! In recent years, Miss Airy,
+daughter of the late astronomer-royal, has seen twelve, and
+Carrington and Denning fourteen. But to most eyes probably
+six only are visible with any certainty. There is a tradition
+that, although seven stars were originally visible, one disappeared
+at the taking of Troy. Professor Pickering has
+recently discovered that the spectrum of Pleione, which forms
+a wide pair with Atlas, bears a striking resemblance to that
+of P Cygni, the so-called “temporary star” of 1600. This
+similarity of spectra suggests the idea that Pleione may possibly—like
+the star in Cygnus—be subject to occasional fluctuations
+<span class='pageno' id='Page_497'>497</span>of light, which might perhaps account for its visibility to the
+naked eye in ancient times.</p>
+
+<p class='c007'>The grouping of even six stars visible to the naked eye in
+so small a space is very remarkable. Considering the total
+number of stars visible without optical aid, Mitchell—writing
+in 1767—calculated by the mathematical theory of probability
+that the chances are 500,000 to one against the close arrangement
+of six stars in the Pleiades being merely the result of
+accident. He therefore concludes “that this distribution was
+the result of design, or that there is reason or cause for such an
+assemblage.”</p>
+
+<p class='c007'>Although to a casual observer the component stars may
+appear of merely equal magnitude, there is considerable
+difference in their relative brilliancy. Measures with a photometer
+show that Alcyone—the brightest of the group—is of
+the third magnitude, Maia, Electra, and Atlas of the fourth,
+Merope about 4⅓, Taygeta 4½, Celæno about 5⅓, and Asterope
+about the sixth. Pleione is about 5½, according to the photometric
+measures made at Oxford, but it lies so close to Atlas
+that to most eyes the two will probably appear as one star.
+About thirty more range from the sixth to the ninth magnitude,
+and this is about the number visible with an opera-glass.
+Galileo counted thirty-six stars with his small telescopes, but
+with modern instruments the number is largely increased.
+Some years since, M. Wolf, the distinguished French astronomer,
+published a chart of the Pleiades, showing about 500
+stars made from his own observations. Photography has
+further added to the number of stars visible in this interesting
+group. On a photograph taken at the Paris Observatory in
+1887, with an exposure of three hours, no less than 2,326 stars
+can be distinctly counted on a space of about three square
+degrees. The fainter stars on this photograph are supposed
+to be of the seventeenth magnitude. Now, as Alcyone, the
+brightest star of the group, is of the third magnitude, we have
+a difference of fourteen magnitudes between the brightest and
+the faintest. This implies that Alcyone is 398,100 times
+brighter than the faintest stars visible on the photographic
+<span class='pageno' id='Page_498'>498</span>plate. If we could conclude that the fainter stars really
+belonged to the cluster, they would be at practically the same
+distance from the earth, and the great difference of brightness
+would be very remarkable, and would suggest that Alcyone is
+a vastly larger body than the smallest stars of the group.
+The difference of brilliancy given above would indicate that
+the diameter of Alcyone is 631 times greater than that of the
+faintest stars revealed by photography. This is of course on
+the assumption that all the stars of the cluster are, surface for
+surface, of the same intrinsic brilliancy, and that this apparent
+brightness to the eye depends simply on their diameter. As
+spheres vary in volume as the cubes of their diameters, we
+have the volume of Alcyone equal to the cube of 631, or over
+250 million times the volume of the faintest stars of the group.
+This startling result was very difficult to explain, for either we
+must assume that Alcyone is an enormously vast body, or else
+that the faint stars of the group are exceedingly small. If we
+take the diameter of Alcyone as 1,400,000 miles, then the
+diameter of the faintest stars in the group would be only
+2,200 miles, or about the size of our moon, and it seems highly
+improbable, if not impossible, that such small bodies should
+shine with inherent light of their own. They would indeed
+be “miniature suns.” On the other hand, if we assume that
+the faintest stars are of about the same size as the planet
+Jupiter, or about 87,000 miles, the diameter of Alcyone would
+be nearly 55 millions of miles, a result which is also highly
+improbable. The difficulty has, I think, been satisfactorily
+cleared up by some photographs recently taken by Professor
+Barnard at the Lick Observatory. A photograph taken with
+a lens of six inches aperture, and 31 inches focal length, and
+an exposure of 10 hours 15 minutes, shows that the sky
+surrounding the Pleiades is, on all sides, as thickly studded
+with small stars as the cluster itself. It seems clear, therefore,
+that the faint stars in the Pleiades are merely some of the
+“hosts of heaven” which happen to lie in that direction, and
+have probably no connexion with the cluster, which is merely
+projected on a starry background of faint and distant stars.</p>
+
+<p class='c007'><span class='pageno' id='Page_499'>499</span>The brilliancy of the Pleiades cluster would naturally suggest
+a comparative proximity to the earth. Attempts to determine
+their distance have, however, hitherto proved unsuccessful.
+This would indicate that the distance is very great, and would,
+of course, lead to the conclusion that the group is of vast
+dimensions. An effort has been made to determine the distance
+indirectly by a consideration of the “proper motion” of
+the principal stars. Professor Newcomb finds a proper motion
+for Alcyone of about 5·8 seconds of arc per century. This
+motion is in a direction nearly opposite to that of the sun’s
+motion in space, and may possibly be due to that cause. If
+we assume that this apparent motion of Alcyone is wholly due
+to the effect of the sun’s real motion at the rate of, say, fourteen
+miles a second, the distance of Alcyone would correspond
+to a “light journey” of about 267 years! Our sun, placed at
+this vast distance, would, I find, be reduced in brilliancy to a
+star of about the ninth magnitude, or six magnitudes fainter
+than Alcyone. This would imply that Alcyone is about 250
+times brighter than the sun! As, however, the spectrum of
+Alcyone is of the first or Sirian type, it cannot properly be
+compared with the sun.</p>
+
+<p class='c007'>There are six other small stars in the Pleiades having
+proper motions similar in amount and direction to that of
+Alcyone. As the other bright stars of the group have much
+smaller motions, it has been suggested that the seven stars
+with comparatively large, proper motions do not really belong
+to the group, but are only optically associated with it. This
+would imply that the real cluster lies much farther from us
+than Alcyone, and the comparative brilliancy of some of its
+component stars would still denote enormous size.</p>
+
+<p class='c007'>In the year 1859, the well-known astronomer, Tempel, announced
+his discovery of a faint nebulosity extending in a
+southerly direction from Merope, the nearest bright star
+to Alcyone. This interesting discovery was practically confirmed
+by other astronomers; but from its visibility to some
+observers with small telescopes, and the failure of others to
+detect it with much larger instruments, the variability of its
+<span class='pageno' id='Page_500'>500</span>light was strongly suspected. The question remained in
+doubt for many years, but has now been finally set at rest by
+photography, which shows not only a mass of nebulous light
+surrounding Merope, but other nebulous spots involving
+Alcyone, Maia, and Electra. Indeed, a photograph taken by
+Dr. Roberts in 1889 shows that all the brighter stars of the
+group are more or less surrounded by nebulosity. The nebula
+surrounding Maia is of a somewhat spiral form, and its existence
+was not even suspected until it was revealed by photography.
+It was afterwards seen with the great 30-inch
+refractor of the Pulkowa Observatory. Had, however, its
+existence been unknown, it would probably have escaped
+detection, even with this large telescope, as it is one thing to
+see a faint object known to exist and another to discover it
+independently. Maia is surrounded by several faint stars of
+the twelfth to the fourteenth magnitude; and the Russian observers
+believe that one of these is variable in light, as it was
+seen distinctly on February 5, 1886, when its magnitude was
+carefully determined with reference to the neighbouring stars;
+but on February 24 of the same year, it could not be seen
+with a telescope of 15 inches aperture. Some of the other
+stars in the group seem to be connected by nebulous rays with
+the principal nebulous centres, and in looking at this wonderful
+Paris chart it seems impossible to avoid the conclusion that
+the stars and nebulous masses are actually mixed up together,
+and not merely placed accidentally in the same direction.
+Indeed, Professor Barnard’s photograph referred to above
+shows the whole group involved in dense nebulosity.</p>
+
+<p class='c007'>Other well-known clusters or groups of stars are the
+Hyades, marked by the bright, reddish star, Aldebaran, the
+Præsepe, or Beehive, in Cancer, and Comæ Berenices, but these
+are larger and more scattered.</p>
+
+<div id='i_503'  class='figcenter id001'>
+<span class='pageno' id='Page_501'>501</span>
+<img src='images/i_503.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 9.</span>—<i>The Double Star Cluster in Perseus.</i><br>  <br>  (From “Scenery of the Heavens.”)</p>
+</div>
+</div>
+
+<p class='c007'>Of other irregular clusters, somewhat similar to the Pleiades,
+but not so bright, may be mentioned the double cluster in
+Perseus, which is visible to the naked eye on a clear night as a
+hazy spot of light in the midst of the Milky Way. Admiral
+Smyth says they form “one of the most brilliant telescopic
+objects in the heavens.” They may be seen with a binocular
+field-glass, but, of course, a good telescope is necessary to see
+them well. They have been beautifully photographed at the
+Paris Observatory, the photograph showing no trace of
+nebulosity. They have also been photographed by Dr.
+Roberts, who says, “The photograph presents to the eye the
+stars in the two clusters, and in the surrounding parts of the
+sky, with a completeness and accuracy of detail never before
+seen. The stars are shown in their true relative positions and
+magnitudes to about the sixteenth, and among them are many
+<span class='pageno' id='Page_502'>502</span>apparent double, triple, and multiple stars. They also appear
+to be arranged in clusters, curves, festoons, and patterns that
+are suggestive of some physical connexion existing between
+the groups; but it is premature to assert that these appearances
+are not due to perspective effect by the eye arranging
+numerous close points of light into various patterns. Similar
+photographs to this, taken at intervals of several years between
+them, will determine the reality, or otherwise, of these
+remarkable groupings of the stars.”</p>
+
+<div id='i_504'  class='figcenter id001'>
+<img src='images/i_504.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 10.</span>—<i>Star Cluster in Gemini.</i><br>  <br>  (From “Scenery of the Heavens.”)</p>
+</div>
+</div>
+
+<p class='c007'>A little north of the star Eta Geminorum is a pretty cluster
+of small stars known as 35 Messier, which is just visible to the
+naked eye. The component stars may be well seen with a
+telescope of moderate power. This cluster has been also
+<span class='pageno' id='Page_503'>503</span>photographed at the Paris Observatory, and shows a well-marked
+clustering tendency in the component stars. Admiral
+Smyth says: “It presents a gorgeous field of stars from the
+ninth to the sixteenth magnitude, but with the centre of the
+mass less rich than the rest. From the small stars being inclined
+to form curves of three or four, and often with a large
+one at the root of the curve, it somewhat reminds one of the
+bursting of a sky rocket.”</p>
+
+<div id='i_505'  class='figcenter id001'>
+<img src='images/i_505.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 11.</span>—<i>37 Messier.</i><br>  <br>  (From “Worlds of Space.”)</p>
+</div>
+</div>
+
+<p class='c007'>About ten degrees to the north of the cluster just described
+is another fine cluster known as 37 Messier. The accompanying
+photograph will show its telescopic appearance.</p>
+
+<p class='c007'><span class='pageno' id='Page_504'>504</span>In the Southern Hemisphere there is a magnificent cluster of
+small stars surrounding the star Kappa Crucis, a reddish star
+of the seventh magnitude. It was thus described by Sir John
+Herschel: “A most vivid and beautiful cluster of 50 to 100
+stars. Among the larger there are one or two evidently
+greenish. South of the red star is one, 13 minutes, also red,
+and near it one, 12 minutes, bluish&#160;... though neither a large
+nor a rich one, is yet an extremely brilliant and beautiful
+object when viewed through an instrument of sufficient
+aperture to show distinctly the very different colours of its
+constituent stars, which gives it the effect of a superb piece of
+fancy jewellery.” He gives the positions of 110 stars, from
+the seventh to the sixteenth magnitude. It lies near the
+northern edge of the well-known “coal sack,” and Dr. Gould
+says of it: “The exquisitely beautiful cluster, κ <i>Crucis</i>, contains
+a large number of stars of various tints and hues, contrasting
+wonderfully with each other, when viewed with a
+telescope of large aperture.” Mr. Russell’s drawing of this
+cluster, made at Sydney (N.S.W.) in 1872, shows several
+changes in the relative positions of the stars as laid down by
+Sir John Herschel, probably the result of proper motion.</p>
+
+<p class='c007'>About 2½° north of the star M Velorum, Sir John
+Herschel describes “an enormous cluster, of a degree and a
+half in diameter, very rich in stars of all magnitudes, from 8
+minutes downwards, a sort of telescopic Præsepe.”</p>
+
+<p class='c007'>Another fine cluster is that known as 11 Messier. It lies a
+little to the west of the star Lambda Aquilæ, and is just
+visible to the naked eye on a clear night. It consists of stars
+of about the eleventh magnitude, and Admiral Smyth compared
+it to a “flight of wild ducks.” It has been beautifully
+photographed by Dr. Roberts, who says: “The negative
+shows the stars individually, though the print, owing to their
+closeness, does not separate them.... It is entirely free from
+nebulosity.”</p>
+
+<p class='c007'>There are many other similar objects in both hemispheres
+too numerous to mention here, but those described are interesting
+objects of their class.</p>
+
+<div id='i_507'  class='figcenter id001'>
+<span class='pageno' id='Page_505'>505</span>
+<img src='images/i_507.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 12.</span>—<i>Star Cluster in Hercules.</i><br>  <br>  (From “Scenery of the Heavens.”)</p>
+</div>
+</div>
+
+<p class='c007'>We now come to the “globular clusters.” This term has
+been applied to those clusters of stars which evidently occupy
+a space of more or less spherical form. Some of these “balls
+of stars,” as they have been called, are truly wonderful, and
+are among the most interesting objects visible in the sidereal
+heavens. Good specimens of the class are, however, rather
+rare objects, and there are not many in the Northern Hemisphere.
+The most remarkable, perhaps, is that called “the
+Hercules cluster,” but known to astronomers as 13 Messier, it
+being No. 13 in the first catalogue of remarkable “nebulæ”
+formed by Messier, the famous discoverer of comets. It was
+discovered by Halley in 1714. This wonderful object lies
+between the stars Zeta and Eta in Hercules, nearer to the
+latter star. It may be seen with a binocular or good opera-glass
+as a hazy star of the sixth magnitude. Messier was
+<span class='pageno' id='Page_506'>506</span>certain that it contained no stars; but when examined with a
+good telescope it is at once resolved into a multitude of small
+stars, which can be individually seen, and even counted, with
+large telescopes. According to Admiral Smyth, “No plate
+can give a fitting representation of this magnificent cluster.
+It is indeed truly glorious, and enlarges on the eye by studious
+gazing.” And Dr. Nichol says: “Perhaps no one ever saw it
+for the first time through a telescope without uttering a shout
+of wonder.” The number of stars included in the cluster
+was estimated by Sir William Herschel at 14,000; but
+the real number is probably much smaller. Were the number
+so great as Herschel supposed, I find that the cluster would
+form a much brighter object than it does. Assuming the
+average magnitude of the component stars at 12½, I find that
+an aggregation of 14,000 stars would shine as a star of about
+the second magnitude. But the cluster is only as bright
+as a star of about the sixth magnitude, and, with this magnitude,
+I find that the total number would be about 400.
+Examining it with his giant telescope, Lord Rosse observed
+three dark rifts radiating from the centre. These were afterwards
+seen by Buffham with a 9-inch reflector, and also by
+Webb. They were also observed at Ann Arbor Observatory
+(U.S.A.), in April, 1887, by Professor Harrington and Mr.
+Schaeberle, using telescopes of six and twelve inches aperture.
+It has been well photographed at the Paris Observatory, and
+also by Dr. Roberts and Mr. Wilson. In some of these
+photographs the dark rifts are perceptible to some extent, but
+owing to the over exposure of the central portion of the
+cluster, they are not so distinct as in drawings made at the
+telescope. Dr. Huggins, examining it with the spectroscope,
+finds that the spectrum is not gaseous; but spectroscopic
+evidence is not necessary to prove that the cluster consists of
+small stars, as these are distinctly seen as points of light with
+telescopes of moderate power, and with the great Lick telescope
+the component stars are visible even in the central
+portion of the cluster. Its globular shape is evident at a
+glance, and we cannot doubt that the stars composing it form
+<span class='pageno' id='Page_507'>507</span>a gigantic system, probably isolated in space. Many people
+might think that this cluster was a mass of double and
+multiple stars; but this is not so. The components, close as
+they are, are too far apart to constitute true double stars.
+Mr. Burnham, the famous double star observer, finds <i>one</i> close
+double star near the centre, and notes the remarkable absence
+of close double stars in bright and apparently compressed
+clusters.</p>
+
+<p class='c007'>In the same constellation, Hercules, between the stars Eta
+and Iota, but nearer the latter, will be found another object of
+the globular class, but not so bright or so easily resolvable
+into stars as the cluster described above. It is known as 92
+Messier. Buffham, with a 9-inch mirror, thought the component
+stars brighter and more compressed than in 13 Messier.
+Sir William Herschel found it seven or eight minutes of arc in
+diameter. The brighter components are easily visible in
+telescopes of moderate power, but even Lord Rosse’s giant
+telescope failed to resolve the central blaze. This object was
+photographed by Dr. Roberts in May, 1891, with a 20-inch
+reflecting telescope, and an exposure of one hour. He says:
+“The photograph shows the cluster to be involved in dense
+nebulosity, which, on the negative, almost prevents the stars
+being seen through it, and on the print quite obscures the
+stars. The stars in this, as in all other globular clusters, are
+arranged in various patterns, and many of them appear to be
+nebulous.”</p>
+
+<p class='c007'>About three degrees north preceding the star 9 Boötis, is
+another fine globular cluster, known as 3 Messier. Smyth
+describes it as “a brilliant and beautiful globular congregation
+of not less than 1,000 stars, between the southern Hound
+and the knee of Boötis; it blazes splendidly towards the
+centre, and has outliers.... This mass is one of those balls of
+compact and wedged stars, whose laws of aggregation it is so
+impossible to assign.” The idea of the component stars
+being “compact and wedged” is, however, a mistake, as I
+have shown elsewhere.<a id='r130'></a><a href='#f130' class='c027'><sup>[130]</sup></a> Sir John Herschel described it as a
+<span class='pageno' id='Page_508'>508</span>remarkable object, exceedingly bright and very large, with
+stars of the eleventh magnitude. Buffham found it resolved
+even in the centre with a 9-inch mirror. It was photographed
+by Dr. Roberts in May, 1891, with an exposure of two hours,
+and the photograph confirms the general descriptions given of
+the cluster, though “the print fails to show the stars that
+on the negative crowd the space covered by the dense
+nebulosity.” Dr. Roberts remarks that “nebulosity seems
+invariably to be present in globular clusters.” From photographs
+of this cluster, taken at Arequipa in Peru, Professor
+Bailey finds 87 stars of the cluster to be variable in light, the
+variability amounting in some cases to two magnitudes, with
+usually short periods.</p>
+
+<p class='c007'>Another fine globular cluster is that known as 5 Messier.
+It lies closely north of the fifth magnitude star, 5 Serpentis.
+It was discovered by Kirch in 1702, and was observed in 1764
+by Messier, who found he could see it with a telescope of one
+foot in length, but could not resolve it into stars. Smyth
+says: “This superb object is a noble mass, refreshing to the
+senses after searching for faint objects, with outliers in all
+directions, and a bright central blaze, which even exceeds 3
+Messier in concentration.” Sir William Herschel, with his
+40-foot telescope, could count about 200 stars, but could not
+distinguish the stars near the central blaze. Sir John Herschel
+describes it as an excessively compressed cluster of a globular
+form, with stars from the eleventh to the fifteenth magnitude,
+condensed into a blaze at the centre. Lord Rosse found it
+more than seven or eight minutes of arc in diameter, with a
+nebulous appearance in the centre. This cluster was photographed
+by Dr. Roberts in April, 1892. “The photograph
+shows the stars to about the fifteenth magnitude, and the
+cluster is involved in dense nebulosity about the centre. The
+nebulosity hides the stars even on the negative.” With
+reference to this latter remark, however, Dr. Common says<a id='r131'></a><a href='#f131' class='c027'><sup>[131]</sup></a>
+that, in photographs of this cluster taken with a larger instrument,
+“the stars are quite distinct, though the exposure was
+<span class='pageno' id='Page_509'>509</span>much longer, a result that might fairly be expected.” From
+photographs of this cluster taken at Arequipa, Peru, by
+Professor Bailey, he finds that the cluster contains about 750
+stars, of which 46 are variable in light, or about 6 per cent.
+of the whole. This is remarkable, for, of the stars visible to
+the naked eye, less than 1 per cent. are variable, so far as is at
+present known. A further examination of the photographs
+made by Miss Leland shows that the periods of these variables
+are in general very short, not exceeding a few hours.<a id='r132'></a><a href='#f132' class='c027'><sup>[132]</sup></a> One
+star, situated about eight minutes of arc from the centre of the
+cluster, has a probable period of 11 hours, 7 minutes, 52 seconds,
+and varies from about magnitudes 13·50 to 14·73. The star
+remains at the minimum light for about half the period, and
+the maximum brightness is of comparatively short duration.
+The rate of increase is more rapid than the decrease—as in
+most short period variables—but in other respects the character
+of the light fluctuations does not seem to be similar to
+that of any other known variable star.</p>
+
+<p class='c007'>Another fine object of this class is that known as 15
+Messier in Pegasus, discovered by Maraldi in 1745. Sir
+John Herschel describes it as a remarkable globular cluster,
+very bright and large, and blazing in the centre. Webb found
+it a glorious object with a nine and one-third inch mirror.
+It was photographed by Dr. Roberts in November, 1890, with
+an exposure of two hours. He says: “The photograph
+confirms the general descriptions, and the negative shows,
+separately, the stars of which the cluster is composed distinctly
+through the nebulosity in the centre. Many of the stars
+have a nebulous appearance, and they are arranged in curves,
+lines, and patterns of various forms, with lanes or spaces
+between them.”</p>
+
+<p class='c007'>We may also mention the globular cluster known as 2
+Messier, which is situated about five degrees north of the
+star Beta Aquarii. It was discovered by Maraldi in 1746
+while looking for Cheseaux’s comet. Sir William Herschel,
+with his forty-foot telescope, could “actually see and distinguish
+<span class='pageno' id='Page_510'>510</span>the stars even in the central blaze.” Sir John
+Herschel compared it to a mass of luminous sand, and
+estimated the stars to be of the fifteenth magnitude. It is
+about five or six minutes of arc in diameter, and Smyth says:
+“This magnificent ball of stars condenses to the centre, and
+presents so fine a spherical figure that imagination cannot but
+figure the inconceivable brilliancy of the visible heavens to its
+animated myriads.” Taking Sir John Herschel’s estimate of
+the component stars at fifteenth magnitude, and the total
+light of the cluster at sixth magnitude, I find that the total
+number of stars it contains would be about 4,000.</p>
+
+<div id='i_512'  class='figcenter id001'>
+<img src='images/i_512.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 13.</span>—<i>The Star Cluster, Omega Centauri.</i><br>  <br>  (From “Worlds of Space.”)</p>
+</div>
+</div>
+
+<p class='c007'>In the Southern Hemisphere there are some magnificent
+examples of globular clusters, and indeed, this hemisphere
+<span class='pageno' id='Page_511'>511</span>seems to be richer in these objects than the northern sky.
+Among these southern clusters is the truly marvellous object
+known as Omega Centauri. Its apparent size is very large—about
+two-thirds of the moon’s diameter—and it is distinctly
+visible to the naked eye as a hazy star of the fourth magnitude,
+and I have often so seen it in the Punjab sky. Sir John
+Herschel, observing it with a large telescope at the Cape of
+Good Hope, describes it as “beyond all comparison, the
+richest and largest object of its kind in the heavens. The
+stars are literally innumerable.... All clearly resolved into
+stars of two sizes, <i>viz.</i>, 13 and 15; the larger lying in
+lines and ridges over the smaller.... The larger form rings
+like lace-work on it. One of these rings, 1½″ diameter, is so
+marked as to give the appearance of comparative darkness,
+like a hole in the centre.... On further attention, the hole
+is double, or an oval space crossed by a bridge of stars....
+Altogether, this object is truly astonishing.” This wonderful
+object has recently been photographed by Dr. Gill, at the
+Royal Observatory, Cape of Good Hope, and also at Arequipa,
+Peru, with a telescope of thirteen inches aperture. On the
+latter photograph, the individual stars can be distinctly seen
+and counted. The enumeration has been made by Professor
+and Mrs. Bailey, and a mean of their counts gives 6,389 for
+the number of stars in the cluster, but they consider that the
+real number is considerably greater.</p>
+
+<p class='c007'>Another wonderful object is that known as 41 Toucani,
+which lies near the smaller “Magellanic Cloud” in the
+Southern Hemisphere. Humboldt found it very visible to
+the naked eye in Peru, and mistook it for a comet.<a id='r133'></a><a href='#f133' class='c027'><sup>[133]</sup></a> Sir John
+Herschel describes it as “a most magnificent globular cluster.
+It fills the field with its outskirts; but within its more compressed
+part I can insulate a tolerably defined circular space
+of 90″ diameter, wherein the compression is much more
+decided, and the stars seem to run together, and this part has,
+I think, a pale pinkish or rose colour,&#160;... which contrasts
+evidently with the white light of the rest.... The stars are
+<span class='pageno' id='Page_512'>512</span>equal, fourteen magnitude, immensely numerous, and compressed....
+It is <i>completely insulated</i>. After it has passed,
+the ground of the sky is perfectly black throughout the whole
+breadth of the sweep. There is a double star of eleventh
+magnitude preceding the centre,&#160;... condensation in three
+distinct stages.... A stupendous object.” Dr. Gould calls
+it one of the most impressive, and perhaps the grandest, of its
+kind in either hemisphere, and he estimated its apparent
+magnitude at 4½, as seen with the naked eye.</p>
+
+<p class='c007'>Another remarkable globular cluster is that known as 22
+Messier, which lies about midway between Mu and Sigma
+Sagittarii. Sir John Herschel says: “The stars are of two
+sizes, <i>viz.</i>, 15&#160;... 16 and 12m; and, what is very remarkable,
+the largest of these latter are visibly reddish, one in particular,
+the largest of all (12–11m) south following the middle, is
+decidedly a ruddy star, and so, I think, are all the other
+larger ones&#160;... very rich, very much compressed, gradually
+much brighter in the middle, but not to a nucleus&#160;... consists
+of stars of two sizes&#160;... with none intermediate, as if
+consisting of two layers, or one shell over another. A noble
+object” I saw the larger stars well with a 3-inch refractor in
+the Punjab.</p>
+
+<p class='c007'>Sir John Herschel remarks “the frequent association of
+nebulæ in pairs forming double nebulæ,” and in his “Cape
+Observations” he figures several examples of this class. One
+of these is evidently a globular cluster, with two centres of
+condensation, one nucleus being much brighter than the other.
+Two others, much smaller, show two distinct nuclei. Another
+drawing shows apparently two globular clusters in contact.
+There are other examples in the Northern Hemisphere. Dr.
+See considers that some of these double nebulæ represent an
+early stage in the evolution of binary or revolving double
+stars, and certainly some of the drawings of these nebulæ are
+very remarkable and suggestive.</p>
+
+<p class='c007'>The actual dimensions of the globular clusters is an interesting
+question. Are they composed of stars comparable in size
+and mass with our sun? or are the component stars really
+<span class='pageno' id='Page_513'>513</span>small and comparatively close together? This is a difficult
+question to answer satisfactorily, as the distance of these
+objects from the earth has not yet been determined. They
+may, on the one hand, be collections of suns similar to ours in
+size and brightness, and situated at vast distances from the
+earth; or, on the other hand, the stars composing them may
+be comparatively small objects, lying at a distance from the
+earth not exceeding that of some stars visible to the naked
+eye. Perhaps the latter hypothesis may be considered the
+more probable of the two. But there is really no reason to
+suppose that these collections of suns are comparatively near
+our system. The probability seems to be in favour of their
+great distance from the earth. The question of the absolute
+size of the component stars is one which, I think, has not been
+hitherto sufficiently considered. Let us examine both alternatives,
+and let us take the cluster Omega Centauri as one in
+which the number of the component stars has been <i>actually
+counted</i>. Assuming that the real number of stars in this
+cluster is 10,000, and that they are individually equal,
+on an average, to our sun in mass and volume, we may
+estimate the probable distance and dimensions of the cluster.
+Taking the stellar magnitude of Omega Centauri as four (as
+estimated at the Cordoba Observatory), I find that, with the
+number 10,000, the average magnitude of the component
+stars would be fourteen. This agrees with Sir John Herschel’s
+estimate of thirteenth to fifteenth magnitude. Now, to reduce
+the sun to a star of the fourteenth magnitude, I find that,
+assuming the sun to be 28 magnitudes brighter than an average
+star of the first magnitude, it would be necessary to remove
+it to a distance of about 158,500,000 times the sun’s distance
+from the earth—a distance so great that light would take no
+less than 2,500 years to reach us from the cluster! Taking
+the apparent diameter of the cluster at twenty minutes of arc,
+I find that its real diameter would be 922,000 times the sun’s
+distance from the earth—a distance so great that light would
+take over 14 years to pass across the cluster. These results
+are certainly very startling, and might lead us to suspect
+<span class='pageno' id='Page_514'>514</span>that these globular clusters are external universes.
+Judging, however, from the average distance recently found
+for stars of the first and second magnitude (see p. 423), the
+distance of ordinary stars of the first magnitude—on the
+supposition that they are of the same size and brightness as
+the sun, and that their light is simply reduced by distance—would
+be about five times greater than that found above for
+Omega Centauri. If, then, we increase the distance of the
+cluster five times, it would be necessary to increase the diameters
+of the component stars to five times that of the sun.
+This would give them a volume 125 times that of our sun—a
+result which seems improbable. If, on the other hand,
+we do not like to admit that each of the faint points of light
+composing the cluster is equal in volume to our sun, let us
+diminish the distance ten times. If we do so, we must
+also diminish the diameter of the component stars ten times.
+This would make them about the size of the planet Jupiter,
+and it seems improbable that such comparatively small bodies
+could retain their solar heat for any great length of time.
+They would probably have cooled down, as Jupiter has done—at
+least to a great extent—ages ago, and would not now be
+visible as a cluster of stars. Even this reduction of the
+distance to one-tenth of the value first found would still leave
+the cluster at an immense distance from the earth, a distance
+represented by 250 years of light travel! A reduction of
+the distance to one-tenth of this again, or 25 years of light
+travel, would make the components about the size of the
+earth, and that bodies of this small size could shine with
+stellar light seems to be an untenable hypothesis. We seem,
+therefore, forced to conclude that these globular star clusters
+lie at an immense distance from the earth.</p>
+
+<p class='c007'>There is, however, another point to be considered with
+reference to the size of the bodies composing a globular
+cluster. This is the character of their light. I am not
+aware that the spectrum of a globular cluster has yet been
+thoroughly examined, but if that of Omega Centauri is of the
+first or Sirian type, it would modify the above conclusions to
+<span class='pageno' id='Page_515'>515</span>some extent. It now seems probable that stars having a
+spectrum of the Sirian type are intrinsically brighter than
+our sun, and I have shown already that Sirius is considerably
+brighter than the sun would be if placed at the same distance,
+although the mass of Sirius is but little more than twice the
+sun’s mass. The components of a star cluster, therefore—if
+of the Sirian type of stars—might be as bright as the sun, and
+at the same time have a smaller mass and volume. This, however,
+would not make a very great difference in the computed
+vast distance of the cluster, and the calculations given above
+seem to point to the conclusion that these globular clusters
+are probably composed of stars of average size and mass, and
+that the faintness of the component stars is simply due to
+their immense distance from the earth.</p>
+
+<p class='c007'>We will now consider the nebulæ, properly so-called, that
+is to say, objects which the spectroscope shows to consist of
+glowing gas. These are sometimes large and irregular in
+form, like the great nebula in the “Sword” of Orion, sometimes
+with spiral convolutions, and sometimes of a definite
+shape, like the planetary and annular nebulæ.</p>
+
+<p class='c007'>Of the large and irregular nebulæ, one of the most remarkable
+is that known as “the great nebula in Orion.” It surrounds
+the multiple star, Theta Orionis, which has been
+already referred to in a preceding chapter. It is a curious
+fact that it escaped the searching eye of Galileo, although he
+gave special attention to the constellation of Orion, for even
+with a good opera-glass a nebulous gleam is distinctly visible
+round the central star of the “Sword.” The nebula seems to
+have been discovered by Cysat, a Swiss astronomer, in the
+year 1618, and it was sketched by Huygens in 1656.
+Huygens says: “While I was observing with a refractor of
+twenty-five feet focal length, the variable belts of Jupiter, a
+dark central belt in Mars, and some phases of this planet, my
+attention was attracted by an appearance among the fixed
+stars, which, as far as I know, has not been observed by anyone
+else, and which, indeed, could not be recognised, except
+by such powerful instruments as I employ. Astronomers
+<span class='pageno' id='Page_516'>516</span>enumerate three stars in the Sword of Orion, lying very near
+one another. On one occasion when, in 1656, I was accidentally
+observing the middle one of these stars through my
+telescope, I saw twelve stars instead of a single one, which,
+indeed, not unfrequently happens in using the telescope.
+Three of this number were almost in contact with one
+another, and <i>four</i> of them shone as if through a mist, so that
+the space around them, having the form drawn in the
+appended figure, appeared much brighter than the rest of the
+sky, which was perfectly clear, and looked almost black.
+This appearance looked, therefore, almost as if there were a
+<i>hiatus</i> or interruption. I have frequently observed this
+phenomenon, and up to the present time, as always unchanged
+in form; whence it would appear that this marvellous object,
+be its nature what it may be, is very probably permanently
+situated at this spot. I never observed anything similar to
+this appearance in the other fixed stars.”<a id='r134'></a><a href='#f134' class='c027'><sup>[134]</sup></a> It has been called
+the “fish-mouth” nebula, from the fancied resemblance of
+the centre portion to the mouth of a fish. A number of small
+stars are visible over the surface of the nebula, and at one
+time, Lord Rosse thought it showed indications of resolution
+into stars when examined with his giant telescope; but this is
+now known to have been a mistake, for Dr. Huggins finds,
+with the spectroscope, that it consists of nothing but glowing
+gas, of which hydrogen is certainly one constituent, and he
+has succeeded in photographing the complete series of lines of
+this gas in the spectrum of the nebula.</p>
+
+<p class='c007'>Referring to his earlier observations, Dr. Huggins says:—“The
+light from the brightest parts of the nebula near the
+trapezium was resolved by the prisms into three bright lines,
+in all respects similar to those of the gaseous nebulæ. The
+whole of this great nebula, as far as lies within the power of
+my instrument, emits light which is identical in character.
+The light from one part differs from the light of another in
+intensity alone.” The brightest line in the nebular spectrum—the
+“chief nebular line,” as it is called—has not yet been
+<span class='pageno' id='Page_517'>517</span>identified with that of any terrestrial substance. It was at
+first supposed to be identical with a line of nitrogen, but this
+was afterwards disproved. It was then incorrectly identified
+with a line of lead, and more recently by Lockyer with the
+edge of a “fluting” in the magnesium spectrum. Dr.
+Huggins and Professor Keeler, however, have shown conclusively
+that the nebular line does not coincide with the
+magnesium fluting, although very close to it. Observations
+by Dr. Copeland in 1886 showed the existence of the yellow
+line, know as D<sub>3</sub>, which is visible in the solar spectrum during
+total eclipses of the sun, and indicates the existence of a gas
+in the sun’s surroundings, to which the name “helium” has
+been given. Dr. Copeland says:—“The recurrence of this line
+in the spectrum of a nebula is of great interest, as affording
+another connecting link between gaseous nebula and the sun
+and stars with bright line spectra, especially with that remarkable
+class of stars of which the finest examples were detected
+by M. M. Wolf and Rayet in the constellation of Cygnus.”<a id='r135'></a><a href='#f135' class='c027'><sup>[135]</sup></a>
+As has been already mentioned in the chapter on variable
+and new stars, the bright lines of hydrogen and helium have
+also been observed in the spectra of these remarkable objects.
+The gas, giving the line D<sub>3</sub> in its spectrum, has quite recently
+been discovered by Professor Ramsay in gases obtained by
+heating certain terrestrial minerals, so that the objective
+existence of the gaseous element “helium”—previously only
+suspected—is now definitely established. From recent
+spectroscopic observations of the Orion nebula, Dr. Huggins
+thinks that “the stars of the ‘trapezium’ are not merely
+optically connected with the nebula, but are physically bound
+up with it, and are very probably condensed out of the
+gaseous matter of the nebula.” With reference to this point,
+Professor Keeler, who has carefully examined the spectra of
+the nebula and the associated stars, says:—“The trapezium
+stars have spectra marked by strong absorption bands; they
+have not the direct connexion with the nebula that would be
+indicated by a bright line spectrum, but are, in fact, on
+<span class='pageno' id='Page_518'>518</span>precisely the same footing (spectroscopically) as other stars
+in the constellation of Orion. While their relation to the
+nebula is more certain than ever, they can no longer be regarded
+as necessarily situated <i>in</i> the nebula, but within
+indefinite limits they may be placed anywhere in the line of
+sight.” These results were confirmed by Professor Campbell.
+He finds, “that of the twenty-five bright lines known to exist
+in the spectrum of the Orion nebula, at least nineteen are
+definitely matched by dark lines in the Orion stars, and at
+least fifteen by dark lines in the six faint stars situated in the
+dense parts of the nebula.”</p>
+
+<p class='c007'>Numerous drawings of this wonderful nebula have been
+made. Of these, the best are those by Sir John Herschel,
+made at the Cape of Good Hope in the years 1834–38, by
+Bond in America, and by Lassell at Malta. The difficulty of
+accurately delineating so difficult and delicate an object has
+given rise to discrepancies in the drawings, which have led to
+the idea that changes of form have occurred, but this seems
+improbable. The nebula has been very successfully photographed
+by Dr. Common and Dr. Roberts, and these photographs
+confirm the general accuracy of the later drawings.</p>
+
+<p class='c007'>From a consideration of the apparent size of the Orion
+nebula and its probable mass and distance from the earth, the
+late Mr. Ranyard came to the conclusion that its average
+density “cannot exceed one ten thousand millionth of the
+density of atmospheric air at the sea-level.”<a id='r136'></a><a href='#f136' class='c027'><sup>[136]</sup></a></p>
+
+<p class='c007'>Mr. W. H. Pickering and Dr. Max Wolf have photographed
+another nebula surrounding the star Zeta Orionis—the
+southern star of the “Belt,” which seems to be connected with
+the nebula in the “Sword”; and, Prof. Barnard, using the
+“lens of a cheap oil lantern” of 1½ inch aperture, and 3½ inches
+focal length, has photographed “an enormous curved nebulosity”
+stretching over nearly the whole of the constellation of
+Orion, and involving the “great nebula.”</p>
+
+<div id='i_521'  class='figcenter id001'>
+<span class='pageno' id='Page_519'>519</span>
+<img src='images/i_521.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 14.</span>—<i>The Orion Nebulæ.</i><br>  <br>  (From “Worlds of Space.”)</p>
+</div>
+</div>
+
+<p class='c007'>Prof. Keeler has recently found, with the spectroscope, that
+the Orion nebula is apparently receding from the earth at the
+rate of nearly eleven miles a second, but this motion may be,
+in part at least, due to the sun’s motion in space in the
+opposite direction. Prof. Pickering considers that the parallax
+<span class='pageno' id='Page_520'>520</span>of the nebula is probably not more than 0·″003, which corresponds
+to a thousand years’ journey for light!</p>
+
+<p class='c007'>In the southern constellation, Argo is a magnificent nebula,
+somewhat similar in appearance to the great nebula in Orion.
+It surrounds the famous variable star Eta Argûs, whose remarkable
+fluctuations in light have been already described in
+the chapter on variable stars. It is sometimes spoken of as
+the “key-hole” nebula, owing to a curious opening of that
+shape near its centre. It was carefully drawn by Sir John
+Herschel at the Cape of Good Hope in the years 1834–38.
+It lies in a very brilliant portion of the Milky Way, and Sir
+John Herschel thus describes it: “It is not easy for language
+to convey a full impression of the beauty and sublimity of the
+spectacle which the nebula offers as it enters the field of view
+of a telescope, fixed in right ascension, by the diurnal motion,
+ushered in as it is by so glorious and innumerable a procession
+of stars, to which it forms a sort of climax, and in a part of
+the heavens otherwise full of interest,” and he adds: “In no
+part of its extent does this nebula show any appearance of
+resolvability into stars, being, in this respect, analogous to the
+nebula of Orion. It has, therefore, nothing in common with
+the Milky Way, on the ground of which we see it projected,
+and may therefore be, and not improbably is, placed at an
+immeasurable distance behind that stratum.” Sir John
+Herschel’s conclusion as to its physical constitution has been
+fully confirmed by the spectroscope, which shows it to consist
+of luminous gas. As in the Orion nebula, there are numerous
+stars scattered over it. Some of these may possibly have a
+physical connexion with the nebula, while others may belong
+to the Milky Way. The nebula is of great extent, covering
+an apparent space about five times the area of the full moon,
+and its real dimensions must be enormous. It was photographed
+by Mr. Russell, director of the Sydney Observatory,
+in July, 1890, and the photograph shows that “one of the
+brightest and most conspicuous parts of the nebula”—the
+swan-shaped form near the centre of Herschel’s drawing—has
+“wholly disappeared,” and its place is now occupied by “a
+<span class='pageno' id='Page_521'>521</span>great, dark oval.” Mr. Russell first missed the vanished
+portion of the nebula in the year 1871, while examining it with
+a telescope of 11½ inches aperture, and the photograph now
+confirms the disappearance, which is very remarkable, and
+shows that changes are actually in progress in these wonderful
+nebulæ, changes which may be detected after a comparatively
+short interval of time.</p>
+
+<div id='i_523'  class='figcenter id001'>
+<img src='images/i_523.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 15.</span>—<i>Sir John Herschel’s drawing of the Nebula round Eta Argus.</i><br>  <br>  (From Flammarion’s “Popular Astronomy.”)</p>
+</div>
+</div>
+
+<p class='c007'><span class='pageno' id='Page_522'>522</span>Smaller than the nebula in Argo, but somewhat similar in
+general appearance, is that known as 30 Doradus, which forms
+one of the numerous and diverse objects which together constitute
+the greater Magellanic Cloud. Sir John Herschel
+drew it carefully at the Cape of Good Hope, and describes it
+as “one of the most singular and extraordinary objects which
+the heavens present,” and he says “it is unique even in the
+system to which it belongs, there being no other object in
+either nubecula to which it bears the least resemblance.” It
+is sometimes called the “looped nebula,” from the curious
+openings it contains. One of these is somewhat similar to the
+“key-hole” opening in the Argo nebula. Near its centre is a
+small cluster of stars, and scattered over the nebula are many
+faint stars, of which Sir John Herschel gives a catalogue of
+105 ranging from the ninth to the seventeenth magnitude. I
+do not know whether this nebula has been examined with the
+spectroscope, but its appearance would suggest that it is
+gaseous. It is remarkable as being the only object of its class
+which is found outside the zone of the Milky Way.</p>
+
+<p class='c007'>Among the nebula of irregular shape, although its spectrum
+is said to be not gaseous, may be mentioned that known as
+the “trifid nebula,” or 20 Messier. It lies closely north of the
+star 4 Sagittarii in a magnificent region of the heavens. As
+will be seen in the drawing made by Sir John Herschel at the
+Cape of Good Hope, the principal portion consists of three
+masses of nebulous matter separated by dark “lanes” or
+“rifts.” Near the junction of the three “rifts” is a triple star.
+A beautiful drawing of this nebula has also been made by
+Trouvelot. It agrees fairly well with that of Sir John Herschel,
+but shows more detail.</p>
+
+<div id='i_525'  class='figcenter id001'>
+<span class='pageno' id='Page_523'>523</span>
+<img src='images/i_525.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 16.</span>—<i>The Trifid Nebula, Sagittarius.</i><br>  <br>  (From “Scenery of the Heavens.”)</p>
+</div>
+</div>
+
+<p class='c007'>Among other gaseous nebula may be mentioned that called
+by Sir John Herschel the “dumb-bell” nebula. It lies a little
+south of the sixth magnitude star 14 Vulpeculæ, and was discovered
+by Messier in 1779, while observing Bode’s comet of
+that year. In small telescopes it has the appearance of a
+dumb-bell, or hour-glass, but in larger telescopes the outline is
+filled in with fainter nebulous light, giving to the whole an
+elliptical form. Several faint stars have been seen in it, but
+these probably belong to the Milky Way, as Dr. Huggins
+finds the spectrum gaseous. Dr. Roberts has photographed
+<span class='pageno' id='Page_524'>524</span>it, and he thinks that “the nebula is probably a globular mass
+of nebular matter, which is undergoing the process of condensation
+into stars, and the faint protrusions of nebulosity in the
+<i>south following</i> and <i>north preceding</i> ends are the projections of
+a broad ring of nebulosity which surrounds the globular mass.
+This ring, not being sufficiently dense to obscure the light of
+the central region of the globular mass, is dense enough to obscure
+those parts of it that are hidden by the increased
+thickness of the nebulosity, thus producing the ‘dumb-bell’
+appearance. If these inferences are true, we may proceed yet
+a step, or a series of steps, farther, and predict that the consummation
+of the life-history of this nebula will be its
+reduction to a globular cluster of stars.”</p>
+
+<p class='c007'>Among the gaseous nebula may also be included those
+known as “annular nebulæ.” These are very rare objects,
+only a few being known in the whole heavens. The most
+remarkable is that known as 57 Messier, which lies between
+the stars Beta and Gamma Lyræ, south of the bright star
+Vega. It was discovered by Darquier, at Toulouse, in 1779,
+while following Bode’s comet of that year. Lord Rosse
+thought it resolvable into stars, and so did Chacornac and
+Secchi, but no stars are perceptible with the great American
+telescopes, and Dr. Huggins finds it to be gaseous. The
+central portion is not absolutely dark, but contains some faint
+nebulous light. Examined with the great telescope of the
+Lick Observatory, Professor Barnard finds that the opening of
+the ring is filled in with fainter light “about midway in brightness
+between the brightness of the ring and the darkness of
+the adjacent sky.”<a id='r137'></a><a href='#f137' class='c027'><sup>[137]</sup></a> “The aperture was more nearly circular
+than the outer boundary of the nebula, so that the ends of the
+ring were thicker than the sides.” The entire nebula was of a
+milky colour. A central star, noticed by some observers, was
+usually seen by Professor Barnard, but was never a conspicuous
+object. He found the extreme dimensions of the nebula
+about 81″ in length by about 59″ in width, or more than
+double the apparent area of Jupiter’s disc. It has been
+<span class='pageno' id='Page_525'>525</span>beautifully photographed by Dr. Roberts, and he says “the
+photograph shows the nebula and the interior of the ring more
+elliptical than the drawings and descriptions indicate; and the
+star of the <i>following</i> side is nearer to the ring than the distance
+given. The nebulosity on the <i>preceding</i> and <i>following</i> ends of
+the ring protrudes a little, and is less dense than on the <i>north</i>
+and <i>south</i> sides. This probably suggested the filamentous appearance
+which Lord Rosse shows. Some photographs of the
+nebula have been taken between 1887 and 1891, and the
+central star is strongly shown on some of them, but on others
+it is scarcely visible, which points to the star being variable.”
+On a photograph taken by MM. Androyer and Montaugerand
+of the Toulouse Observatory, with an exposure of nine
+hours (in multiple exposures), about 4,800 stars are visible on
+and near the nebula in an area of three square degrees.</p>
+
+<p class='c007'>Another object of the annular class will be found a little to
+the south-west of the star Lambda Scorpii. It is thus
+described by Sir John Herschel: “A delicate, extremely faint,
+but perfectly well defined, annulus. The field crowded with
+stars, two of which are on the nebula. A beautiful, delicate
+ring, of a faint, ghost-like appearance, about 40″ in diameter
+in a field of about 150 stars, eleven and twelve magnitude
+and under.”</p>
+
+<p class='c007'>Near the stars 44 and 51 Ophiuchi is another object of the
+annular class, which Sir John Herschel describes as “exactly
+round, pretty faint, 12″ diameter, well terminated, but a little
+cottony at the edge, and with a decided darkness in the
+middle, equal to a tenth magnitude star at the most. Few
+stars in the field, a beautiful specimen of the planetary annular
+class of nebula.”</p>
+
+<p class='c007'>The Planetary Nebulæ form an interesting class. They
+were so named by Sir William Herschel from their resemblance
+to the discs of the planets, but, of course, much fainter. They
+are generally of uniform brightness, without any nucleus or
+brighter part in the centre. There are numerous examples of
+this class, one of the most remarkable being that known as
+97 Messier, which is situated about two degrees south-east of
+<span class='pageno' id='Page_526'>526</span>Beta Ursæ Majoris—the southern of the two “pointers” in
+the Plough. It is of considerable apparent size, and even
+supposing its distance to be not greater than that of 61 Cygni,
+its real dimensions must be enormous. Lord Rosse observed
+two openings in the centre with a star in each opening, and
+from this appearance he called it the “owl nebula.” One of
+the stars seems to have disappeared since 1850, and a photograph
+recently taken by Dr. Roberts confirms the disappearance.</p>
+
+<p class='c007'>Another fine object of the planetary class is one which lies
+close to the pole of the ecliptic. Webb saw it “like a considerable
+star out of focus.” Smyth found it pale blue in
+colour. Dr. Huggins finds a gaseous spectrum, the first discovery
+of the kind made. Professor Holden, observing it with
+the great Lick telescope, finds its structure extraordinary.
+He says it “is apparently composed of rings overlying each
+other, and it is difficult to resist the conviction that these are
+arranged in space in the form of a true helix,” and he ranks
+it in a new class which he calls “helical nebulæ.”</p>
+
+<p class='c007'>A somewhat similar nebula lies a little to the west of the
+star Nu Aquarii. Secchi believed it to be in reality a cluster
+of small stars, but Dr. Huggins finds its spectrum gaseous.
+A small nebula on each side gives it an appearance somewhat
+similar to the planet Saturn, with the rings seen edgeways.
+The great Lick telescope shows it as a wonderful object—“a
+central ring lies upon an oval of much fainter nebulosity.”
+Professor Holden says “the colour is a pale blue,” and he
+compares the appearance of the central ring “to that of a footprint
+left in the wet sand on a sea beach.”</p>
+
+<p class='c007'>About two degrees south of the star Mu Hydræ is another
+planetary nebula, which Smyth describes as resembling the
+planet Jupiter in “size, equable light and colour.” Webb saw
+it of “a steady, pale blue light,” and Sir John Herschel, at the
+Cape of Good Hope, speaks of its colour as “a decided blue—at
+all events, a good sky-blue,” a colour which seems characteristic
+of these curious objects. Although Sir William
+Herschel, with his large telescopes, failed to resolve it into
+<span class='pageno' id='Page_527'>527</span>stars, Secchi thought he saw it breaking up into stars with a
+“sparkling ring.” Dr. Huggins, however, finds the spectrum
+to be gaseous, so that the luminous points seen by Secchi
+could not have been stellar.</p>
+
+<p class='c007'>Sir John Herschel, in his “Cape Observations,” describes a
+planetary nebula which lies between the stars Pi Centauri and
+Delta Crucis. He says it is “perfectly round, very planetary,
+colour fine blue&#160;... very like Uranus, only about half as
+large again, and blue.... It is of the most decided independent
+blue colour when in the field by itself, and with no lamplight
+and no bright star. About 10′ north of it is an orange-coloured
+star, eighth magnitude. When this is brought into
+view, the blue colour of the nebula becomes intense&#160;...
+colour, a beautiful rich blue, between Prussian blue and
+verditer green.”</p>
+
+<p class='c007'>There are some rare objects called “nebulous stars.” The
+star Epsilon Orionis—the centre star of Orion’s Belt—is involved
+in a great nebulous atmosphere. The triple star Iota
+Orionis is surrounded by a nebulous haze. The star Beta in
+Canes Venatici is a 4½ magnitude star surrounded by a
+nebulous atmosphere.</p>
+
+<p class='c007'>The term elliptical nebulæ has been applied to those of an
+elliptical or elongated shape. This form is probably due in
+many cases to the effect of perspective, their real shape being
+circular, or nearly so. Perhaps the most remarkable object
+of this class is the well-known “nebula in Andromeda,”
+known to astronomers as 31 Messier. It can be just seen with
+the naked eye, on a clear moonless night, as a hazy spot of
+light near the star Nu Andromedæ, and it is curious that it is
+not mentioned by the ancients, although it must have been
+very visible to their keen eyesight in the clear Eastern skies.
+It was, however, certainly seen so far back as 905 <span class='fss'>A.D.</span>, and it
+Is referred to as a familiar object by the Persian astronomer,
+Al-Sûfi, who wrote a description of the heavens about the
+middle of the tenth century. Tycho Brahé and Bayer failed
+to notice it, but Simon Marius saw it in December, 1612, and
+described it “as a light seen from a great distance through
+<span class='pageno' id='Page_528'>528</span>half-transparent horn plates.” It was also observed by
+Bullialdus, in 1664, while following the comet of that year.
+It has frequently been mistaken for a comet by amateur observers
+in recent years. Closely north-west of the great
+nebula is a smaller one discovered by Le Gentil in 1749, and
+another to the south, detected by Miss Caroline Herschel in
+1783. The great nebula is of an elliptical shape and considerable
+apparent size. The American astronomer, Bond,
+using a telescope of 15 inches aperture, traced it to a length
+of about four degrees, and a width of two and a half degrees.
+A beautiful photograph taken by Dr. Roberts in December, 1888
+(see p. 398), shows an extension of nearly two degrees in length,
+and about half a degree in width, or considerably larger than
+the apparent size of the full moon. Bond could not see any
+symptom of resolution into stars, but noticed two dark rifts or
+channels running nearly parallel to the length of the nebula.
+In Dr. Roberts’ photograph these rifts are seen to be really
+dark intervals between consecutive nebulous rings into which
+the nebula is divided. Dr. Roberts says: “A photograph
+which I took with the 20-inch reflector on October 10, 1887,
+revealed for the first time the true character of the great
+nebula, and one of the features exhibited was that the dark
+bands, referred to by Bond, formed parts of divisions between
+symmetrical rings of nebulous matter surrounding the large
+diffuse centre of the nebula. Other photographs were taken
+in 1887, November 15; 1888, October 1; 1888, October 2;
+1888, December 29; besides several others taken since, upon
+all of which the rings of nebulosity are identically shown, and
+thus the photographs confirm the accuracy of each other, and
+the objective reality of the details shown of the structure of
+the nebula.” Dr. Roberts adds: “These photographs throw
+a strong light on the probable truth of the <i>Nebular Hypothesis</i>,
+for they show what appears to be the progressive evolution of
+a gigantic stellar system.”</p>
+
+<p class='c007'>The largest telescopes have hitherto completely failed to
+resolve this wonderful object into stars. Dr. Huggins, however,
+finds that the spectrum is <i>not</i> gaseous, so that if the
+<span class='pageno' id='Page_529'>529</span>nebula really consists of stellar points, they must be of very
+small dimensions. Assuming a parallax of one-fiftieth of a
+second of arc—corresponding to 163 years of “light travel”—I
+find that our sun, placed at this distance, would be reduced
+in brightness to a star of about the eighth magnitude. If we
+assume the components to have only one-hundredth of the
+sun’s diameter, they would shine as stars of only the eighteenth
+magnitude, which no telescope yet constructed would show as
+separate points of light. A more probable explanation, however,
+seems to be that the nebula may consist of masses of
+nebulous matter partially condensed into the solid form, but
+not yet arrived at the stage in which our sun is at present.
+In other words, the whole nebulous mass may be in a fluid or
+viscous state, which might perhaps account for the continuous
+spectrum found by Dr. Huggins.</p>
+
+<p class='c007'>The question may be asked, What is the probable size and
+distance of this wonderful nebula? and could it be an
+external universe? Possibly its distance from the earth may
+be even greater than that indicated by the small parallax I
+have assumed above, but taking this parallax and the
+apparent dimensions of the nebula as shown by Dr. Huggins’
+photograph, I find that its real distance would be no less than
+330,000 times the sun’s diameter from the earth, a diameter so
+great that light would take over five years to pass from one
+side of the nebula to the other! This result might lead us to
+imagine that the nebula may be really an external universe.
+But let us consider the matter a little further. The diameter
+found above is not very much greater than the distance of
+the <i>nearest</i> fixed star, Alpha Centauri, from the earth, and
+the limits of <i>our</i> universe are certainly far beyond Alpha
+Centauri. If we diminish the parallax to, say ¹⁄₂₀₀th of a
+second, or a “light journey” of 652 years, the diameter of the
+nebula would be increased to 1,320,000 times the sun’s
+distance from the earth, or about five times the distance of
+Alpha Centauri, and there are probably many faint stars
+belonging to our system much farther from the earth than
+this.</p>
+
+<p class='c007'><span class='pageno' id='Page_530'>530</span>The temporary star which appeared near the nucleus of the
+nebula in August, 1885—already referred to in the chapter
+on variable stars—was of the seventh magnitude. I find that
+our sun, if placed at the distance indicated by a parallax of
+¹⁄₂₀₀th of a second, would be reduced to a star of about
+the eleventh magnitude, or four magnitudes fainter than
+the temporary star appeared to us. That is to say, the star
+would have been—with the assumed distance—about forty
+times brighter than the sun. With any greater distance, the
+star would have been proportionately brighter, compared with
+the sun. This seems improbable, and tends to the conclusion
+that the nebula is <i>not</i> an external galaxy, but a member of
+our own sidereal system, a system which probably includes
+all the stars and nebulæ visible in our largest telescopes.
+Dr. Common, indeed, suggests that it may be comparatively
+near our system. He says: “It is difficult to imagine that
+such an enormous object, as the Andromeda nebula must be,
+is not very near to us; perhaps it may be found to be the
+nearest celestial object of all beyond the solar system. It is
+one that offers the best chance of the detection of parallax, as
+it seems to be projected on a crowd of stars, and there are
+well defined points that might be taken as fiducial points for
+measurement,” and he adds: “Apart from the great promise
+this nebula seems to give of determining parallax, there is a
+fair presumption that in the course of time, the rotation of the
+outer portion may perhaps be detected by observation of the
+positions of the two outer detached portions in relation to the
+neighbouring stars.”<a id='r138'></a><a href='#f138' class='c027'><sup>[138]</sup></a> Prof. Hall’s failure to detect any
+parallax in the temporary star, as mentioned in the last
+chapter, is, of course, against Dr. Common’s idea of its
+proximity to the earth. Referring to the latter portion of
+Dr. Common’s remarks, Mr. C. Easton points out<a id='r139'></a><a href='#f139' class='c027'><sup>[139]</sup></a> that a
+comparison of a drawing by Trouvelot, in 1874, with Dr.
+Roberts’ photograph, suggests that the small elongated nebula—<i>h</i>
+44—which lies to the north of the great nebula, “has
+<span class='pageno' id='Page_531'>531</span>turned about 15° from left to right. The globular nebula
+(M 32), to the other side of M 31, seems to have slightly
+shifted its position.”</p>
+
+<div id='i_533'  class='figcenter id001'>
+<img src='images/i_533.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 17.</span>—<i>Spiral Nebula, 51 Messier.</i><br>  <br>  (From “The Visible Universe.”)</p>
+</div>
+</div>
+
+<p class='c007'>The spiral nebulæ are wonderful objects, and were discovered
+by the late Lord Rosse, with his great six-foot
+telescope. Their character has been fully confirmed by photographs
+taken by Dr. Roberts. One of the most remarkable
+of these extraordinary objects is that known as 51 Messier.
+It lies about three degrees south-west of the bright star Eta
+Ursæ Majoris—the star at the end of the Great Bear’s tail.
+It was discovered by Messier while comet-hunting on October
+13, 1773. Telescopes of moderate power merely show two
+<span class='pageno' id='Page_532'>532</span>nebulæ nearly in contact, but Lord Rosse saw it as a wonderful
+spiral, and his drawing agrees fairly well with a photograph
+taken by Dr. Roberts in April, 1889. The nebula has also
+been photographed by Dr. Common. Dr. Roberts says:
+“The photograph shows both nuclei of the nebula to be
+stellar, surrounded by dense nebulosity, and the convolutions
+of the spiral in this as in other spiral nebulæ are broken up
+into star-like condensations with nebulosity around them.
+Those stars that do not conform to the trends of the spiral
+have nebulous trails attached to them, and seem as if they
+had broken away from the spirals.” A tendency to a spiral
+structure in the smaller nebula is also visible on the original
+negative. Dr. Huggins finds that the spectrum is <i>not</i> gaseous.</p>
+
+<p class='c007'>The nebulæ known as 99 Messier is of the spiral form. It
+lies on the borders of Virgo and Coma Berenices, near the
+star 6 Comæ. In large telescopes it somewhat resembles a
+“Catherine wheel.” D’Arrest and Key thought it resolvable
+into stars. It has been photographed by M. Von Gothard.</p>
+
+<p class='c007'>Among the clusters and nebulæ, we may class the Magellanic
+Clouds, or Nubeculæ in the Southern Hemisphere, as
+they consist of stars, clusters, and nebulæ. These very remarkable
+objects form two bright spots of milky light, which,
+at first sight, look like luminous patches of the Milky Way,
+but are in no way connected with the Galaxy. Sir John
+Herschel, speaking of the larger cloud, says: “The immediate
+neighbourhood of the Nubecula Major is somewhat less
+barren of stars than that of the Minor, but it is by no means
+rich, nor does any branch of the Milky Way whatever form
+any certain or conspicuous junction with, or include, it,” and
+again he says, with reference to the smaller cloud: “Neither
+with the naked eye, nor with a telescope, is any connexion to
+be traced either with the greater Nubecula, or with the Milky
+Way.” The Nubeculæ are roughly circular in form, and,
+viewed with the naked eye, they very much resemble irresolvable
+nebulæ as seen in a telescope. The larger cloud,
+or Nubecula Major, as it is called, is of considerable extent,
+and covers about 42 square degrees, or over two hundred
+<span class='pageno' id='Page_533'>533</span>times the apparent size of the full moon. It was called by
+the Arabs <i>el-baker</i>, or “the White Ox,” and is referred to by
+Al-Sûfi in his “Description of the Heavens,” written in the
+tenth century. When examined with a good telescope, it is
+found to consist of about six hundred stars of the sixth to the
+tenth magnitude, with many fainter ones, and about three
+hundred clusters and nebulæ. Sir John Herschel, in his “Cape
+Observations,” says: “The Nubeculæ Major, like the Minor,
+consists partly of large tracts and ill-defined patches of irresolvable
+nebula, and of nebulosity in every stage of resolution,
+up to perfectly resolved stars like the Milky Way, as
+also of regular and irregular nebulæ properly so-called, of
+globular clusters in every stage of resolvability, and of clustering
+groups sufficiently insulated and condensed to come under
+the designation of ‘clusters of stars.’... It is evident, from
+the intermixture of stars and unresolved nebulosity, which
+probably might be resolved with a higher optical power, that
+the nubeculæ are to be regarded as systems <i>sui generis</i>, and
+which have no analogues in our hemisphere.”</p>
+
+<p class='c007'>The smaller Magellanic Cloud, or Nubecula Minor, is
+fainter to the eye, and not so rich in the telescope. It covers
+about 10 square degrees, or about fifty times the area of the full
+moon. Sir John Herschel, in his “Cape Observations,” describes
+it as “a fine large cluster of very small stars, 12&#160;... 18
+magnitude, which fills more than many fields, and is broken
+into many knots, groups, and straggling branches, but <i>the
+whole</i> (<i>i.e.</i>, the whole of the clustering part) is clearly resolved.”
+It is surrounded by a barren region remarkably devoid of
+stars. Sir John Herschel says: “The access to the Nubecula
+Minor is on all sides through a desert.”... “It is preceded
+at a few minutes in R. A. by the magnificent globular cluster,
+47 Toucani (Bode), but is completely cut off from all connexion
+with it; and with this exception, its situation is in one of the
+most barren regions in the heavens.” Herschel found the
+middle of the cloud clearly resolved into stars, while its edges
+remained irresolvable with his large reflector. He says:
+“The edge of the smaller <i>cloud</i> comes on as a mere nebula....
+<span class='pageno' id='Page_534'>534</span>We are now <i>in the cloud</i>. The field begins to be full of a
+faint light perfectly irresolvable.... I should consider about
+this place to be the body of the cloud which is here fairly
+resolved into excessively minute stars.... It is not like the
+stippled ground of the sky. The borders fade away, quite insensibly,
+and are less or not at all resolved.” Herschel gives
+a catalogue of 244 objects in the Nubecula Minor. Of these
+about 200 are stars, and the remainder nebula and clusters.
+From this it appears that the smaller nubecula contains a
+much larger proportion of stars than the larger cloud.</p>
+
+<p class='c007'>Judging from their roughly globular form, the dimensions
+of the Magellanic Clouds are probably small compared with
+their distance from the earth, so that in these remarkable
+objects—particularly in the larger cloud—we see stars of the
+seventh, eighth, ninth, and tenth magnitude, apparently mixed
+up with fainter stars, and “clusters of all degrees of resolvability,”
+and Sir John Herschel says: “It must therefore
+be taken as a demonstrated fact, that stars of the seventh or
+eighth magnitude, and irresolvable nebulæ, may co-exist within
+limits of distance not differing in proportion more than as 9
+to 10.”<a id='r140'></a><a href='#f140' class='c027'><sup>[140]</sup></a> It should be remembered, however, that possibly
+some of the fainter stars may—as in the Pleiades—lie far out
+in space beyond the greater Magellanic Cloud.</p>
+
+<p class='c007'>The Magellanic Clouds have recently been photographed by
+Mr. Russell at the Sydney Observatory. He finds the larger
+cloud—the Nubecula Major—to be of a most complex form,
+with evidence of a spiral structure, a feature also traceable, but
+not so clearly, in a photograph of the Nubecula Minor, or
+smaller cloud.</p>
+
+<p class='c007'>Dr. Dreyer’s new index catalogue of recent discoveries of
+nebulæ, together with the general catalogue previously published,
+gives the position of 9,369 nebulæ.<a id='r141'></a><a href='#f141' class='c027'><sup>[141]</sup></a> A very small proportion
+of the new discoveries have been made by photography,
+and more than half of them were found by M. Javelle
+with the great refractor of the Nice Observatory. Most of the
+<span class='pageno' id='Page_535'>535</span>new objects are very small and faint, and form probably “only
+a small portion of the number visible in large telescopes.”</p>
+
+<div id='i_537'  class='figcenter id001'>
+<img src='images/i_537.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 18.</span>—<i>Magellanic Clouds.</i><br>  <br>  (From “Worlds of Space.”)</p>
+</div>
+</div>
+
+<p class='c007'>Several nebulæ have been suspected of variation in light.
+One discovered by Dr. Hind in 1852 near the variable star T
+Tauri was found to be an easy object with the great Lick
+telescope in February, 1895, but in September of the same
+<span class='pageno' id='Page_536'>536</span>year it had “entirely vanished.” In the same instrument, “T
+Tauri was involved in a small hazy nebulosity, but the definite
+nebula in which it shone in 1890 did not exist in September,
+1895.”<a id='r142'></a><a href='#f142' class='c027'><sup>[142]</sup></a></p>
+
+<h3 class='c004'>CHAPTER VII.<br> <span class='c014'>THE CONSTRUCTION OF THE HEAVENS.</span></h3>
+
+<p class='c003'>The construction of the visible universe is one of great
+interest, but of considerable difficulty. If we reflect that in
+viewing the starry heavens we are placed at the centre of
+a hollow sphere of indefinite extent, and that the distance of
+only a few of the stars from the earth has hitherto been
+ascertained with any approach to accuracy, the great difficulty
+of framing a satisfactory theory of the construction of the
+heavens will be easily understood.</p>
+
+<p class='c007'>In considering the subject, let us first inquire as to the
+probable number of stars visible in our largest telescopes.
+Are the visible stars infinite or limited in number? The
+reply to this question is easy. As the number of stars visible
+to the naked eye is limited, so the number of stars visible in
+the largest telescopes is limited also. Those who do not give
+the subject sufficient consideration seem to think that the
+number of the stars is practically infinite, or at least that the
+number is so great that it cannot be estimated. But this idea
+is totally incorrect, and due to complete ignorance of telescopic
+revelations. It is certainly true that, to a certain
+extent, the larger the telescope used in the examination of the
+heavens, the more the number of the stars seems to increase;
+but we now know that there is a limit to this increase of
+telescopic vision. And the evidence clearly shows that we
+<span class='pageno' id='Page_537'>537</span>are rapidly approaching this limit. Although the number of
+stars visible in the Pleiades rapidly increases at first with
+increase in the size of the telescope used, and although photography
+has still further increased the number of stars in this
+remarkable cluster, it has recently been found that an increased
+length of exposure—beyond three hours—adds very few stars
+to the number visible on the photograph taken at the Paris
+Observatory in 1885, on which over 2,000 stars can be counted.
+Even with this great number on so small an area of the
+heavens, comparatively large vacant spaces are visible between
+the stars, and a glance at the original photograph is sufficient
+to show that there would be ample room for many times the
+number actually visible. I find that, if the whole heavens
+were as rich in stars as the Pleiades, there would be only
+33 millions in both hemispheres.</p>
+
+<p class='c007'>On a photograph of the region surrounding Gamma Cassiopeiæ,
+taken by Dr. Roberts in December, 1895, with a reflecting
+telescope of 20 inches aperture, and an exposure of two
+hours and twelve minutes, he finds 17,100 stars on an area of
+four square degrees. This would give for the whole area of
+the heavens—if equally rich in stars—a total of about 176
+millions; but Gamma Cassiopeiæ lies in a rich region of the
+Milky Way, and probably the great majority of the stars
+shown on Dr. Roberts’ photograph belong to the Galaxy,
+which we know to be especially rich in stars. One thing is
+certain, that the heavens as a whole are not nearly so rich as
+this particular spot. There may, perhaps, be richer spots elsewhere
+in the Milky Way, but in other parts of the sky there
+are many regions considerably poorer.</p>
+
+<p class='c007'>Let us consider a still more extreme case of stellar richness.
+On a photograph of the great globular cluster, Omega Centauri,
+recently taken in Peru, a count of the stars has been
+carefully made by Professor and Mrs. Bailey, and, as stated in
+the last chapter, the number of stars contained in the cluster
+may be taken as 10,000. Now, if the whole sky were as
+thickly studded with stars as in this cluster, the total number
+visible in the whole heavens would be 1,650 millions, a very
+<span class='pageno' id='Page_538'>538</span>large number, of course, but not much in excess of the present
+population of the earth, and I am not aware that the number
+of the earth’s inhabitants has ever been described as “infinite.”</p>
+
+<p class='c007'>Clusters, such as the Pleiades and Omega Centauri, are, of
+course, remarkable, and rare exceptions to the general rule of
+stellar distribution, and the heavens in general are not—even
+in the richest portions of the Milky Way—nearly so rich in
+stars as the globular clusters. The fact of these clusters being
+remarkable objects, proves that they are unusually rich in stars,
+and there is strong evidence—evidence amounting to absolute
+proof in the case of the globular clusters—that these collections
+of stars are really, and not apparently, close, and that
+they are actually systems of suns, and occupy a comparatively
+limited volume in space. We cannot, then, estimate the probable
+number of the visible stars by counting those visible in
+one of the globular clusters.</p>
+
+<p class='c007'>That the number of the visible stars will not probably be
+largely increased by any increase in telescopic power, is indicated
+by the fact that Celoria, using a small telescope, of
+power barely sufficient to show stars to the eleventh magnitude,
+found that he could see almost exactly the same number
+of stars near the north pole of the Milky Way as were visible
+in Sir William Herschel’s great telescope! thus indicating that,
+here at least, no increase of optical power will materially increase
+the number of stars visible in that direction; for
+Herschel’s large telescope certainly showed far fainter stars
+than those of the eleventh magnitude in other portions of the
+heavens. It should therefore have shown fainter stars at the
+pole of the Milky Way also, if such stars existed in that
+region of space. Their absence, therefore, seems certain proof
+that very faint stars do <i>not</i> exist in that direction, and that,
+here at least, our sidereal universe is limited in extent A
+photograph, taken by Dr. Roberts not very far from the spot
+in question, shows only 178 stars to the square degree. This
+rate of distribution would give a total of only 7,343,000 stars
+for both hemispheres!</p>
+
+<p class='c007'><span class='pageno' id='Page_539'>539</span>An examination by Miss Clerke of Professor Pickering’s
+catalogue of stars surrounding the north pole of the heavens
+shows that “the small stars are overwhelmingly too few for
+the space they must occupy, if of average brightness; and
+they are too few in a constantly increasing ratio.”<a id='r143'></a><a href='#f143' class='c027'><sup>[143]</sup></a> Here
+again, a “thinning out” of the stellar hosts seems clearly
+indicated, and suggests that a limit will soon be reached,
+beyond which our most powerful telescopes and photographic
+plates will fail to reveal any further stars.</p>
+
+<p class='c007'>Let us now consider the number of stars actually visible.
+Maps of the northern portion of the heavens have been published
+by Argelander and Heis, and charts of the southern
+sky by Behrmann and Gould. Heis shows stars to about
+magnitude 6⅓, and Behrmann to about the same brightness.
+I find that the total number shown by both observers, as
+visible to the naked eye, is 7,249. The total number, to the
+sixth magnitude inclusive, shown by both observers, is 4,181.
+Argelander gives 5,000 stars to the sixth magnitude inclusive,
+and for stars to the ninth magnitude, the following numbers
+in each magnitude:—First magnitude, 20; second magnitude,
+65; third magnitude, 190; fourth magnitude, 425; fifth
+magnitude, 1,100; sixth magnitude, 3,200; seventh magnitude,
+13,000; eighth magnitude, 40,000; and ninth magnitude,
+142,000, or a total of “200,000 for the entire number of stars
+from the first to the ninth magnitude inclusive.”<a id='r144'></a><a href='#f144' class='c027'><sup>[144]</sup></a> This result
+agrees closely with an estimate previously made by
+Struve. From a formula given by Dr. Gould, deduced from
+observations in the Southern Hemisphere, I find the number
+of stars to the ninth magnitude inclusive would be 215,674, so
+that Argelanders estimate of 200,000 stars to the ninth
+magnitude inclusive cannot be far from the truth. It will be
+seen from Argelanders figures that the number of stars in
+each class of magnitude is roughly three times that in the
+class one magnitude brighter. Supposing this progressive
+increase continued to the seventeenth magnitude—the faintest
+<span class='pageno' id='Page_540'>540</span>visible in the great Lick telescope—I find that the total
+number of stars would be nearly 1,400 millions, or less than
+the number found from a consideration of the cluster Omega
+Centauri. But it is evident from Celoria’s observation, referred
+to above, and from Professor Pickering’s photographs
+of stars near the North Pole, that the fainter stars do <i>not</i>
+increase in the ratio assumed above. We must therefore
+conclude that there is a “thinning out” of the fainter stars at
+some point below the ninth magnitude. Taking into consideration
+the rich regions of the Milky Way, and the
+comparatively poor portions of the sky, it is now generally
+admitted by astronomers, who have studied this particular
+question, that the probable number of stars visible in our
+largest telescopes does not exceed 100 millions, a number
+which, large as it absolutely is, may be considered as relatively
+very small, and even utterly insignificant, when compared
+with an “infinite number.”</p>
+
+<p class='c007'>Let us see what richness of stellar distribution is implied
+by this number of 100 millions of visible stars. It may be
+easily shown that the area of the whole sky, in both hemispheres,
+is 41,253 square degrees, or about 200,000 times the
+area of the full moon. This gives 2,424 stars to the square
+degree. The moon’s apparent diameter being slightly over
+half a degree (31′ 5″), the area of its disc is about one-fifth of
+a square degree. Hence, for 100 millions of stars in the
+whole star sphere, we have 485 stars to each space of sky,
+equal in area to the full moon. This seems a large number,
+but stars scattered even as thickly as this would appear at a
+considerable distance apart when viewed with a large telescope
+and a high power. As the area of the moon’s disc
+contains about 760 square minutes of arc, there would not be
+an average of even one star to each square minute. A pair of
+stars half a minute, or 30 seconds, apart, would form a very
+wide double star, and with stars placed at even this distance,
+the moon’s disc would cover about 3,000, or over six times the
+actual number visible in the largest telescopes. In Dr.
+Roberts’ photograph of the region surrounding Gamma
+<span class='pageno' id='Page_541'>541</span>Cassiopeiæ, which shows over 17,000 stars, on four square
+degrees, or over 4,000 stars to the square degree, the stars do
+not seem very crowded, and there is a good deal of black sky
+visible between them.</p>
+
+<p class='c007'>But, in addition to the conclusive evidence as to the limited
+number of the visible stars derived from actual observation
+and the results of photography, we have indisputable evidence
+from mathematical considerations that the number of the
+visible stars <i>must necessarily</i> be limited. For were the stars
+infinite in number, and scattered through infinite space with
+any approach to uniformity, it may be proved that the whole
+heavens would shine with the brightness of the sun. As the
+surface of a sphere varies as the square of its radius, and light
+inversely as the square of the distance (or radius of the star
+sphere at any point), we have the diminished light of the stars
+exactly counterbalanced by the increased number at any
+given distance. For a distance of say ten times the distance
+of the nearest fixed star, the light of each star would be
+diminished by the square of 10 or 100 times, but the total
+number of stars would be 100 times greater, so that the total
+star light would be the same. This would be true for <i>all</i>
+distances. The total light would therefore—by addition—be
+proportional to the distance, and hence, for an infinite distance
+we should have an infinite amount of light For an infinite
+number of stars, therefore, we should have a continuous blaze
+of light over the whole surface of the visible heavens. Far
+from this being the case, the amount of light afforded by the
+stars on the clearest nights is, on the contrary, comparatively
+small, and the blackness of the background, “the darkness
+behind the stars,” is very obvious. According to Miss Clerke
+(“System of the Stars,” p. 7), the total light of all the stars, to
+magnitude 9½, is about one-eightieth of full moonlight. M. G.
+l’Hermite found for the total amount of starlight one-tenth of
+moonlight; but this estimate is evidently too high. Assuming
+the sun’s brightness as 28 magnitudes brighter than a star
+of the first magnitude,<a id='r145'></a><a href='#f145' class='c027'><sup>[145]</sup></a> and Zöllner’s estimate that sunlight is
+<span class='pageno' id='Page_542'>542</span>618,000 times that of moonlight, I find that the total light of
+the stars to magnitude 9½, as stated by Miss Clerke, would be
+equivalent to the combined light of about 320,000 stars of the
+sixth magnitude, or 3,200 stars of the first magnitude. Even
+taking M. l’Hermite’s high estimate of one-tenth of moonlight,
+the total starlight would be represented by 25,600 stars of the
+first magnitude.</p>
+
+<p class='c007'>To explain the limited number of the visible stars, several
+hypothesis have been advanced. If space be really infinite,
+as we seem compelled to suppose, it would be reasonable to
+expect that the number of the stars would be practically infinite
+also. But, as I have shown above, the number of the
+<i>visible</i> stars is certainly finite, and the number visible and
+invisible must be finite also, for otherwise the amount of starlight
+would be much greater than it is. To account for the
+limited number of visible stars, it has been suggested that
+beyond a certain distance in space, there may be an “extinction
+of light,” caused by absorption in the luminiferous ether.
+In a recent paper on this subject, Schiaparelli, the famous
+Italian astronomer, suggests that if any extinction of light
+really takes place, it may probably be due, not to absorption
+in the ether, but to fine particles of matter scattered through
+interstellar space. In support of this hypothesis, he refers to
+the supposed constitution of comets’ tails, of falling stars, and
+meteorites, and he shows that the quantity of matter necessary
+to produce the required extinction would be very small—so
+small, indeed, that a quantity of this matter scattered through
+a volume equal to that of the earth, if collected into one mass,
+would only form a ball of less than one inch in diameter. We
+can readily admit the existence of such a minute quantity of
+matter in a fine state of subdivision scattered through space,
+but it seems to me much more probable that the limited
+number of the visible stars is due, not to any extinction of
+their light by absorption in the ether, or by fine particles
+scattered through space, but to a real thinning out of the stars
+as we approach the limits of our sidereal universe. Celoria’s
+observation, mentioned above, seems to prove that near the
+<span class='pageno' id='Page_543'>543</span>pole of the Milky Way very few stars fainter than the eleventh
+magnitude are visible, even in a large telescope, and Dr.
+Roberts’ photographs, taken in the vicinity of the celestial
+pole, confirm this conclusion. Now, this paucity of stars of the
+fainter magnitudes cannot be due to any absorption of light
+in the ether, for numerous stars of the sixteenth magnitude, or
+perhaps fainter, are visible in other parts of the heavens, and
+if in one place, why not in another? Sir John Herschel’s
+observations of the Milky Way in the Southern Hemisphere
+appear to render the hypothesis of any extinction of light very
+improbable. He says that the hypothesis, “if applicable to
+any, is equally so to every part of the Galaxy. We are not
+at liberty to argue that at one part of its circumference our
+view is limited by this sort of cosmical veil, which extinguishes
+the smaller magnitudes, cuts off the nebulous light of distant
+masses, and closes our view in impenetrable darkness; while at
+another we are compelled, by the clearest evidence telescopes
+can afford, to believe that star-strewn vistas <i>lie open</i>, exhausting
+their powers, and stretching out beyond their utmost
+reach, as is proved by that very phænomenon which the
+existence of such a veil would render impossible, <i>viz.</i>, infinite
+increase of number and diminution of magnitude, terminating
+in complete irresolvable nebulosity.”</p>
+
+<p class='c007'>How then are we to explain the limited number of the
+visible stars? If space be infinite, as we seem compelled to
+suppose, the number of the stars would probably be infinite
+also, or at least vastly greater than the number actually visible.
+It has been suggested that, owing to the progressive motion
+of light, the light of very distant stars may probably not yet
+have reached the earth, although travelling through space for
+thousands of years. But considering the vast periods of time
+during which the stellar universe has probably been in existence,
+this hypothesis seems very unsatisfactory. The most
+probable hypothesis seems to be that all the stars, clusters and
+nebulæ, visible in our largest telescopes, form together one vast
+system, which constitutes our visible universe, and that this
+system is isolated by a starless void from other similar systems
+<span class='pageno' id='Page_544'>544</span>which probably exist in infinite space. The distance between
+these separate systems—or “island universes,” as they have
+been called—may be very great, compared with the diameter
+of each system, in the same way that the diameter of our
+visible universe is very great compared with the diameter
+of the solar system. As the sun is a star, and the stars are
+suns, and as our sun is separated from his neighbour suns in
+space by a sunless void, so may our universe be separated
+from other universes by a vast and starless abyss. On this
+hypothesis, the supposed extinction of light—which may have
+little or no perceptible effect within the limits of our visible
+universe—may possibly come into play across the vast and
+immeasurable distances which probably separate the different
+universes from each other, and may perhaps extinguish their
+light altogether.</p>
+
+<p class='c007'>Another hypothesis which also seems possible is that the
+luminiferous ether which extends throughout our visible
+universe may perhaps be confined to this universe itself, and
+that beyond its confines, the ether may thin out, as our atmosphere
+does at a certain distance from the earth, and finally
+cease to exist altogether, ending in an <i>absolute</i> vacuum, which
+would, of course, arrest the passage of all light from outer
+space, and thus produce “the darkness behind the stars.”</p>
+
+<p class='c007'>Let us now consider the apparent distribution of the stars
+and nebulæ on the celestial vault, and their probable relation
+to each other in space. As already stated, Argelander considered
+the number of stars of the first magnitude to be about
+twenty, but modern photometric measures have reduced this
+number to thirteen or fourteen. According to the Harvard
+measures, the fourteen brightest stars in the heavens, in order
+of magnitude, are: Sirius, Canopus, Arcturus, Capella, Vega,
+Alpha Centauri, Rigel, Procyon, Achernar, Beta Centauri,
+Betelgeuse, Altair, Aldebaran and Alpha Crucis. Seven of
+these are in the Northern Hemisphere, namely: Arcturus,
+Capella, Vega, Procyon, Betelgeuse, Altair, and Aldebaran;
+and seven in the Southern Hemisphere: Sirius, Canopus, Alpha
+Centauri, Rigel, Achernar, Beta Centauri, and Alpha Crucis,
+<span class='pageno' id='Page_545'>545</span>so that the brightest stars are pretty evenly distributed between
+the two hemispheres. Of these bright stars, no less
+than twelve lie in or near the Milky Way, Arcturus and
+Achernar being the only two at any considerable distance
+from the Galaxy. This is very remarkable and suggestive,
+as the area covered by the Milky Way is probably not more
+than one-fourth of the whole star sphere.</p>
+
+<p class='c007'>Of the stars fainter than the first magnitude, but brighter
+than magnitude 2·0, there are about 10 in the Northern
+Hemisphere, of which 4 lie in or near the Milky Way, and
+about 19 in the Southern Hemisphere, of which no less than
+14 are situated in or near the Galaxy.</p>
+
+<p class='c007'>Of those brighter than magnitude 3·0, I find 33 stars in or
+near the Milky Way out of a total of about 95 in both
+hemispheres. To extend this investigation to all stars visible
+to the naked eye, I made, some years since, an examination
+of all the stars in Heis’ atlas that lie in the Milky Way, and
+found that number to be 1,186 out of a total of 5,356, or a
+percentage of about 22. At my request, Col. Markwick,
+F.R.A.S., made a similar count for the stars in Dr. Gould’s
+charts of the Southern Hemisphere (<i>Uranometria Argentina</i>),
+and found that, down to the fourth magnitude, there are 121
+stars on the Milky Way out of 228, or a percentage of 53, and
+for all stars to the seventh magnitude inclusive, there are
+3,072 on the Milky Way out of a total of 6,694, or a percentage
+of nearly 46. Col. Markwick finds that the Milky
+Way in the Southern Hemisphere, as shown on Gould’s charts,
+covers about one-third of the whole hemisphere. As will be
+seen by the above figures, the percentage of stars, even to the
+fourth magnitude, lying on the Milky Way is considerably
+greater than this proportion.</p>
+
+<p class='c007'>The above results show that the brighter stars which are
+apparently projected on the Milky Way probably belong to
+that zone, and are not merely fortuitously scattered over the
+surface of the heavens.</p>
+
+<p class='c007'>To extend the investigation still further, and include stars
+to the eighth magnitude, I made an examination of the stars
+<span class='pageno' id='Page_546'>546</span>shown on Harding’s charts to that magnitude, in a zone of
+30° in width—15° degrees on each side of the Equator—and
+found a marked increase in the number of stars where the
+zone crossed the Milky Way. The numbers per hour of
+Right Ascension varied from a minimum of 275 (hours I. and
+II.) to maxima of 601 in the Milky Way in Monoceros, and
+611 in the Galaxy in Serpens and Aquila. A valuable investigation
+by the late Mr. Proctor went further still. He plotted
+all the stars shown in the charts of Argelander’s <cite>Durchmusterung</cite>,
+which contains stars to 9½ or 10th magnitude. In
+this remarkable chart the course of the Milky Way is clearly
+defined by a marked increase of stellar density. Proctor says:
+“In the very regions where the Herschelian gauges showed
+the minutest telescopic stars to be most crowded, my chart of
+324,198 stars shows the stars of the higher orders (down to
+the eleventh magnitude) to be so crowded that, by their mere
+aggregation within the mass, they show the Milky Way with
+all its streams and clusterings. This evidence, I venture to
+affirm, is altogether decisive as to the main question, whether
+large and small stars are really intermixed in many regions of
+space, or whether the small stars are excessively remote. It
+is utterly impossible that excessively remote stars could seem
+to be clustered exactly where relatively near stars are richly
+spread. This might happen, no doubt, in a single instance;
+but that it could be repeated over and over again, so as to
+account for all the complicated features seen in my chart of
+324,198 stars, I maintain to be utterly incredible.”<a id='r146'></a><a href='#f146' class='c027'><sup>[146]</sup></a></p>
+
+<p class='c007'>From a careful examination of the Milky Way in Aquila
+and Cygnus, Mr. Easton finds that “(1) In the zones considered,
+the distribution of stars down to 9·5 magnitude
+corresponds to the greater or less intensity of galactic light.
+(2) There is a real correspondence of the general outlines of
+the galactic forms with the distribution of 11 magnitude stars,
+and with those of stars between 10 and 15 magnitude.
+(3) Thus, in general, for the zones considered, the faint stars
+which form the Milky Way are thickly or sparsely scattered
+<span class='pageno' id='Page_547'>547</span>in respectively the same regions as the stars in Argelander’s
+last class; it follows, therefore, with a great degree of probability,
+that there is a real connexion between the distribution
+of 9 and 10 magnitude stars and that of the very faint
+stars of the Milky Way. Consequently, the very faint stars
+are at a distance which does not greatly exceed that of 9–10
+magnitude stars. If stars of 13–15 magnitude were at their
+theoretical distance, there would be no reason why they should
+have the same apparent distribution in galactic latitude and
+longitude as 9–10 magnitude stars separated from them by
+enormous intervals.”<a id='r147'></a><a href='#f147' class='c027'><sup>[147]</sup></a></p>
+
+<p class='c007'>There are some regions in both hemispheres especially rich
+in naked eye stars. Of these the following may be mentioned
+in the Northern Hemisphere:—the region including the
+Pleiades, and Hyades in Taurus, the Northern portion of
+Orion, and the adjoining part of Gemini, the constellation
+Lyra, the northern portion of Cygnus, Cassiopeia’s Chair,
+and Coma Berenices. In the Southern Hemisphere there are
+several rich spots. A rich region extends from Canis Major
+to the Southern Cross, and nearly coincides with the course
+of the Milky Way. The richest spot of all, and perhaps
+the richest in the whole heavens in naked eye stars—with
+exception of the Pleiades—is that including the Southern
+Cross. This spot has an average of three stars to five square
+degrees, and if the whole heavens were as richly studded with
+stars there would be about 24,000 visible to the naked eye!
+The poverty of the adjoining “coal sack” is very remarkable.
+Another rich spot surrounds the variable star Eta Argûs, and
+the great nebula in Argo. There is another rich spot in the
+constellation Hydrus, not far from the greater Magellanic
+Cloud, and another will be found in Centaurus and Lupus,
+with its centre about Alpha of the latter constellation.
+According to Gould’s maps of the Southern Hemisphere, the
+richest region in stars down to the seventh magnitude is the
+southern portion of that part of the constellation Argo,
+known as Puppis.</p>
+
+<p class='c007'><span class='pageno' id='Page_548'>548</span>In contrast to these rich regions, and in many cases closely
+adjoining them, are some barren regions, very poor in naked
+eye stars. For example, closely following the rich spot in
+Cassiopeia and between Iota Cassiopeiæ and Eta Persei is a
+remarkably poor spot, where a space of some sixty square
+degrees does not contain a single star brighter than the sixth
+magnitude! There is another poor region south of Alpha
+Hydræ, and another in the southern portion of the constellation
+Cetus.</p>
+
+<p class='c007'>A region of considerable extent, remarkably deficient in
+bright stars, will be noticed in the Northern Hemisphere.
+This comparatively barren region, which contains no star
+brighter than the fourth magnitude, is bounded by Cepheus,
+Cassiopeia, Perseus, Auriga, Gemini, Ursa Major, Draco, and
+Ursa Minor, and forms a conspicuous feature in the north-eastern
+portion of the sky in the early winter evenings. It
+will be noticed that the surrounding constellations all contain
+bright stars.</p>
+
+<p class='c007'>Whether the apparent crowding of stars in certain regions
+of the heavens is caused by a real proximity in space, or
+whether it is merely due to their being placed accidentally in
+the line of sight, is a question difficult to determine. In the
+case of star clusters, and especially the globular clusters,
+there is a high mathematical probability, amounting almost
+to absolute certainty, that they are comparatively close
+together, but in groups scattered over a considerable area, like
+those referred to above, the probability in favour of proximity
+is not so great. As we know the distance of so few stars
+from the earth, it is impossible to say whether the crowding
+is real or only apparent, but the probability seems to be that
+it is to some extent real.</p>
+
+<p class='c007'>A tendency to an arrangement of stars in streams was
+pointed out by Proctor in his “Universe and the Coming
+Transits.” This tendency to stream formation may be noticed
+on a large scale among the naked eye stars, for example, in
+Pisces, Scorpio, the River Eridanus, Aquarius, and the festoon
+of stars in Perseus. In some of these cases, of course, the
+<span class='pageno' id='Page_549'>549</span>stars are so far apart that the formation may be more apparent
+than real, but the tendency can also be clearly recognised
+among the fainter stars, and even among those only
+visible in telescopes and stellar photographs. This tendency
+to run in streams is well marked on the photographs taken at
+the Paris Observatory, and on those taken by Professor
+Barnard, Dr. Max Wolf, and others. It is a suggestive fact
+that these star streams are also very noticeable in star clusters,
+where there can be little or no doubt of a physical connexion
+between the component stars. With reference to a photograph
+of the southern portion of Aquila taken by Dr. Max
+Wolf in July, 1892, the late Mr. Ranyard, remarked: “Some
+of the streams of fainter stars in this region are very striking,
+and must convince the most sceptical of their reality. It is
+possible to draw an arc of a circle through any three stars,
+and a conic section through any five; but where we find ten
+or twenty stars falling into line, not once, but in many cases,
+and that there is a curious similarity between the strange
+curves and branching streams which these phalanges of stars
+mark out on the heavens, there is no room left for doubt that
+the mind is not being led away by a tendency of the imagination
+similar to that which finds faces in the fire, or sees a man
+carrying sticks on the face of the moon. If it is proved that
+a group of stars is arranged in line or marshalled in any
+order, it would follow that the individuals of the group must
+be actually as well as apparently close to one another, and
+that they form some kind of system, having all of them had a
+common origin, or been subject to some common influence.”<a id='r148'></a><a href='#f148' class='c027'><sup>[148]</sup></a></p>
+
+<p class='c007'>The great majority of the star clusters are found along the
+course of the Milky Way, while the irresolvable nebulæ seem
+to congregate towards the poles of the galactic zone.</p>
+
+<p class='c007'>Dr. Gould is of opinion that “a belt or stream of bright
+stars appears to girdle the heavens very nearly in a great
+circle, which intersects the Milky Way at about the points of
+its highest declination, and forms with it an angle not far
+from 20°; the southern node being near the margin of the
+<span class='pageno' id='Page_550'>550</span>Cross, and the northern in Cassiopeia.” According to Gould,
+this belt covers Orion, Canis Major, Columba, Puppis, Carina,
+the Southern Cross, Centaurus, Lupus, and the head of
+Scorpion in the Southern Hemisphere, its northern course
+being indicated by the brightest stars in Taurus, Perseus,
+Cassiopeia, Cepheus, Cygnus, and Lyra. Dr. Gould considers
+that our sun may possibly be a member of this belt of stars,
+which perhaps numbers less than 500, and which constitute
+“a small cluster, distinct from the vast organisation of that
+which forms the Milky Way, and of a flattened and somewhat
+bifid form. The southern portion of this supposed stream of
+bright stars had been previously recognised by Sir John
+Herschel, who says in his ‘Cape Observations,’ (p. 385), ‘It
+is about this region, or, perhaps, somewhat earlier, in the interval
+between η Argus and α Crucis, that the galactic circle, or
+medial line of the Milky Way may be considered as crossed
+by that zone of large stars, which is marked out by the
+brilliant constellation of <i>Orion</i>, the bright stars of Canis
+Major, and almost all the more conspicuous stars of <i>Argo</i>, the
+Cross, the Centaur, Lupus, and <i>Scorpion</i>. A great circle passing
+through ε Orionis and α Crucis will mark out the axis of
+the zone in question, whose inclination to the galactic circle
+is, therefore, about 20°, and whose appearance would lead us
+to suspect that our nearest neighbours in the sidereal system
+(if really such) form part of a subordinate sheet or stratum
+deviating to that extent from parallelism to the general mass
+which, seen projected on the heavens, forms the Milky Way.’”</p>
+
+<p class='c007'>These conclusions might seem probable enough when we
+compare the supposed zone of bright stars with the very
+diagrammatic drawings of the Milky Way as shown in many
+star maps; but when we consider the stars referred to with
+reference to the more artistic and accurate delineations of the
+Milky Way as drawn by Boeddicker, and even by Gould
+himself, we see that most of them are involved in the milky
+light of the Galaxy, and their connexion with the Milky Way
+itself seems quite as probable as that they form a belt distinct
+from the galactic zone. The apparent connexion of the stars
+<span class='pageno' id='Page_551'>551</span>in question with the Milky Way does not, however, disprove
+the existence of Dr. Gould’s belt or zone of bright stars. If
+the plane of the supposed belt nearly coincided with that of
+the Milky Way, the apparent connexion might not be real.</p>
+
+<p class='c007'>Mr. J. R. Sutton advances the theory<a id='r149'></a><a href='#f149' class='c027'><sup>[149]</sup></a> that the Milky Way
+consists of “a great ring of large stars”—Dr. Gould’s solar
+cluster above referred to—“intersecting an equal ring of small
+ones (the Milky Way) at the extremities of a common
+diameter.” He considers that “the great star belt is a
+genuine girdle of stars in space, in which also the foundations
+of the sidereal system are laid, the Milky Way being an
+appendant to it of lesser rank.”</p>
+
+<p class='c007'>That the Milky Way really forms a ring of stars in space
+there is strong evidence to show. Sir William Herschel’s
+original theory that the galactic gleam is due to our sun being
+situated near the centre of an indefinite stratum of stars—the
+“disc theory,” as it is termed—was abandoned by its illustrious
+author in his later writings, and is now considered to be wholly
+untenable by nearly all astronomers who have studied the
+subject. Sir John Herschel remarks that the general aspect
+of the galaxy near the Southern Cross indicates “that the
+Milky Way, in this neighbourhood, at any rate, is really what
+it appears to be, a belt or zone of stars separated from us by
+a starless interval.” It certainly seems utterly improbable
+that the nearly circular blank space near the Southern Cross,
+known as “the coal sack,” should represent a tunnel through
+a disc, of which the thickness is comparatively small, while its
+diameter, on the “disc theory,” stretches out almost to infinity.
+A straight, tunnel-shaped opening of great length, pointing
+directly towards the earth, would form an extraordinary
+phenomenon even in a solitary instance; yet there are several
+somewhat similar openings to be found in the Milky Way, as
+viewed both with the naked eye and with a telescope. That
+<i>all</i> these openings should represent tunnels radiating from a
+common centre is quite beyond the bounds of probability, and,
+indeed, such an hypothesis does not deserve serious consideration.
+<span class='pageno' id='Page_552'>552</span>With reference to a photograph of the Milky Way in
+the constellation Cepheus, Professor Barnard says, “the sky
+(or Milky Way) is broken up into numerous black cracks or
+crevices. Looking at these peculiar features, I cannot well
+see how one can avoid the conclusion that they are necessarily
+real vacancies in the Milky Way, through which we look out into
+the blackness of space.”<a id='r150'></a><a href='#f150' class='c027'><sup>[150]</sup></a> Using a telescope with a low power,
+Mr. S. M. Baird Gemmill says, “December 1, 1886. In sweeping
+over the constellation of Monoceros, I was much struck
+with the reticulated character of the arrangement of the
+brighter stars upon the glimmering background, and the way
+in which this background seemed to follow the reticulation.
+By ‘brighter stars’ are meant stars of from 8 to 10 magnitude,
+for it was among these that I noticed this peculiarity of
+arrangement. It put me in mind of M. M. Henry’s photographs
+of Cygnus. The region seemed, in fact, a vast network
+of stars, the reticulations of which were separated by
+desert, or comparatively desert spaces.”<a id='r151'></a><a href='#f151' class='c027'><sup>[151]</sup></a> I have noticed the
+same thing myself while examining the Milky Way with a
+binocular field-glass. On October 26, 1889, I noted as follows:
+“North of Alpha Cygni, and near Xi and Nu Cygni, the
+nebulous light of the Milky Way seems to cling round and
+follow streams of small stars in a very remarkable way;
+numerous small ‘coal sacks’ and rifts are visible, in which
+comparatively few stars are to be seen with the binocular.”
+This observation has been fully confirmed by photographs of
+this region, taken by Dr. Max Wolf in 1891.</p>
+
+<div id='i_555'  class='figcenter id001'>
+<span class='pageno' id='Page_553'>553</span>
+<img src='images/i_555.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 19.</span>—<i>Photograph of Milky Way, Sagittarius.</i><br>  <br>  (From “Visible Universe.”)</p>
+</div>
+</div>
+
+<p class='c007'>That the Milky Way is not indefinitely extended in the
+line of sight seems clearly shown by Sir John Herschel’s
+observations in the Southern Hemisphere. In his “Outlines of
+Astronomy” (p. 578), he says: “When examined with
+powerful telescopes, the constitution of this wonderful zone
+is found to be no less various than its aspect to the eye is
+irregular. In some regions, the stars of which it is wholly
+composed are scattered with remarkable uniformity over
+immense tracts, while in others the irregularity of their
+distribution is quite as striking, exhibiting a rapid succession
+of closely clustering rich patches, separated by comparatively
+poor intervals, and indeed, in some instances, by spaces
+absolutely dark <i>and completely void of any star</i>,<a id='r152'></a><a href='#f152' class='c027'><sup>[152]</sup></a> even of the
+smallest telescopic magnitude.... In some, for instance,
+extremely minute stars, though never altogether wanting,
+occur in numbers so moderate, as to lead us irresistibly to the
+conclusion that, in those regions, we see <i>fairly through</i> the starry
+stratum, since it is impossible otherwise (supposing their light
+not intercepted), that the members of the smaller magnitude
+<span class='pageno' id='Page_554'>554</span>should not go on increasing <i>ad infinitum</i>. In such cases,
+moreover, the ground of the heavens, as seen between the stars,
+is for the most part perfectly dark, which again would not be
+the case if innumerable multitudes of stars, too minute to be
+individually discernible, existed beyond. In other regions
+we are presented with the phænomenon of an almost uniform
+degree of brightness of the individual stars, accompanied with
+a very even distribution of them over the ground of the
+heavens, both the larger and smaller magnitudes being strikingly
+deficient. In such cases it is equally impossible not to
+perceive that we are looking <i>through</i> a sheet of stars nearly of
+a size and of no great thickness compared with the distance
+which separates them from us. Were it otherwise, we should
+be driven to suppose the more distant stars uniformly the
+larger, so as to compensate by their greater intrinsic brightness
+for their greater distance, a supposition contrary to all
+probability. In others again, and that not unfrequently, we
+are presented with a double phænomenon of the same kind,
+<i>viz.</i>, a tissue, as it were, of large stars spread over another of
+very small ones, the intermediate magnitude being wanting.
+The conclusion here seems equally evident that in such cases
+we look through two sidereal sheets separated by a starless
+interval.”</p>
+
+<p class='c007'>An examination of the evidence at present available, with
+reference to the distribution of the visible stars in space, has
+recently been undertaken by Professor Kapteyn of Groningen,
+and an account of the conclusions he has arrived at may prove
+of interest to the reader.</p>
+
+<div id='i_557'  class='figcenter id001'>
+<span class='pageno' id='Page_555'>555</span>
+<img src='images/i_557.jpg' alt='' class='ig001'>
+<div class='ic001'>
+<p><span class='sc'>Fig. 20.</span>—<i>The Milky Way.</i><br>  <br>  (From <cite>Knowledge</cite>, Nov., 1894.)</p>
+</div>
+</div>
+
+<p class='c007'>We must first explain that in order to obtain a clear view
+of the construction of the visible universe, it would be necessary
+to know the relative distances of a large number of stars; but
+as the distances of only a few stars from the earth have yet
+been determined by actual measurement, and the results
+hitherto obtained are open to much uncertainty, we must have
+recourse to some other method of estimating the distances.
+While travelling in a railway carriage, if we fix our attention
+on trees, buildings, and other objects we pass on our journey,
+<span class='pageno' id='Page_556'>556</span>it will be noticed that all objects apparently move past us in
+the opposite direction to that in which we are travelling, and
+that the nearer the object is the faster it seems to move with
+reference to distant objects near the horizon. So it is with the
+stars. As we showed in Chapter III., the sun is moving
+through space, carrying along with the earth all the planets,
+satellites, and comets, forming the solar system. The effect
+of this motion is to cause an apparent small motion of the
+stars in the opposite direction, and the nearer the star is to
+the earth, the greater will this apparent motion seem to be
+as in the case of the railway train. In addition to this
+apparent motion, the stars are themselves—like the sun—moving
+through space, and this <i>real</i> motion is also visible. If
+this real motion takes place in the <i>opposite</i> direction to that in
+which the sun and earth are moving, it will add to the
+apparent motion, and will increase the star’s “proper motion,”
+as it is termed. If, on the other hand, the real motion is in
+the <i>same</i> direction as the earth’s motion, the proper motion
+will be diminished. In either case, the nearer the star is to the
+earth, the greater will be its apparent annual displacement on
+the background of the heavens. The amount of the “proper
+motion” is, therefore, considered by astronomers to form a reliable
+criterion of the star’s distance from the earth, and the actual
+measures of distance which have been made show that this
+assumption is approximately true. Of fourteen stars which have
+proper motion of over three seconds of arc per annum, eleven
+have yielded a measurable parallax, or displacement, due to the
+earth’s annual motion round the sun; that is to say, eleven out
+of fourteen fast-moving stars are within a measurable distance
+of the earth, and are, therefore, near us, when compared with
+the great majority of stars which are not within measurable
+distance, or, at least, are beyond the reach of our present
+methods of measurement.</p>
+
+<p class='c007'>In the case of small groups of stars, we may assume that the
+real motions of the individual stars take place indifferently in
+all directions, and that consequently, taking an average of all
+the motions of the stars composing the group, the effects due
+<span class='pageno' id='Page_557'>557</span>to the real motions will destroy each other, and there will remain,
+as the most reliable criterion, the effect due to the sun’s
+motion in space. If, however, we compare the proper motions
+of groups situated in <i>different parts</i> of the sky, there is a consideration
+which, to a great extent, vitiates this conclusion.
+For, near the point of the heavens, towards which the sun and
+earth are moving, known as the “apex of the solar way,” and
+probably situated not far from the bright star Vega, as indicated
+by recent researches, and near the point away <i>from</i>
+which the sun is moving known as the <i>ant-apex</i>, about 15°
+south of Sirius, there will be no apparent displacement due to
+the solar motion through space, as this motion takes place in
+the line of sight with reference to these points of the sky. The
+observed proper motion at these points will, therefore, be
+solely due to the real motions of the stars themselves in those
+regions. In other parts of the heavens, however, the total
+proper motion will be a combination of the apparent and real
+motions of the stars, and for stars in different parts of the sky,
+it will not follow that stars having equal proper motions are
+necessarily at the same distance from the earth. To make
+this point clearer, let us suppose that there are two stars at
+absolutely the same distance from the earth, one situated at or
+near the solar “apex,” and the other at a point 90° from the
+apex, and let us suppose that both stars are moving through
+space with exactly the same velocity and in the same direction,
+say at right angles to the direction of the solar motion. Then
+in the case of the star near the apex, the observed “proper
+motion” will be solely due to the star’s real motion, and in the
+star 90° distant from the apex, the proper motion will be solely
+due to the solar motion, as the star’s <i>real motion</i>, being in the
+line of sight, will not be visible. Now, unless the stellar
+motion and the solar motion happen to be equal, the observed
+“proper motions” will not be equal, although both stars are
+at the same distance from the earth. If both the stars are
+really at rest, the star at the apex will have no proper motion,
+while the star 90° distant will have an apparent proper motion
+due to the sun’s motion. To overcome this source of error in
+<span class='pageno' id='Page_558'>558</span>estimating the distance of a star from its proper motion, Professor
+Kapteyn made use of another measure, which is independent
+of the solar motion. This is the component of the
+proper motion measured at right angles to a great circle of the
+sphere passing through a star and the solar apex. The
+amount of motion in this direction will evidently not be
+affected by the sun’s motion, and from a discussion of the stars,
+contained in the Draper “Catalogue of Stellar Spectra,” which
+were observed by Bradley (and of which the proper motions
+are now known with accuracy), Professor Kapteyn finds that
+this motion is “nearly inversely proportional to the distance,”
+that is, the greater the motion, the less the distance of the
+stars, and the smaller the motion, the greater the distance.
+Excluding stars with proper motions greater than half a
+second of arc per annum, Professor Kapteyn found that for
+stars at various distances from the Milky Way this component
+of the “proper motion” forms a good measure of distance.</p>
+
+<p class='c007'>As the result of his investigations on the subject, Professor
+Kapteyn arrives at the following conclusions. Neglecting
+stars with small or imperceptible proper motions, we have a
+group of stars which no longer show any condensation in a
+plane. Stars with very small or no proper motions show a
+condensation towards the plane of the Milky Way. This
+applies to stars of the second or solar type, as well as to
+those of the first or Sirian type of spectrum, and evidently
+indicates that the stars composing the Milky Way lie at a
+great distance from the earth. The extreme faintness of the
+majority of the stars composing the Galaxy seems in favour
+of this conclusion. The condensation of stars of the first type
+is more marked than those of the second, and this agrees with
+the fact which has been noticed by Professor Pickering, that
+the majority of the brighter stars of the Milky Way have
+spectra of the Sirian type.</p>
+
+<p class='c007'>Professor Kapteyn finds that this condensation of stars with
+small proper motions is very perceptible even for stars visible
+to the naked eye, and is as well marked in those stars which
+have spectra of the second type as for all the stars of the ninth
+<span class='pageno' id='Page_559'>559</span>magnitude; but for stars of the first type the condensation is
+still more marked. He considers that this condensation is
+either partly real, or that there is a real thinning out of stars
+near the pole of the Milky Way. As already mentioned (in
+the beginning of this chapter), Celoria’s observations with a
+small telescope, compared with Sir William Herschel’s observations
+with a large telescope, indicate clearly that there
+<i>is a real thinning out</i> of stars near the poles of the Galaxy.</p>
+
+<p class='c007'>Professor Kapteyn concludes that the arrangement of the
+stars suggested by Struve—a modification of the “disc
+theory”—has no real existence.<a id='r153'></a><a href='#f153' class='c027'><sup>[153]</sup></a> He attributes the fallacy in
+Struve’s hypothesis to the fact that the mean distance of stars
+of a given magnitude in the Milky Way, and outside it, is not
+the same.</p>
+
+<p class='c007'>Professor Kapteyn finds that the vicinity of the sun is
+almost exclusively occupied by stars of the second or solar
+type, a conclusion which evidently tends to strengthen Dr.
+Gould’s theory of a “solar cluster.” He finds that the number
+of Sirian type stars increases gradually with the distance, and
+that beyond a distance corresponding to a proper motion of
+about ¹⁄₁₄th of a second of arc per annum, the Sirian stars
+largely predominate. In the group of stars known as the
+Hyades, however, the components of which have a common
+proper motion both in amount and direction, stars of the first
+and second types appear to be mixed, and Professor Kapteyn
+assumes that the two types represent different phases of
+evolution, and that as the brightest stars of the group are
+chiefly of the solar type, these stars must be the largest of the
+group. From this fact he concludes the solar type stars are
+in a less advanced stage of evolution than those of the Sirian
+type. This does not agree with the generally accepted view.
+Professor Vogel considers the Sirian stars to represent an
+earlier stage of stellar evolution. Mr. Proctor held the same
+opinion, and in Professor Lockyer’s hypothesis of increasing
+and decreasing temperatures in stars of various types, he
+<span class='pageno' id='Page_560'>560</span>places the Sirian stars at the summit of the evolution curve,
+and the sun and solar stars just below them on the descending
+branch of the curve.<a id='r154'></a><a href='#f154' class='c027'><sup>[154]</sup></a> These hypotheses are in conformity
+also with the current opinion that the sun is a cooling body.
+The discrepancy may perhaps be explained by supposing that
+the <i>brighter</i> stars of the Hyades form a connected group, and
+that some, at least, of the fainter stars do not belong to the
+group, but lie at a great distance behind it. In the case of
+the Pleiades, which form a more evident cluster, I find from
+the Draper “Catalogue of Stellar Spectra” that the great
+majority of the brighter stars have spectra of the Sirian type.
+Most of the stars in the Pleiades have a very similar proper
+motion, both in amount and in direction, and there can be no
+doubt that most of the brighter stars, at least, form a connected
+system. As already stated, it seems highly probable
+that the fainter stars in the Pleiades lie far beyond the brighter
+components, and have merely an optical connexion with
+them, and the same may be the case in the Hyades. The
+superior brilliancy of the stars composing the Hyades would
+suggest that they are nearer to the earth than the Pleiades
+group, and they may possibly form members of Gould’s
+“solar cluster.”</p>
+
+<p class='c007'>Assuming that the distances are inversely proportional to
+the proper motions, Professor Kapteyn computes the relative
+volumes of the spherical shells which contain the stars with
+different proper motions (from one-tenth of a second to one
+second of arc and more). Comparing these volumes with the
+corresponding number of stars, we arrive at an estimate of the
+density of star distribution at various distances. The result
+of this calculation shows that the distribution of stars of the
+Sirian type approaches uniformity when a large number of the
+faint stars (ninth magnitude) are considered. With reference
+to the stars of the second type, however, the larger the proper
+motion the greater the number of the stars; or, in other
+words, the second type, or solar stars, are crowded together in
+the sun’s vicinity. Evidence in favour of this conclusion is
+<span class='pageno' id='Page_561'>561</span>afforded by the fact that, of eight stars having the largest
+measured parallax (and whose spectrum has been determined),
+I find that seven have spectra of the solar type. The exception
+is Sirius, which is evidently an exceptional star with
+reference to its brightness and comparative proximity to the
+earth, no other star of the first magnitude having nearly so
+large a parallax. Indeed, the average distance of all the first
+magnitude stars is about forty times the distance of Sirius.</p>
+
+<p class='c007'>Professor Kapteyn finds that the centre of greatest condensation
+of the solar type stars lies near a point situated about
+ten degrees to the west of the great nebula in Andromeda,
+and that this centre nearly coincides with the point which, according
+to Struve and Herschel, represents the apparent centre
+of the Milky Way considered as a ring. This would indicate
+that the sun and solar system lie a little to the north of the
+Milky Way, and towards a point situated in the northern portion
+of the constellation of the Centaur. The fact is worth
+noting, that the nearest fixed star to the earth, Alpha Centauri,
+lies not very far from this point. Possibly there may
+be other stars in this direction having a measured parallax,
+as the southern portion of the heavens has not yet been
+thoroughly explored.</p>
+
+<p class='c007'>Professor Kapteyn finds that for stars of equal brightness,
+those of the Sirian type are, on an average, about two and
+three-quarter times farther from the earth than those of the
+solar type. Now, as light varies inversely as the square of the
+distance, this would imply that the Sirian stars are intrinsically
+brighter than those of the solar type. This conclusion is confirmed
+by the great brilliancy of Sirius and other stars of the
+same type in proportion to their mass. I have shown in
+Chapter IV. that Sirius is about ten times brighter than the
+sun would be if placed at the same distance, although its mass
+is only twice the sun’s mass, as computed from the orbit of its
+satellite.</p>
+
+<p class='c007'>The general conclusions to be derived from the above
+results seems to be that the sun is a member of a cluster of
+stars, possibly distributed in the form of a ring, and that outside
+<span class='pageno' id='Page_562'>562</span>this ring, at a much greater distance from us than the
+stars of the solar cluster, lies a considerably richer ring-shaped
+cluster, the light of which, reduced to nebulosity by immensity
+of distance, produces the Milky Way gleam of our midnight
+skies.</p>
+
+<div class='chapter'>
+  <span class='pageno' id='Page_564'>564</span>
+  <h2 class='c006'>INDEX</h2>
+</div>
+
+<ul class='index c002'>
+  <li class='center'>A</li>
+  <li class='c033'>Aberration of light, discovered, <a href='#Page_18'>18</a>;
+    <ul>
+      <li>a proof of the earth’s revolution, <a href='#Page_57'>57</a>;</li>
+      <li>of meteor-radiants, <a href='#Page_396'>396</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Aboul Wefa, the moon’s variation, <a href='#Page_5'>5</a></li>
+  <li class='c033'>Acceleration, <a href='#Page_152'>152</a></li>
+  <li class='c033'>Achromatic lens, <a href='#Page_177'>177</a></li>
+  <li class='c033'>Adams, <a href='#Page_449'>449</a>;
+    <ul>
+      <li>discovery of Neptune, <a href='#Page_32'>32</a>, <a href='#Page_349'>349</a>;</li>
+      <li>orbit of November meteors, <a href='#Page_393'>393</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Aerolites. <i>See</i> Meteorites</li>
+  <li class='c033'>Airy, reduction of Greenwich observations, <a href='#Page_19'>19</a>;
+    <ul>
+      <li>search for Neptune, <a href='#Page_32'>32</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Albategnius, movement of the sun’s apogee, <a href='#Page_5'>5</a></li>
+  <li class='c033'>Albedo of Mercury, <a href='#Page_274'>274</a>;
+    <ul>
+      <li>of Venus, <a href='#Page_278'>278</a>;</li>
+      <li>of the earth, <a href='#Page_289'>289</a>;</li>
+      <li>of the moon, <a href='#Page_290'>290</a>;</li>
+      <li>of Mars, <a href='#Page_298'>298</a>, <a href='#Page_334'>334</a>;</li>
+      <li>of asteroids, <a href='#Page_312'>312</a>;</li>
+      <li>of Jupiter, <a href='#Page_320'>320</a>;</li>
+      <li>of Jupiter’s satellites, <a href='#Page_330'>330</a>, <a href='#Page_332'>332</a>;</li>
+      <li>of Saturn, <a href='#Page_334'>334</a>;</li>
+      <li>of rings, <a href='#Page_338'>338</a>;</li>
+      <li>of Titan, <a href='#Page_342'>342</a>;</li>
+      <li>of Uranus, <a href='#Page_345'>345</a>;</li>
+      <li>of Neptune, <a href='#Page_349'>349</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Alcor, <a href='#Page_402'>402</a></li>
+  <li class='c033'>Alcyone, <a href='#Page_499'>499</a>–502</li>
+  <li class='c033'>Aldebaran, <a href='#Page_403'>403</a>, <a href='#Page_404'>404</a>, <a href='#Page_407'>407</a>, <a href='#Page_415'>415</a>, <a href='#Page_421'>421</a>, <a href='#Page_423'>423</a>, <a href='#Page_427'>427</a></li>
+  <li class='c033'>Algol, <a href='#Page_407'>407</a>, <a href='#Page_415'>415</a>, <a href='#Page_453'>453</a>, <a href='#Page_457'>457</a>, <a href='#Page_469'>469</a>–474</li>
+  <li class='c033'>Almagest, <a href='#Page_4'>4</a>, <a href='#Page_6'>6</a></li>
+  <li class='c033'>Al-Mamûm’s school of astronomy at Baghdad, <a href='#Page_5'>5</a></li>
+  <li class='c033'>Alphard, <a href='#Page_409'>409</a>, <a href='#Page_415'>415</a></li>
+  <li class='c033'>Alphonsine tables, <a href='#Page_6'>6</a></li>
+  <li class='c033'>Al-Sûfi, description of the stars, <a href='#Page_5'>5</a>;
+    <ul>
+      <li>Alphard, red, <a href='#Page_415'>415</a>;</li>
+      <li>Algol, red, <a href='#Page_472'>472</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Altair, <a href='#Page_404'>404</a>, <a href='#Page_427'>427</a></li>
+  <li class='c033'>Altazimuth, <a href='#Page_184'>184</a>, <a href='#Page_202'>202</a></li>
+  <li class='c033'>Altitude, <a href='#Page_65'>65</a></li>
+  <li class='c033'>Amplitude, <a href='#Page_66'>66</a></li>
+  <li class='c033'>Anderson, Dr., discovery of new star, <a href='#Page_489'>489</a></li>
+  <li class='c033'>Andromeda nebula, <a href='#Page_409'>409</a>, <a href='#Page_529'>529</a>–532</li>
+  <li class='c033'>Andromedæ, Gamma, <a href='#Page_412'>412</a>, <a href='#Page_417'>417</a></li>
+  <li class='c033'>— Nova, <a href='#Page_489'>489</a>, <a href='#Page_491'>491</a></li>
+  <li class='c033'>Andromede meteor-showers, <a href='#Page_393'>393</a>, <a href='#Page_394'>394</a></li>
+  <li class='c033'>Angelot, lunar volcanic action, <a href='#Page_293'>293</a></li>
+  <li class='c033'>Annular eclipse, <a href='#Page_113'>113</a></li>
+  <li class='c033'>— nebulæ, <a href='#Page_526'>526</a>, <a href='#Page_527'>527</a></li>
+  <li class='c033'>Antares, <a href='#Page_404'>404</a>, <a href='#Page_415'>415</a></li>
+  <li class='c033'>Anthelmus, new star, <a href='#Page_484'>484</a></li>
+  <li class='c033'>Antlia, <a href='#Page_468'>468</a></li>
+  <li class='c033'>Aphelion, <a href='#Page_75'>75</a></li>
+  <li class='c033'>Apogee, <a href='#Page_89'>89</a></li>
+  <li class='c033'>Apse Line, <a href='#Page_75'>75</a></li>
+  <li class='c033'>Aquilæ, Eta, <a href='#Page_467'>467</a></li>
+  <li class='c033'>Arago, nature of meteorites, <a href='#Page_392'>392</a>;
+    <ul>
+      <li>parallax of 61 Cygni, <a href='#Page_422'>422</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Arc of meridian, <a href='#Page_130'>130</a></li>
+  <li class='c033'>Arcturus, <a href='#Page_403'>403</a>, <a href='#Page_405'>405</a>, <a href='#Page_406'>406</a>, <a href='#Page_415'>415</a>, <a href='#Page_423'>423</a>, <a href='#Page_427'>427</a></li>
+  <li class='c033'>Argelander, solar translation, <a href='#Page_28'>28</a>;
+    <ul>
+      <li>survey of the heavens, <a href='#Page_38'>38</a>;</li>
+      <li>comet of 1811, <a href='#Page_357'>357</a>;</li>
+      <li>estimate of stars of ninth magnitude, <a href='#Page_541'>541</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Argo Nebula, <a href='#Page_522'>522</a>, <a href='#Page_523'>523</a>, <a href='#Page_549'>549</a></li>
+  <li class='c033'>— Eta, <a href='#Page_462'>462</a>–464</li>
+  <li class='c033'>Argon, not a solar element, <a href='#Page_250'>250</a>;
+    <ul>
+      <li>peculiar qualities, <a href='#Page_255'>255</a>;</li>
+      <li>found in meteorites, <a href='#Page_389'>389</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Aries, first point of, <a href='#Page_67'>67</a></li>
+  <li class='c033'>Aristarchus, heliocentric system, <a href='#Page_4'>4</a></li>
+  <li class='c033'>Aristotle, description of a comet, <a href='#Page_358'>358</a></li>
+  <li class='c033'>Asteroids, position in solar system, <a href='#Page_229'>229</a>, <a href='#Page_230'>230</a>, <a href='#Page_310'>310</a>;
+    <ul>
+      <li>discoveries, <a href='#Page_311'>311</a>, <a href='#Page_314'>314</a>;</li>
+      <li>diameters, <a href='#Page_312'>312</a>, <a href='#Page_315'>315</a>;</li>
+      <li>computation of orbits, <a href='#Page_314'>314</a>;</li>
+      <li>numbers and joint mass, <a href='#Page_315'>315</a>;</li>
+      <li><span class='pageno' id='Page_565'>565</span>distribution, <a href='#Page_316'>316</a>;</li>
+      <li>groups, <a href='#Page_317'>317</a>;</li>
+      <li>origin, <a href='#Page_318'>318</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Asterope, <a href='#Page_498'>498</a>, <a href='#Page_499'>499</a></li>
+  <li class='c033'>Astronomy, Greek, <a href='#Page_3'>3</a>, <a href='#Page_4'>4</a>;
+    <ul>
+      <li>Arab, <a href='#Page_4'>4</a>–6;</li>
+      <li>Tartar, <a href='#Page_5'>5</a>;</li>
+      <li>of the Invisible, <a href='#Page_31'>31</a>;</li>
+      <li>gravitational, <a href='#Page_11'>11</a>, <a href='#Page_33'>33</a>;</li>
+      <li>spectroscopic, <a href='#Page_33'>33</a>–36;</li>
+      <li>photographic, <a href='#Page_36'>36</a>–38</li>
+    </ul>
+  </li>
+  <li class='c033'>Astrophysics, foundation of, <a href='#Page_36'>36</a></li>
+  <li class='c033'>Atmosphere, of the sun, <a href='#Page_240'>240</a>, <a href='#Page_271'>271</a>;
+    <ul>
+      <li>of Mercury, <a href='#Page_277'>277</a>;</li>
+      <li>of Venus, <a href='#Page_278'>278</a>, <a href='#Page_279'>279</a>;</li>
+      <li>of the earth, <a href='#Page_286'>286</a>, <a href='#Page_313'>313</a>;</li>
+      <li>of the moon, <a href='#Page_294'>294</a>, <a href='#Page_313'>313</a>;</li>
+      <li>of Mars, <a href='#Page_299'>299</a>, <a href='#Page_307'>307</a>;</li>
+      <li>of Vesta, <a href='#Page_312'>312</a>–313;</li>
+      <li>presence dependant upon mass, <a href='#Page_313'>313</a>;</li>
+      <li>of Jupiter, <a href='#Page_326'>326</a>;</li>
+      <li>of Uranus, <a href='#Page_345'>345</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Atmospheric refraction, <a href='#Page_52'>52</a></li>
+  <li class='c033'>Augmentation of moon’s diameter, <a href='#Page_144'>144</a></li>
+  <li class='c033'>Aurigæ Beta, <a href='#Page_404'>404</a>, <a href='#Page_454'>454</a>, <a href='#Page_456'>456</a>, <a href='#Page_457'>457</a></li>
+  <li class='c033'>— New Star, <a href='#Page_489'>489</a></li>
+  <li class='c033'>Auroræ, magnetic relations, <a href='#Page_17'>17</a>, <a href='#Page_288'>288</a></li>
+  <li class='c033'>Auwers’ reduction of Bradley’s observations, <a href='#Page_19'>19</a>;
+    <ul>
+      <li>proper motion of Sirius, <a href='#Page_437'>437</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Azimuth, <a href='#Page_65'>65</a></li>
+  <li class='center'>B</li>
+  <li class='c033'>Babinet, rarity of cometary matter, <a href='#Page_366'>366</a></li>
+  <li class='c033'>Baden-Powell, Sir George, eclipse-expedition, <a href='#Page_259'>259</a>;
+    <ul>
+      <li>coronal photographs, <a href='#Page_271'>271</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Bailey, Prof., <a href='#Page_441'>441</a>, <a href='#Page_464'>464</a>, <a href='#Page_511'>511</a>, <a href='#Page_513'>513</a>, <a href='#Page_539'>539</a></li>
+  <li class='c033'>Ball, Sir Robert, <a href='#Page_422'>422</a>, <a href='#Page_433'>433</a></li>
+  <li class='c033'>Barnard, Prof., photograph of corona of January 1, 1889, <a href='#Page_268'>268</a>–9;
+    <ul>
+      <li>effect of totality, <a href='#Page_270'>270</a>;</li>
+      <li>zodiacal counterglow, <a href='#Page_272'>272</a>;</li>
+      <li>photograph of eclipsed moon, <a href='#Page_296'>296</a>;</li>
+      <li>drawing of Mars, <a href='#Page_302'>302</a>;</li>
+      <li>seas of Mars, <a href='#Page_306'>306</a>;</li>
+      <li>measurements of asteroids, <a href='#Page_312'>312</a>;</li>
+      <li>markings on Jupiter’s satellites, <a href='#Page_330'>330</a>;</li>
+      <li>discovery of fifth satellite, <a href='#Page_331'>331</a>;</li>
+      <li>measures of Saturn, <a href='#Page_335'>335</a>;</li>
+      <li>of ring-system, <a href='#Page_330'>330</a>;</li>
+      <li>disappearance of rings, <a href='#Page_337'>337</a>;</li>
+      <li>eclipse of Japetus, <a href='#Page_338'>338</a>;</li>
+      <li>compression of Uranus, <a href='#Page_343'>343</a>, <a href='#Page_344'>344</a>;</li>
+      <li>Encke’s comet, <a href='#Page_366'>366</a>;</li>
+      <li>comet-photographs, <a href='#Page_378'>378</a>–381;</li>
+      <li>Swift’s comet, <a href='#Page_383'>383</a>;</li>
+      <li>Nova in Auriga, <a href='#Page_494'>494</a>;</li>
+      <li>Alcyone, <a href='#Page_500'>500</a>;</li>
+      <li>curved nebulosity stretching over constellation of Orion, <a href='#Page_520'>520</a>;</li>
+      <li>annular nebulæ, <a href='#Page_526'>526</a>;</li>
+      <li>stars in streams, <a href='#Page_551'>551</a>;</li>
+      <li>vacancies in the Milky Way, <a href='#Page_554'>554</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Base line, <a href='#Page_131'>131</a></li>
+  <li class='c033'>Baxendell, <a href='#Page_484'>484</a></li>
+  <li class='c033'>Bayer, <a href='#Page_404'>404</a>, <a href='#Page_529'>529</a></li>
+  <li class='c033'>Behrmann, <a href='#Page_400'>400</a>, <a href='#Page_433'>433</a>, <a href='#Page_541'>541</a></li>
+  <li class='c033'>Bellatrix, <a href='#Page_408'>408</a></li>
+  <li class='c033'>Bélopolsky, spectrographic determination of Jupiter’s rotation, <a href='#Page_325'>325</a>;
+    <ul>
+      <li>absolute velocity of 61 Cygni, <a href='#Page_427'>427</a>;</li>
+      <li>spectroscopic examination of Castor, <a href='#Page_451'>451</a>;</li>
+      <li>observation of Delta Cephei, <a href='#Page_456'>456</a>;</li>
+      <li>Beta Lyræ, <a href='#Page_466'>466</a>–467</li>
+    </ul>
+  </li>
+  <li class='c033'>Berberich, variability of Encke’s comet, <a href='#Page_360'>360</a></li>
+  <li class='c033'>Berson, aeronautic ascent, <a href='#Page_286'>286</a></li>
+  <li class='c033'>Bessel, <cite>Fundamenta Astronomiæ</cite>, <a href='#Page_19'>19</a>;
+    <ul>
+      <li>astronomy of the invisible, <a href='#Page_31'>31</a>, <a href='#Page_32'>32</a>;</li>
+      <li>measurement of the Pleiades, <a href='#Page_37'>37</a>;</li>
+      <li>Halley’s comet, <a href='#Page_355'>355</a>;</li>
+      <li>comet of 1807, <a href='#Page_362'>362</a>;</li>
+      <li>Epsilon Lyræ, <a href='#Page_411'>411</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Betelgeuse, <a href='#Page_404'>404</a>, <a href='#Page_408'>408</a>, <a href='#Page_415'>415</a>, <a href='#Page_427'>427</a></li>
+  <li class='c033'>Bianchi, <a href='#Page_459'>459</a></li>
+  <li class='c033'>Bianchini, rotation of Venus, <a href='#Page_280'>280</a></li>
+  <li class='c033'>Biela, discovery of a comet, <a href='#Page_365'>365</a></li>
+  <li class='c033'>Bigelow, theory of Zodiacal Light, <a href='#Page_272'>272</a></li>
+  <li class='c033'>Binary stars, <a href='#Page_431'>431</a></li>
+  <li class='c033'>Biot, meteoric fall, <a href='#Page_387'>387</a></li>
+  <li class='c033'>Bird, quadrants, <a href='#Page_19'>19</a>, <a href='#Page_20'>20</a></li>
+  <li class='c033'>“Bird, Red,” <a href='#Page_415'>415</a></li>
+  <li class='c033'>Birmingham, <a href='#Page_417'>417</a>, <a href='#Page_485'>485</a>, <a href='#Page_486'>486</a></li>
+  <li class='c033'>“Blaze Star,” <a href='#Page_485'>485</a>, <a href='#Page_487'>487</a></li>
+  <li class='c033'>Bliss, astronomer-royal, <a href='#Page_19'>19</a></li>
+  <li class='c033'>Bode’s law, <a href='#Page_145'>145</a>, <a href='#Page_232'>232</a>, <a href='#Page_311'>311</a>, <a href='#Page_317'>317</a>, <a href='#Page_349'>349</a></li>
+  <li class='c033'>Boeddicker, Dr., heat-phases of eclipsed moon, <a href='#Page_295'>295</a></li>
+  <li class='c033'>Bolometer, <a href='#Page_226'>226</a>, <a href='#Page_239'>239</a></li>
+  <li class='c033'>Bompas, <a href='#Page_430'>430</a></li>
+  <li class='c033'>Bond, W. C., discoveries of Hyperion and of Saturn’s dusky ring, <a href='#Page_25'>25</a>, <a href='#Page_336'>336</a>, <a href='#Page_341'>341</a>;
+    <ul>
+      <li>celestial photography, <a href='#Page_36'>36</a>, <a href='#Page_37'>37</a>;</li>
+      <li>the great nebula, <a href='#Page_530'>530</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Bradley, discoveries of aberration and nutation, <a href='#Page_18'>18</a>, <a href='#Page_20'>20</a>;
+    <ul>
+      <li>reduction of his observations, <a href='#Page_19'>19</a>;</li>
+      <li>Saturn’s rings, <a href='#Page_337'>337</a>;</li>
+      <li>the distance of stars, <a href='#Page_419'>419</a>–420;</li>
+      <li>Gamma Virginis, <a href='#Page_445'>445</a>–446</li>
+    </ul>
+  </li>
+  <li class='c033'>Brahé, Tycho, the moon’s variation, <a href='#Page_5'>5</a>;
+    <ul>
+      <li>career, <a href='#Page_8'>8</a>;</li>
+      <li>scheme of the celestial movements, <a href='#Page_9'>9</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Bredichin, theory of comets’ tails, <a href='#Page_369'>369</a>, <a href='#Page_370'>370</a>.
+    <ul>
+      <li>(<i>See also</i> Tycho.)</li>
+    </ul>
+  </li>
+  <li class='c033'>Brenner, ashen light of Venus, <a href='#Page_279'>279</a>;
+    <ul>
+      <li>rotation of Venus, <a href='#Page_280'>280</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Brightest stars, <a href='#Page_403'>403</a>, <a href='#Page_404'>404</a>, <a href='#Page_546'>546</a></li>
+  <li class='c033'>Brinkley, <a href='#Page_422'>422</a></li>
+  <li class='c033'>British catalogue, <a href='#Page_15'>15</a>, <a href='#Page_16'>16</a></li>
+  <li class='c033'>Brooks’ cometary discoveries, <a href='#Page_365'>365</a>, <a href='#Page_371'>371</a>, <a href='#Page_380'>380</a></li>
+  <li class='c033'><span class='pageno' id='Page_566'>566</span>Bunsen, foundation of spectrum analysis, <a href='#Page_33'>33</a></li>
+  <li class='c033'>Burnham, <a href='#Page_433'>433</a>, <a href='#Page_437'>437</a>, <a href='#Page_441'>441</a>, <a href='#Page_448'>448</a>, <a href='#Page_509'>509</a></li>
+  <li class='center'>C</li>
+  <li class='c033'>Calcium, represented in Fraunhofer spectrum, <a href='#Page_230'>230</a>;
+    <ul>
+      <li>in chromospheric and prominence-spectra, <a href='#Page_258'>258</a>, <a href='#Page_261'>261</a>, <a href='#Page_262'>262</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Calendar, <a href='#Page_86'>86</a></li>
+  <li class='c033'>Callandreau, capture of comets, <a href='#Page_372'>372</a></li>
+  <li class='c033'>Campbell, Prof., spectrum of Mars, <a href='#Page_306'>306</a>;
+    <ul>
+      <li>mountains on, <a href='#Page_307'>307</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Canals of Mars, <a href='#Page_301'>301</a>–305</li>
+  <li class='c033'>Cancri, S., <a href='#Page_474'>474</a></li>
+  <li class='c033'>— Zeta, <a href='#Page_439'>439</a>, <a href='#Page_440'>440</a></li>
+  <li class='c033'>Canis Majoris, R, <a href='#Page_473'>473</a></li>
+  <li class='c033'>Canopus, <a href='#Page_403'>403</a></li>
+  <li class='c033'>Capella, <a href='#Page_403'>403</a>, <a href='#Page_406'>406</a>, <a href='#Page_415'>415</a>, <a href='#Page_427'>427</a></li>
+  <li class='c033'>Capricornus, <a href='#Page_411'>411</a></li>
+  <li class='c033'>Capture-theory of comets, <a href='#Page_372'>372</a></li>
+  <li class='c033'>Carbon in sun, <a href='#Page_242'>242</a>, <a href='#Page_250'>250</a>;
+    <ul>
+      <li>in comets, <a href='#Page_368'>368</a>;</li>
+      <li>in meteorites, <a href='#Page_389'>389</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Cardinal points, <a href='#Page_51'>51</a></li>
+  <li class='c033'>Carrington, sun-spot zones, <a href='#Page_247'>247</a>;
+    <ul>
+      <li>sun’s rotation, <a href='#Page_248'>248</a>, <a href='#Page_249'>249</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Casey, <a href='#Page_433'>433</a></li>
+  <li class='c033'>Cassegrain telescope, <a href='#Page_180'>180</a></li>
+  <li class='c033'>Cassini, rotation of Venus, <a href='#Page_280'>280</a>;
+    <ul>
+      <li>red spot on Jupiter, <a href='#Page_323'>323</a>;</li>
+      <li>division of Saturn’s rings, <a href='#Page_336'>336</a>;</li>
+      <li>discoveries of Saturnian satellites, <a href='#Page_341'>341</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Cassiopeia, Chair of, <a href='#Page_405'>405</a>, <a href='#Page_481'>481</a>, <a href='#Page_549'>549</a></li>
+  <li class='c033'>Cassiopeiæ, Eta, <a href='#Page_413'>413</a>, <a href='#Page_450'>450</a></li>
+  <li class='c033'>Castor, <a href='#Page_404'>404</a>, <a href='#Page_406'>406</a>, <a href='#Page_413'>413</a>, <a href='#Page_450'>450</a>, <a href='#Page_451'>451</a></li>
+  <li class='c033'>Catalogues of stars, <a href='#Page_70'>70</a></li>
+  <li class='c033'>Celoria, <a href='#Page_433'>433</a>, <a href='#Page_442'>442</a>, <a href='#Page_540'>540</a></li>
+  <li class='c033'>Centauri, Alpha, <a href='#Page_410'>410</a>, <a href='#Page_413'>413</a>, <a href='#Page_422'>422</a>, <a href='#Page_440'>440</a>, <a href='#Page_441'>441</a></li>
+  <li class='c033'>— Omega, <a href='#Page_512'>512</a>, <a href='#Page_513'>513</a>, <a href='#Page_516'>516</a>, <a href='#Page_539'>539</a></li>
+  <li class='c033'>— R, <a href='#Page_478'>478</a></li>
+  <li class='c033'>Cephii Delta, <a href='#Page_417'>417</a>, <a href='#Page_456'>456</a>, <a href='#Page_466'>466</a></li>
+  <li class='c033'>— U, <a href='#Page_474'>474</a></li>
+  <li class='c033'>Ceraski, luminous night-clouds, <a href='#Page_286'>286</a>;
+    <ul>
+      <li>discovery of U Cephei, <a href='#Page_474'>474</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Ceres, discovery, <a href='#Page_311'>311</a>;
+    <ul>
+      <li>diameter, <a href='#Page_312'>312</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Cerulli, rotation of Venus, <a href='#Page_280'>280</a></li>
+  <li class='c033'>Ceti, Mira, <a href='#Page_458'>458</a></li>
+  <li class='c033'>Challis, search for Neptune, <a href='#Page_32'>32</a></li>
+  <li class='c033'>Chandler, <a href='#Page_462'>462</a>, <a href='#Page_472'>472</a>, <a href='#Page_476'>476</a></li>
+  <li class='c033'>Charlois, asteroidal discoveries, <a href='#Page_314'>314</a></li>
+  <li class='c033'>Chemistry, universal, <a href='#Page_35'>35</a>, <a href='#Page_36'>36</a>;
+    <ul>
+      <li>solar, <a href='#Page_250'>250</a>, <a href='#Page_255'>255</a>;</li>
+      <li>of prominences, <a href='#Page_256'>256</a>;</li>
+      <li>of chromosphere, <a href='#Page_258'>258</a>;</li>
+      <li>of comets, <a href='#Page_368'>368</a>, <a href='#Page_370'>370</a>, <a href='#Page_384'>384</a>;</li>
+      <li>of meteorites, <a href='#Page_389'>389</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Chromosphere, <a href='#Page_253'>253</a>, <a href='#Page_258'>258</a></li>
+  <li class='c033'>Chronograph, <a href='#Page_175'>175</a></li>
+  <li class='c033'>Chronometer, <a href='#Page_175'>175</a></li>
+  <li class='c033'>Circle, meridian, <a href='#Page_198'>198</a>;
+    <ul>
+      <li>transit, <a href='#Page_198'>198</a>;</li>
+      <li>position, <a href='#Page_208'>208</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Circumpolar stars, <a href='#Page_46'>46</a></li>
+  <li class='c033'>Clairaut, verification of Newton’s law, <a href='#Page_11'>11</a>;
+    <ul>
+      <li>calculation of Halley’s comet, <a href='#Page_16'>16</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Clark, Alvan, great refractors, <a href='#Page_26'>26</a></li>
+  <li class='c033'>— — G., detection of the companion of Sirius, <a href='#Page_26'>26</a>, <a href='#Page_437'>437</a></li>
+  <li class='c033'>Clarke, dimensions of earth, <a href='#Page_134'>134</a></li>
+  <li class='c033'>Clausen, groups of comets, <a href='#Page_361'>361</a></li>
+  <li class='c033'>Clerke, Agnes, appearance of R Sculptoris, <a href='#Page_416'>416</a>;
+    <ul>
+      <li>examination of Pickering’s catalogue of stars, <a href='#Page_541'>541</a>;</li>
+      <li>estimate of total light of stars to magnitude 9½, <a href='#Page_543'>543</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Clock, astronomical, <a href='#Page_174'>174</a>;
+    <ul>
+      <li>driving, <a href='#Page_186'>186</a>;</li>
+      <li>sidereal, <a href='#Page_68'>68</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Clock stars, <a href='#Page_82'>82</a></li>
+  <li class='c033'>Clusters, globular, <a href='#Page_507'>507</a>–517;
+    <ul>
+      <li>irregular, <a href='#Page_497'>497</a>–507</li>
+    </ul>
+  </li>
+  <li class='c033'>“Coal sacks” in Milky Way, <a href='#Page_554'>554</a></li>
+  <li class='c033'>Coelostat, <a href='#Page_194'>194</a></li>
+  <li class='c033'>Collimation of transit instrument, <a href='#Page_200'>200</a></li>
+  <li class='c033'>Collimator of spectroscope, <a href='#Page_215'>215</a></li>
+  <li class='c033'>Colours of double stars, <a href='#Page_417'>417</a></li>
+  <li class='c033'>Comæ Berenices, <a href='#Page_434'>434</a>, <a href='#Page_502'>502</a>, <a href='#Page_549'>549</a></li>
+  <li class='c033'>Comet, Aristotle’s, <a href='#Page_352'>352</a>, <a href='#Page_353'>353</a>;
+    <ul>
+      <li>of 1743, <a href='#Page_354'>354</a>;</li>
+      <li>Newton’s, <a href='#Page_355'>355</a>;</li>
+      <li>of 1843, <a href='#Page_358'>358</a>, <a href='#Page_359'>359</a>;</li>
+      <li>Tebbutt’s, <a href='#Page_362'>362</a>, <a href='#Page_368'>368</a>;</li>
+      <li>Donati’s, <a href='#Page_362'>362</a>, <a href='#Page_369'>369</a>;</li>
+      <li>Lexell’s, <a href='#Page_365'>365</a>, <a href='#Page_370'>370</a>, <a href='#Page_371'>371</a>;</li>
+      <li>Brooks’, of 1889, <a href='#Page_365'>365</a>;</li>
+      <li>of 1893, <a href='#Page_380'>380</a>;</li>
+      <li>Winnecke’s, <a href='#Page_368'>368</a>, <a href='#Page_371'>371</a>, <a href='#Page_372'>372</a>;</li>
+      <li>Brorsen’s, <a href='#Page_370'>370</a>;</li>
+      <li>Tuttle’s, <a href='#Page_372'>372</a>, <a href='#Page_393'>393</a>;</li>
+      <li>Wolf’s, <a href='#Page_377'>377</a>;</li>
+      <li>Rordame’s, <a href='#Page_383'>383</a>;</li>
+      <li>Gale’s, <a href='#Page_383'>383</a>;</li>
+      <li>Leonid, <a href='#Page_393'>393</a>, <a href='#Page_395'>395</a></li>
+    </ul>
+  </li>
+  <li class='c033'>— Halley’s, return in 1759, <a href='#Page_16'>16</a>, <a href='#Page_17'>17</a>;
+    <ul>
+      <li>status in solar system, <a href='#Page_230'>230</a>, <a href='#Page_232'>232</a>;</li>
+      <li>return in 1835, <a href='#Page_335'>335</a>, <a href='#Page_336'>336</a>;</li>
+      <li>type of tail, <a href='#Page_369'>369</a>;</li>
+      <li>a client of Neptune, <a href='#Page_371'>371</a>, <a href='#Page_372'>372</a></li>
+    </ul>
+  </li>
+  <li class='c033'>— Encke’s, disturbed by Mercury, <a href='#Page_273'>273</a>;
+    <ul>
+      <li>rarefaction, <a href='#Page_366'>366</a>;</li>
+      <li>acceleration, <a href='#Page_307'>307</a>;</li>
+      <li>exempt from Jupiter’s influence, <a href='#Page_371'>371</a></li>
+    </ul>
+  </li>
+  <li class='c033'>— of 1811, structure, <a href='#Page_356'>356</a>, <a href='#Page_357'>357</a>;
+    <ul>
+      <li>type of tail, <a href='#Page_369'>369</a>;</li>
+      <li>bulk, <a href='#Page_383'>383</a></li>
+    </ul>
+  </li>
+  <li class='c033'>— of 1843, surprising appearance, <a href='#Page_358'>358</a>;
+    <ul>
+      <li>conditions of movement, <a href='#Page_359'>359</a></li>
+    </ul>
+  </li>
+  <li class='c033'>— of 1882, photographs, <a href='#Page_38'>38</a>, <a href='#Page_361'>361</a>;
+    <ul>
+      <li>transit, <a href='#Page_359'>359</a>, <a href='#Page_361'>361</a>;</li>
+      <li>period, <a href='#Page_300'>300</a>;</li>
+      <li>spectrum, <a href='#Page_369'>369</a></li>
+    </ul>
+  </li>
+  <li class='c033'><span class='pageno' id='Page_567'>567</span>Comet, Biela’s, discovery, <a href='#Page_365'>365</a>;
+    <ul>
+      <li>duplication, <a href='#Page_366'>366</a>;</li>
+      <li>related meteor-swarm, <a href='#Page_393'>393</a>, <a href='#Page_394'>394</a></li>
+    </ul>
+  </li>
+  <li class='c033'>— Wells, spectrum, <a href='#Page_368'>368</a></li>
+  <li class='c033'>— photographically detected, <a href='#Page_377'>377</a></li>
+  <li class='c033'>Comets, orbits of, <a href='#Page_108'>108</a>;
+    <ul>
+      <li>periodic, <a href='#Page_109'>109</a>;</li>
+      <li>domiciled in solar system, <a href='#Page_230'>230</a>, <a href='#Page_371'>371</a>, <a href='#Page_372'>372</a>;</li>
+      <li>granular nuclei, <a href='#Page_353'>353</a>, <a href='#Page_379'>379</a>, <a href='#Page_384'>384</a>;</li>
+      <li>tenuity, <a href='#Page_354'>354</a>, <a href='#Page_384'>384</a>;</li>
+      <li>classification by Olbers, <a href='#Page_358'>358</a>, <a href='#Page_383'>383</a>;</li>
+      <li>groups, <a href='#Page_359'>359</a>, <a href='#Page_361'>361</a>, <a href='#Page_362'>362</a>;</li>
+      <li>disruption, <a href='#Page_360'>360</a>, <a href='#Page_366'>366</a>, <a href='#Page_379'>379</a>;</li>
+      <li>photographs, <a href='#Page_361'>361</a>, <a href='#Page_377'>377</a>, <a href='#Page_380'>380</a>;</li>
+      <li>chemistry, <a href='#Page_368'>368</a>, <a href='#Page_370'>370</a>, <a href='#Page_385'>385</a>;</li>
+      <li>luminous by electricity, <a href='#Page_309'>309</a>, <a href='#Page_384'>384</a>;</li>
+      <li>lost, <a href='#Page_370'>370</a>;</li>
+      <li>short-period, <a href='#Page_370'>370</a>, <a href='#Page_371'>371</a>;</li>
+      <li>capture by planets, <a href='#Page_371'>371</a>, <a href='#Page_372'>372</a>, <a href='#Page_384'>384</a>;</li>
+      <li>share sun’s translation, <a href='#Page_372'>372</a>;</li>
+      <li>meteoric relationships, <a href='#Page_384'>384</a>, <a href='#Page_393'>393</a>, <a href='#Page_394'>394</a></li>
+    </ul>
+  </li>
+  <li class='c033'>— tails, multiple, <a href='#Page_354'>354</a>, <a href='#Page_355'>355</a>, <a href='#Page_361'>361</a>, <a href='#Page_377'>377</a>;
+    <ul>
+      <li>electrical theory, <a href='#Page_357'>357</a>, <a href='#Page_369'>369</a>, <a href='#Page_383'>383</a>;</li>
+      <li>passage of the earth through, <a href='#Page_302'>302</a>, <a href='#Page_365'>365</a>;</li>
+      <li>three types, <a href='#Page_369'>369</a>;</li>
+      <li>structure shown in photographs, <a href='#Page_377'>377</a>, <a href='#Page_383'>383</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Common, Dr., <a href='#Page_25'>25</a>, <a href='#Page_510'>510</a>, <a href='#Page_520'>520</a>, <a href='#Page_532'>532</a>, <a href='#Page_534'>534</a></li>
+  <li class='c033'>Conjunctions, <a href='#Page_99'>99</a>, <a href='#Page_103'>103</a></li>
+  <li class='c033'>Constant of aberration, <a href='#Page_59'>59</a></li>
+  <li class='c033'>Constellations, <a href='#Page_45'>45</a></li>
+  <li class='c033'>Contacts in eclipse, <a href='#Page_114'>114</a></li>
+  <li class='c033'>Copeland, Dr., cometary spectra, <a href='#Page_368'>368</a>;
+    <ul>
+      <li>Nova in Auriga, <a href='#Page_490'>490</a>;</li>
+      <li>helium, <a href='#Page_519'>519</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Copernicus, residence in Italy, <a href='#Page_7'>7</a>;
+    <ul>
+      <li>theory of planetary revolutions, <a href='#Page_8'>8</a>, <a href='#Page_9'>9</a>, <a href='#Page_418'>418</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Cornelius, Gamma, <a href='#Page_479'>479</a></li>
+  <li class='c033'>Corona Borealis, Eta, <a href='#Page_437'>437</a></li>
+  <li class='c033'>— — Gamma, <a href='#Page_442'>442</a></li>
+  <li class='c033'>— solar, <a href='#Page_253'>253</a>;
+    <ul>
+      <li>compound nature of light, <a href='#Page_202'>202</a>;</li>
+      <li>daylight photography, <a href='#Page_267'>267</a>;</li>
+      <li>periodicity of type, <a href='#Page_208'>208</a>, <a href='#Page_270'>270</a>, <a href='#Page_272'>272</a>;</li>
+      <li>photographs, <a href='#Page_268'>268</a>–271;</li>
+      <li>rarefaction, <a href='#Page_271'>271</a>, <a href='#Page_361'>361</a>;</li>
+      <li>connexion with Zodiacal Light, <a href='#Page_272'>272</a>, <a href='#Page_273'>273</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Coronium, <a href='#Page_238'>238</a>, <a href='#Page_262'>262</a></li>
+  <li class='c033'>Co-tidal lines, <a href='#Page_165'>165</a></li>
+  <li class='c033'>Coudé telescope, <a href='#Page_27'>27</a>, <a href='#Page_296'>296</a></li>
+  <li class='c033'>Crateris, R, <a href='#Page_478'>478</a></li>
+  <li class='c033'>Craters, lunar, <a href='#Page_292'>292</a>, <a href='#Page_307'>307</a></li>
+  <li class='c033'>Crema meteorite, <a href='#Page_386'>386</a></li>
+  <li class='c033'>Cross, Southern, <a href='#Page_410'>410</a>, <a href='#Page_416'>416</a>, <a href='#Page_549'>549</a></li>
+  <li class='c033'>Crosswires, <a href='#Page_195'>195</a>, <a href='#Page_199'>199</a>, <a href='#Page_206'>206</a></li>
+  <li class='c033'>Crucis, Kappa, <a href='#Page_506'>506</a></li>
+  <li class='c033'>Cygni Beta, <a href='#Page_417'>417</a></li>
+  <li class='c033'>— Chi, <a href='#Page_460'>460</a></li>
+  <li class='c033'>— (<i>34</i>), <a href='#Page_482'>482</a></li>
+  <li class='c033'>— (<i>61</i>), <a href='#Page_422'>422</a>, <a href='#Page_427'>427</a></li>
+  <li class='c033'>— Rho, new star near, <a href='#Page_486'>486</a></li>
+  <li class='c033'>— Y, <a href='#Page_474'>474</a></li>
+  <li class='c033'>Cygnus, <a href='#Page_407'>407</a></li>
+  <li class='center'>D</li>
+  <li class='c033'>D’Alembert, verification of Newton’s Law, <a href='#Page_11'>11</a></li>
+  <li class='c033'>D’Arrest, asteroidal orbits, <a href='#Page_316'>316</a>;
+    <ul>
+      <li>comet, <a href='#Page_371'>371</a>, <a href='#Page_372'>372</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Darwin, G. H., tidal friction, <a href='#Page_236'>236</a>;
+    <ul>
+      <li>origin of the moon, <a href='#Page_236'>236</a>, <a href='#Page_237'>237</a>;</li>
+      <li>density of Saturn, <a href='#Page_333'>333</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Day and night, <a href='#Page_52'>52</a></li>
+  <li class='c033'>— apparent solar, <a href='#Page_79'>79</a>;
+    <ul>
+      <li>mean solar, <a href='#Page_79'>79</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Declination, <a href='#Page_66'>66</a></li>
+  <li class='c033'>De la Rue, celestial photography, <a href='#Page_36'>36</a>, <a href='#Page_295'>295</a></li>
+  <li class='c033'>Delphini, Beta, <a href='#Page_435'>435</a></li>
+  <li class='c033'>— Gamma, <a href='#Page_412'>412</a></li>
+  <li class='c033'>Deneb, <a href='#Page_407'>407</a></li>
+  <li class='c033'>Denning, rotation of Saturn, <a href='#Page_334'>334</a>;
+    <ul>
+      <li>discovery of a comet, <a href='#Page_370'>370</a>;</li>
+      <li>August meteors, <a href='#Page_391'>391</a>;</li>
+      <li>meteor-radiants, <a href='#Page_395'>395</a>, <a href='#Page_396'>396</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Density of earth, <a href='#Page_160'>160</a></li>
+  <li class='c033'>Deslandres, prominence-photography, <a href='#Page_261'>261</a>;
+    <ul>
+      <li>photographs of the sun as a bright-line star, <a href='#Page_262'>262</a>;</li>
+      <li>daylight coronal photography, <a href='#Page_267'>267</a>;</li>
+      <li>eclipse of 1893, <a href='#Page_270'>270</a>;</li>
+      <li>rotation of Jupiter, <a href='#Page_325'>325</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Dewar, atmospheric resistance to meteorites, <a href='#Page_388'>388</a></li>
+  <li class='c033'>Dhurmsala meteorite, <a href='#Page_389'>389</a></li>
+  <li class='c033'>Diameters, determination of, <a href='#Page_141'>141</a></li>
+  <li class='c033'>Diamonds in meteorites, <a href='#Page_390'>390</a></li>
+  <li class='c033'>Diffraction grating, <a href='#Page_216'>216</a></li>
+  <li class='c033'>Direct movement, <a href='#Page_89'>89</a></li>
+  <li class='c033'>— vision spectroscope, <a href='#Page_216'>216</a></li>
+  <li class='c033'>Distance of the stars, <a href='#Page_417'>417</a></li>
+  <li class='c033'>Doberck, Dr., <a href='#Page_441'>441</a>, <a href='#Page_442'>442</a>, <a href='#Page_447'>447</a>, <a href='#Page_450'>450</a>, <a href='#Page_451'>451</a></li>
+  <li class='c033'>Dollond, invention of achromatic lenses, <a href='#Page_21'>21</a></li>
+  <li class='c033'>Donati, discovery of a comet, <a href='#Page_362'>362</a>;
+    <ul>
+      <li>cometary spectrum, <a href='#Page_368'>368</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Double-slit method of photography, <a href='#Page_261'>261</a></li>
+  <li class='c033'>Draconis, Gamma, <a href='#Page_419'>419</a>, <a href='#Page_420'>420</a></li>
+  <li class='c033'>Draper, Henry, photograph of the moon, <a href='#Page_36'>36</a></li>
+  <li class='c033'>Dubjago, <a href='#Page_435'>435</a></li>
+  <li class='c033'>Dunér, spectroscopic measurement of the sun’s rotation, <a href='#Page_249'>249</a>;
+    <ul>
+      <li>R Hydræ, <a href='#Page_462'>462</a>;</li>
+      <li>Y Cygni, <a href='#Page_474'>474</a>;</li>
+      <li><span class='pageno' id='Page_568'>568</span>Z Herculis, <a href='#Page_475'>475</a></li>
+    </ul>
+  </li>
+  <li class='center'>E</li>
+  <li class='c033'>Earth, shape of, <a href='#Page_41'>41</a>, <a href='#Page_134'>134</a>;
+    <ul>
+      <li>size of, <a href='#Page_42'>42</a>, <a href='#Page_134'>134</a>;</li>
+      <li>rotation of, <a href='#Page_47'>47</a>, <a href='#Page_48'>48</a>, <a href='#Page_283'>283</a>, <a href='#Page_284'>284</a>;</li>
+      <li>revolution of, <a href='#Page_57'>57</a>;</li>
+      <li>orbit of, <a href='#Page_59'>59</a>, <a href='#Page_72'>72</a>;</li>
+      <li>varying speed of, <a href='#Page_75'>75</a>;</li>
+      <li>real path of, <a href='#Page_77'>77</a>;</li>
+      <li>shadow of, <a href='#Page_110'>110</a>;</li>
+      <li>mass of, <a href='#Page_159'>159</a>;</li>
+      <li>internal heat, <a href='#Page_284'>284</a>, <a href='#Page_285'>285</a>;</li>
+      <li>age, <a href='#Page_285'>285</a>;</li>
+      <li>atmosphere, <a href='#Page_286'>286</a>, <a href='#Page_289'>289</a>;</li>
+      <li>magnetic relations, <a href='#Page_287'>287</a>, <a href='#Page_288'>288</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Easton, <a href='#Page_532'>532</a>, <a href='#Page_548'>548</a></li>
+  <li class='c033'>Eccentricity of ellipse, <a href='#Page_74'>74</a></li>
+  <li class='c033'>Eclipse, solar, of 1842, <a href='#Page_253'>253</a>;
+    <ul>
+      <li>of 1860, <a href='#Page_254'>254</a>;</li>
+      <li>of 1868, <a href='#Page_254'>254</a>;</li>
+      <li>of 1870, <a href='#Page_258'>258</a>;</li>
+      <li>of 1896, <a href='#Page_259'>259</a>, <a href='#Page_271'>271</a>;</li>
+      <li>of 1882, <a href='#Page_260'>260</a>, <a href='#Page_268'>268</a>;</li>
+      <li>of 1878, <a href='#Page_268'>268</a>;</li>
+      <li>of 1889, <a href='#Page_268'>268</a>, <a href='#Page_270'>270</a>;</li>
+      <li>of 1893, <a href='#Page_270'>270</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Eclipses, lunar, <a href='#Page_111'>111</a>;
+    <ul>
+      <li>partial, <a href='#Page_111'>111</a>, <a href='#Page_114'>114</a>;</li>
+      <li>annular, <a href='#Page_113'>113</a>;</li>
+      <li>magnitude of, <a href='#Page_113'>113</a>;</li>
+      <li>total of sun, <a href='#Page_113'>113</a>;</li>
+      <li>duration of solar, <a href='#Page_115'>115</a>;</li>
+      <li>number of in a year, <a href='#Page_118'>118</a>;</li>
+      <li>recurrence of, <a href='#Page_119'>119</a>;</li>
+      <li>of satellites, <a href='#Page_121'>121</a>;</li>
+      <li>varieties of lunar, <a href='#Page_295'>295</a>;</li>
+      <li>of Jupiter’s satellites, <a href='#Page_329'>329</a>;</li>
+      <li>of Saturn’s, <a href='#Page_342'>342</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Ecliptic, <a href='#Page_56'>56</a></li>
+  <li class='c033'>— obliquity of, <a href='#Page_61'>61</a></li>
+  <li class='c033'>Electrical theory of photospheric radiance, <a href='#Page_242'>242</a>;
+    <ul>
+      <li>of corona, <a href='#Page_271'>271</a>, <a href='#Page_272'>272</a>;</li>
+      <li>of comets’ tails, <a href='#Page_357'>357</a>, <a href='#Page_358'>358</a>, <a href='#Page_383'>383</a>;</li>
+      <li>of cometary luminosity, <a href='#Page_369'>369</a>, <a href='#Page_384'>384</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Electra, <a href='#Page_498'>498</a>, <a href='#Page_499'>499</a></li>
+  <li class='c033'>Elements of an orbit, <a href='#Page_106'>106</a></li>
+  <li class='c033'>Elevating floor, <a href='#Page_193'>193</a></li>
+  <li class='c033'>Elger, lunar <i>maria</i>, <a href='#Page_290'>290</a></li>
+  <li class='c033'>Elkin, Dr., transit of great comet, <a href='#Page_359'>359</a>;
+    <ul>
+      <li>meteorograph, <a href='#Page_396'>396</a>;</li>
+      <li>measurements, <a href='#Page_421'>421</a>–423, <a href='#Page_425'>425</a>, <a href='#Page_433'>433</a>, <a href='#Page_438'>438</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Ellipse, properties of, <a href='#Page_73'>73</a>;
+    <ul>
+      <li>eccentricity of, <a href='#Page_74'>74</a>;</li>
+      <li>foci of, <a href='#Page_74'>74</a>;</li>
+      <li>to draw an, <a href='#Page_74'>74</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Elliptical nebulæ, <a href='#Page_529'>529</a>–533</li>
+  <li class='c033'>Elongations, <a href='#Page_99'>99</a></li>
+  <li class='c033'>Encke, discovery of a comet, <a href='#Page_366'>366</a>;
+    <ul>
+      <li>resisting medium, <a href='#Page_367'>367</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Enoch, Book of, <a href='#Page_404'>404</a></li>
+  <li class='c033'>Equation of time, <a href='#Page_79'>79</a></li>
+  <li class='c033'>Equator, terrestrial, <a href='#Page_50'>50</a>;
+    <ul>
+      <li>celestial, <a href='#Page_66'>66</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Equatorial coudé, <a href='#Page_193'>193</a></li>
+  <li class='c033'>Equatorial telescope, <a href='#Page_185'>185</a></li>
+  <li class='c033'>Equinoxes, <a href='#Page_55'>55</a>;
+    <ul>
+      <li>precession of, <a href='#Page_69'>69</a>, <a href='#Page_167'>167</a>, <a href='#Page_170'>170</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Equulei, Delta, <a href='#Page_433'>433</a></li>
+  <li class='c033'>Eridani, (<i>40</i>), <a href='#Page_444'>444</a></li>
+  <li class='c033'>Espin, <a href='#Page_460'>460</a>, <a href='#Page_482'>482</a>, <a href='#Page_494'>494</a></li>
+  <li class='c033'>Establishment of a port, <a href='#Page_165'>165</a></li>
+  <li class='c033'>Ether of space, <a href='#Page_546'>546</a></li>
+  <li class='c033'>Euler, lunar theory, <a href='#Page_11'>11</a></li>
+  <li class='c033'>Evening star, <a href='#Page_100'>100</a></li>
+  <li class='c033'>Evolution, of solar system, <a href='#Page_235'>235</a>, <a href='#Page_310'>310</a>;
+    <ul>
+      <li>of terrestrial, <a href='#Page_236'>236</a>, <a href='#Page_237'>237</a>, <a href='#Page_283'>283</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Eye-pieces, <a href='#Page_182'>182</a></li>
+  <li class='center'>F</li>
+  <li class='c033'>Fabricius, <a href='#Page_458'>458</a>, <a href='#Page_483'>483</a></li>
+  <li class='c033'>Fabry, cometary orbits, <a href='#Page_372'>372</a></li>
+  <li class='c033'>Faculæ, associated with sun-spots, <a href='#Page_244'>244</a>;
+    <ul>
+      <li>rotation, <a href='#Page_249'>249</a>;</li>
+      <li>photographed, <a href='#Page_262'>262</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Faye, planetary origin, <a href='#Page_235'>235</a>, <a href='#Page_350'>350</a>;
+    <ul>
+      <li>water on Mars, <a href='#Page_299'>299</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Fényi, solar eruptions, <a href='#Page_259'>259</a>, <a href='#Page_260'>260</a></li>
+  <li class='c033'>Finder of telescope, <a href='#Page_187'>187</a></li>
+  <li class='c033'>First Point of Aries, <a href='#Page_67'>67</a></li>
+  <li class='c033'>Fixed stars, <a href='#Page_45'>45</a>, <a href='#Page_423'>423</a></li>
+  <li class='c033'>Flammarion, rotation of Venus, <a href='#Page_280'>280</a>;
+    <ul>
+      <li>canals of Mars, <a href='#Page_304'>304</a>;</li>
+      <li>condition of Mars, <a href='#Page_309'>309</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Flamsteed, first astronomer-royal, <a href='#Page_15'>15</a>;
+    <ul>
+      <li>stellar parallax, <a href='#Page_18'>18</a>;</li>
+      <li>Flamsteed’s star, <a href='#Page_461'>461</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Fleming, Mrs., <a href='#Page_460'>460</a>, <a href='#Page_465'>465</a>, <a href='#Page_489'>489</a>, <a href='#Page_494'>494</a>–496</li>
+  <li class='c033'>Fletcher, <a href='#Page_447'>447</a></li>
+  <li class='c033'>Fomalhaut, <a href='#Page_404'>404</a></li>
+  <li class='c033'>Fontana, pseudo-satellite of Venus, <a href='#Page_282'>282</a></li>
+  <li class='c033'>Forbes, ultra-Neptunian planets, <a href='#Page_231'>231</a></li>
+  <li class='c033'>Foucault’s pendulum, <a href='#Page_48'>48</a>–50</li>
+  <li class='c033'>Fraunhofer, improvement of telescopes, <a href='#Page_21'>21</a>;
+    <ul>
+      <li>solar spectrum mapped by, <a href='#Page_34'>34</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Fraunhofer lines, <a href='#Page_34'>34</a>, <a href='#Page_249'>249</a>, <a href='#Page_259'>259</a>, <a href='#Page_271'>271</a>;
+    <ul>
+      <li>interpreted, <a href='#Page_35'>35</a>, <a href='#Page_250'>250</a>;</li>
+      <li>reflected in spectrum of Uranus, <a href='#Page_346'>346</a>;</li>
+      <li>in spectra of comets, <a href='#Page_368'>368</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Fritsche, <a href='#Page_433'>433</a></li>
+  <li class='c033'>Frost, spectrograph of Uranus, <a href='#Page_346'>346</a></li>
+  <li class='c033'>Froley, <a href='#Page_434'>434</a></li>
+  <li class='center'>G</li>
+  <li class='c033'>Galaxy. <i>See</i> Milky Way</li>
+  <li class='c033'>Galileo, telescopic observations, <a href='#Page_9'>9</a>;
+    <ul>
+      <li>double-star method of parallaxes, <a href='#Page_28'>28</a>, <a href='#Page_418'>418</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Gaseous nebula, <a href='#Page_517'>517</a></li>
+  <li class='c033'>Gemini, star cluster in, <a href='#Page_504'>504</a></li>
+  <li class='c033'>Geminorum, Zeta, <a href='#Page_468'>468</a></li>
+  <li class='c033'>Gemma, Cornelius, <a href='#Page_479'>479</a></li>
+  <li class='c033'>Gemmill, <a href='#Page_554'>554</a></li>
+  <li class='c033'><span class='pageno' id='Page_569'>569</span>Geocentric positions, <a href='#Page_70'>70</a></li>
+  <li class='c033'>Geodesy, <a href='#Page_129'>129</a></li>
+  <li class='c033'>Gill, Dr., photographs of comet of 1882, <a href='#Page_38'>38</a>, <a href='#Page_361'>361</a>;
+    <ul>
+      <li>parallax of Sirius, <a href='#Page_421'>421</a>;</li>
+      <li>parallax and velocity, Lacaille, <a href='#Page_424'>424</a>;</li>
+      <li>Omega Centauri, <a href='#Page_513'>513</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Glasenapp, <a href='#Page_433'>433</a>–434</li>
+  <li class='c033'>Gledhill, red spot on Jupiter, <a href='#Page_323'>323</a></li>
+  <li class='c033'>Globular clusters, <a href='#Page_507'>507</a></li>
+  <li class='c033'>Gnomon, <a href='#Page_125'>125</a></li>
+  <li class='c033'>Goodricke, <a href='#Page_465'>465</a>, <a href='#Page_466'>466</a>, <a href='#Page_470'>470</a></li>
+  <li class='c033'>Gould, Dr., photographic measurement of the Pleiades, <a href='#Page_37'>37</a>;
+    <ul>
+      <li>planetary photography, <a href='#Page_327'>327</a>;</li>
+      <li>Pi Gruis and R Sculptoris, <a href='#Page_416'>416</a>;</li>
+      <li>Kappa Crucis, <a href='#Page_506'>506</a>;</li>
+      <li>stars in Southern Hemisphere, <a href='#Page_541'>541</a>;</li>
+      <li>belt of stars intersecting the Milky Way, <a href='#Page_551'>551</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Graduated circles, <a href='#Page_171'>171</a></li>
+  <li class='c033'>Grating spectroscope, <a href='#Page_216'>216</a></li>
+  <li class='c033'>Gravity, surface, on Mercury, <a href='#Page_274'>274</a>;
+    <ul>
+      <li>on Venus, <a href='#Page_278'>278</a>;</li>
+      <li>on the moon, <a href='#Page_293'>293</a>;</li>
+      <li>on Mars, <a href='#Page_298'>298</a>;</li>
+      <li>on Saturn, <a href='#Page_335'>335</a>;</li>
+      <li>on Uranus, <a href='#Page_345'>345</a>;</li>
+      <li>on Neptune, <a href='#Page_349'>349</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Gravitation, laws of, <a href='#Page_153'>153</a>;
+    <ul>
+      <li>universal, <a href='#Page_156'>156</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Greenwich observations, <a href='#Page_15'>15</a>, <a href='#Page_19'>19</a>, <a href='#Page_20'>20</a></li>
+  <li class='c033'>Groombridge, <a href='#Page_424'>424</a></li>
+  <li class='c033'>Grosch, corona of 1867, <a href='#Page_268'>268</a></li>
+  <li class='c033'>Grubb, Sir Howard, great refractors, <a href='#Page_26'>26</a></li>
+  <li class='c033'>— Thomas, Melbourne reflecting telescope, <a href='#Page_24'>24</a></li>
+  <li class='c033'>Guinand, optical glass, <a href='#Page_21'>21</a></li>
+  <li class='c033'>Gully, Ludovic, <a href='#Page_488'>488</a></li>
+  <li class='c033'>Gylden, <a href='#Page_423'>423</a></li>
+  <li class='c033'>Gyroscope, <a href='#Page_50'>50</a></li>
+  <li class='center'>H</li>
+  <li class='c033'>Hadley, improvement of reflecting telescopes, <a href='#Page_21'>21</a></li>
+  <li class='c033'>Hale, spectrographs of prominences, <a href='#Page_261'>261</a>;
+    <ul>
+      <li>calcium light pictures of sun and surroundings, <a href='#Page_262'>262</a>;</li>
+      <li>double-slit method of coronal photography, <a href='#Page_267'>267</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Hall, Prof. Asaph, discovery of the moons of Mars, <a href='#Page_26'>26</a>, <a href='#Page_309'>309</a>;
+    <ul>
+      <li>rotation of Saturn, <a href='#Page_334'>334</a></li>
+    </ul>
+  </li>
+  <li class='c033'>— Chester More, invention of achromatic lenses, <a href='#Page_20'>20</a></li>
+  <li class='c033'>— Maxwell, <a href='#Page_472'>472</a></li>
+  <li class='c033'>Halley, law of gravitation, <a href='#Page_10'>10</a>;
+    <ul>
+      <li>acceleration of the moon, <a href='#Page_12'>12</a>;</li>
+      <li>astronomer-royal, <a href='#Page_16'>16</a>;</li>
+      <li>comet calculated by, <a href='#Page_16'>16</a>;</li>
+      <li>transits of Venus, <a href='#Page_17'>17</a>;</li>
+      <li>discovery of proper motion in stars, <a href='#Page_423'>423</a>;</li>
+      <li>discovery of the star cluster in Hercules, <a href='#Page_507'>507</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Harding, <a href='#Page_548'>548</a></li>
+  <li class='c033'>Hartwig, <a href='#Page_488'>488</a></li>
+  <li class='c033'>Harvest moon, <a href='#Page_95'>95</a></li>
+  <li class='c033'>Heavens, diurnal motion of, <a href='#Page_45'>45</a></li>
+  <li class='c033'>Heis, <a href='#Page_400'>400</a>, <a href='#Page_401'>401</a>, <a href='#Page_541'>541</a></li>
+  <li class='c033'>Heliocentric positions, <a href='#Page_70'>70</a></li>
+  <li class='c033'>Heliometer, <a href='#Page_209'>209</a></li>
+  <li class='c033'>Helium, a chromospheric element, <a href='#Page_255'>255</a>, <a href='#Page_258'>258</a>;
+    <ul>
+      <li>extracted from clevite, <a href='#Page_255'>255</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Helmholtz, maintenance of sun’s heat, <a href='#Page_234'>234</a>;
+    <ul>
+      <li>past duration of sunlight, <a href='#Page_285'>285</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Hencke, asteroidal discoveries, <a href='#Page_314'>314</a></li>
+  <li class='c033'>Henderson, <a href='#Page_422'>422</a>, <a href='#Page_447'>447</a></li>
+  <li class='c033'>Henry’s belts of Uranus, <a href='#Page_343'>343</a></li>
+  <li class='c033'>Hepidannus, <a href='#Page_479'>479</a></li>
+  <li class='c033'>Herculis, Alpha, <a href='#Page_413'>413</a>, <a href='#Page_416'>416</a></li>
+  <li class='c033'>— Zeta, <a href='#Page_435'>435</a></li>
+  <li class='c033'>— Z, <a href='#Page_475'>475</a></li>
+  <li class='c033'>Herschel, Sir John, mathematical analysis at Cambridge, <a href='#Page_15'>15</a>;
+    <ul>
+      <li>observations of nebulæ, <a href='#Page_23'>23</a>, <a href='#Page_31'>31</a>;</li>
+      <li>Magellanic clouds, <a href='#Page_30'>30</a>, <a href='#Page_31'>31</a>;</li>
+      <li>survey of the heavens, <a href='#Page_31'>31</a>;</li>
+      <li>photography of sun-spots, <a href='#Page_36'>36</a>;</li>
+      <li>telescope, <a href='#Page_180'>180</a>;</li>
+      <li>great spot-group in 1837, <a href='#Page_244'>244</a>;</li>
+      <li>cyclonic theory of sun-spots, <a href='#Page_252'>252</a>;</li>
+      <li>Halley’s comet, <a href='#Page_355'>355</a>;</li>
+      <li>comet of 1843, <a href='#Page_358'>358</a>;</li>
+      <li>Biela’s comet, <a href='#Page_365'>365</a>;</li>
+      <li>red stars, <a href='#Page_416'>416</a>;</li>
+      <li>orbit of Gamma Virginis, <a href='#Page_446'>446</a>;</li>
+      <li>Kappa Crucis, <a href='#Page_506'>506</a>;</li>
+      <li>2 Messier, <a href='#Page_511'>511</a>–512;</li>
+      <li>22 Messier, <a href='#Page_514'>514</a>;</li>
+      <li>nebula round Eta Argus, <a href='#Page_522'>522</a>–523;</li>
+      <li>30 Doradus, <a href='#Page_524'>524</a>;</li>
+      <li>the trifid nebula, Sagittarius, <a href='#Page_525'>525</a>;</li>
+      <li>planetary nebula, <a href='#Page_528'>528</a>–529;</li>
+      <li>the Nubecula Major, <a href='#Page_534'>534</a>–536;</li>
+      <li>Milky Way, crossed by zone of large stars, <a href='#Page_552'>552</a>;</li>
+      <li>observations in the Southern Hemisphere, <a href='#Page_554'>554</a></li>
+    </ul>
+  </li>
+  <li class='c033'>— Sir William, the sun’s translation, <a href='#Page_19'>19</a>, <a href='#Page_28'>28</a>;
+    <ul>
+      <li>reflecting telescopes, <a href='#Page_21'>21</a>–23;</li>
+      <li>discovery of Uranus, <a href='#Page_21'>21</a>, <a href='#Page_22'>22</a>;</li>
+      <li>of binary stars, <a href='#Page_28'>28</a>;</li>
+      <li>comprehensive designs, <a href='#Page_27'>27</a>, <a href='#Page_29'>29</a>;</li>
+      <li>nebular theory, <a href='#Page_30'>30</a>, <a href='#Page_35'>35</a>;</li>
+      <li>rotation of Jupiter’s satellites, <a href='#Page_331'>331</a>;</li>
+      <li>variability of Japetus, <a href='#Page_341'>341</a>;</li>
+      <li>discovery of Uranian moons, <a href='#Page_347'>347</a>;</li>
+      <li>binary stars, <a href='#Page_419'>419</a>, <a href='#Page_431'>431</a>;</li>
+      <li>motion real and apparent, <a href='#Page_428'>428</a>;</li>
+      <li>Zeta Herculis, <a href='#Page_435'>435</a>;</li>
+      <li>Xi Ursæ Majoris, <a href='#Page_440'>440</a>;</li>
+      <li>70 Ophiuchi, <a href='#Page_441'>441</a>;</li>
+      <li>5 Messier, <a href='#Page_510'>510</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Hevelius, <a href='#Page_459'>459</a>, <a href='#Page_462'>462</a>, <a href='#Page_484'>484</a></li>
+  <li class='c033'><span class='pageno' id='Page_570'>570</span>Hind, <a href='#Page_433'>433</a>, <a href='#Page_474'>474</a>, <a href='#Page_482'>482</a>, <a href='#Page_484'>484</a></li>
+  <li class='c033'>Hipparchus, construction of a star catalogue, <a href='#Page_3'>3</a>;
+    <ul>
+      <li>mathematical standpoint, <a href='#Page_4'>4</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Holden, Prof., solar rotation, <a href='#Page_249'>249</a>;
+    <ul>
+      <li>names of asteroids, <a href='#Page_315'>315</a>;</li>
+      <li>helical nebulæ, <a href='#Page_528'>528</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Holmes, discovery of a comet, <a href='#Page_379'>379</a></li>
+  <li class='c033'>Holwarda, Phocylides, <a href='#Page_458'>458</a></li>
+  <li class='c033'>Hooke, law of gravitation, <a href='#Page_10'>10</a>;
+    <ul>
+      <li>observations of Greek letter Draconis, <a href='#Page_18'>18</a>;</li>
+      <li>Gamma Arietis, <a href='#Page_412'>412</a>;</li>
+      <li>parallax of Gamma Draconis, <a href='#Page_419'>419</a>–420</li>
+    </ul>
+  </li>
+  <li class='c033'>Horizon, visible, <a href='#Page_41'>41</a>;
+    <ul>
+      <li>sensible, <a href='#Page_44'>44</a>;</li>
+      <li>celestial, <a href='#Page_44'>44</a>;</li>
+      <li>rational, <a href='#Page_44'>44</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Horrebow, satellite of Venus, <a href='#Page_282'>282</a></li>
+  <li class='c033'>Hour circle, <a href='#Page_186'>186</a></li>
+  <li class='c033'>Howlett, depression of sun-spot umbræ, <a href='#Page_251'>251</a></li>
+  <li class='c033'>Huggins, Dr., stellar and nebular spectra, <a href='#Page_35'>35</a>;
+    <ul>
+      <li>photographed, <a href='#Page_37'>37</a>;</li>
+      <li>observations of prominences, <a href='#Page_255'>255</a>;</li>
+      <li>daylight coronal photography, <a href='#Page_267'>267</a>;</li>
+      <li>prismatic occultation of a star, <a href='#Page_294'>294</a>;</li>
+      <li>spectrum of Mars, <a href='#Page_306'>306</a>;</li>
+      <li>of Jupiter, <a href='#Page_326'>326</a>;</li>
+      <li>of Uranus, <a href='#Page_345'>345</a>;</li>
+      <li>of Winnecke’s comet, <a href='#Page_368'>368</a>;</li>
+      <li>spectrograph of Tebbutt’s comet, <a href='#Page_368'>368</a>;</li>
+      <li>measurement of motion in the line of sight, <a href='#Page_426'>426</a>;</li>
+      <li>spectroscopic examination of new star, <a href='#Page_493'>493</a>;</li>
+      <li>spectroscopic examination of the “fish-mouth” nebula, <a href='#Page_518'>518</a>;</li>
+      <li>discovery of gaseous spectrum, <a href='#Page_528'>528</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Humboldt, meteoric shower of 1799, <a href='#Page_392'>392</a>;
+    <ul>
+      <li>temporary star of 1572, <a href='#Page_479'>479</a>–481</li>
+    </ul>
+  </li>
+  <li class='c033'>Hussey, cometary forms, <a href='#Page_380'>380</a>;
+    <ul>
+      <li>photograph of Rordame’s comet, <a href='#Page_383'>383</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Huygens, <a href='#Page_417'>417</a>, <a href='#Page_517'>517</a></li>
+  <li class='c033'>Hyades, <a href='#Page_407'>407</a>, <a href='#Page_549'>549</a></li>
+  <li class='c033'>Hydræ, R, <a href='#Page_462'>462</a></li>
+  <li class='c033'>Hydrogen, ultra-violet spectrum in stars, <a href='#Page_37'>37</a>;
+    <ul>
+      <li>a gaseous metal, <a href='#Page_250'>250</a>;</li>
+      <li>a constituent of prominences and chromosphere, <a href='#Page_255'>255</a>, <a href='#Page_258'>258</a>;</li>
+      <li>velocity of molecules, <a href='#Page_313'>313</a>;</li>
+      <li>free in atmospheres of Uranus and Neptune, <a href='#Page_346'>346</a>, <a href='#Page_349'>349</a>;</li>
+      <li>assumed constituent of comets’ tails, <a href='#Page_369'>369</a>, <a href='#Page_370'>370</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Hypothesis of external galaxies, <a href='#Page_546'>546</a></li>
+  <li class='center'>I</li>
+  <li class='c033'>Infinity of Space, <a href='#Page_546'>546</a></li>
+  <li class='center'>J</li>
+  <li class='c033'>Jacob, <a href='#Page_433'>433</a>, <a href='#Page_447'>447</a></li>
+  <li class='c033'>Jacoby, measures of photographs, <a href='#Page_37'>37</a></li>
+  <li class='c033'>Janssen, photograph of the sun, <a href='#Page_243'>243</a>;
+    <ul>
+      <li>spectroscopic method of prominence-observation, <a href='#Page_254'>254</a>;</li>
+      <li>double-slit method, <a href='#Page_261'>261</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Japetus, remarkable eclipse, <a href='#Page_338'>338</a>;
+    <ul>
+      <li>variability, <a href='#Page_341'>341</a>;</li>
+      <li>plane of orbit, <a href='#Page_342'>342</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Jesse, luminous night-clouds, <a href='#Page_286'>286</a></li>
+  <li class='c033'>Job, Book of, <a href='#Page_404'>404</a></li>
+  <li class='c033'>Johnson, <a href='#Page_450'>450</a></li>
+  <li class='c033'>Juno, discovery, <a href='#Page_311'>311</a>;
+    <ul>
+      <li>diameter and albedo, <a href='#Page_312'>312</a>, <a href='#Page_316'>316</a>;</li>
+      <li>a twin of Clotho, <a href='#Page_317'>317</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Jupiter, long inequality, <a href='#Page_12'>12</a>, <a href='#Page_17'>17</a>;
+    <ul>
+      <li>disturbance of Halley’s comet, <a href='#Page_16'>16</a>;</li>
+      <li>influence upon asteroidal distribution, <a href='#Page_316'>316</a>–318;</li>
+      <li>mass and figure, <a href='#Page_318'>318</a>;</li>
+      <li>rotation, <a href='#Page_318'>318</a>, <a href='#Page_325'>325</a>, <a href='#Page_326'>326</a>;</li>
+      <li>density, <a href='#Page_319'>319</a>, <a href='#Page_326'>326</a>;</li>
+      <li>reflective power, <a href='#Page_320'>320</a>;</li>
+      <li>belts and streamers, <a href='#Page_321'>321</a>, <a href='#Page_322'>322</a>, <a href='#Page_326'>326</a>;</li>
+      <li>spots, <a href='#Page_323'>323</a>, <a href='#Page_325'>325</a>;</li>
+      <li>photographs, <a href='#Page_327'>327</a>;</li>
+      <li>disturbance of comets, <a href='#Page_371'>371</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Jupiter’s satellites, Galilean quartette, <a href='#Page_9'>9</a>, <a href='#Page_327'>327</a>, <a href='#Page_328'>328</a>;
+    <ul>
+      <li>transits, <a href='#Page_329'>329</a>;</li>
+      <li>constitution, <a href='#Page_330'>330</a>;</li>
+      <li>fifth satellite, <a href='#Page_331'>331</a>, <a href='#Page_332'>332</a></li>
+    </ul>
+  </li>
+  <li class='center'>K</li>
+  <li class='c033'>Kapteyn, <a href='#Page_422'>422</a>, <a href='#Page_556'>556</a>, <a href='#Page_561'>561</a>–563</li>
+  <li class='c033'>Keeler, drawings of Jupiter, <a href='#Page_321'>321</a>;
+    <ul>
+      <li>description of markings, <a href='#Page_322'>322</a>;</li>
+      <li>spectroscopic test of the meteoric constitution of Saturn’s rings, <a href='#Page_339'>339</a>;</li>
+      <li>measuring velocities of nebula in line of sight, <a href='#Page_428'>428</a>;</li>
+      <li>spectra of the Orion nebula, <a href='#Page_519'>519</a>–520</li>
+    </ul>
+  </li>
+  <li class='c033'>Kelvin, Lord, subterranean temperature, <a href='#Page_285'>285</a></li>
+  <li class='c033'>Kepler’s Laws, <a href='#Page_10'>10</a>, <a href='#Page_155'>155</a>, <a href='#Page_339'>339</a>, <a href='#Page_417'>417</a></li>
+  <li class='c033'>Kirch, <a href='#Page_460'>460</a>, <a href='#Page_470'>470</a>, <a href='#Page_510'>510</a></li>
+  <li class='c033'>Kirchhoff, spectrum analysis, <a href='#Page_33'>33</a>;
+    <ul>
+      <li>Fraunhofer’s lines, <a href='#Page_34'>34</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Kirkwood, distribution of asteroids, <a href='#Page_316'>316</a>, <a href='#Page_317'>317</a>;
+    <ul>
+      <li>divisions in Saturn’s rings, <a href='#Page_338'>338</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Kleiber, number of shooting stars, <a href='#Page_390'>390</a></li>
+  <li class='c033'>Koch, <a href='#Page_461'>461</a></li>
+  <li class='c033'><span class='pageno' id='Page_571'>571</span>Kreutz, relations of great southern comets, <a href='#Page_360'>360</a></li>
+  <li class='c033'>Krüger, <a href='#Page_442'>442</a></li>
+  <li class='center'>L</li>
+  <li class='c033'>Lacaille, southern nebulæ, <a href='#Page_30'>30</a></li>
+  <li class='c033'>Lagrange, verified principle of gravitation, <a href='#Page_11'>11</a>;
+    <ul>
+      <li>stability of solar system, <a href='#Page_13'>13</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Lajoye, <a href='#Page_488'>488</a></li>
+  <li class='c033'>Lamp, fate of Brorsen’s comet, <a href='#Page_370'>370</a></li>
+  <li class='c033'>Lane’s law, <a href='#Page_242'>242</a></li>
+  <li class='c033'>Langley, solar radiation, <a href='#Page_238'>238</a>, <a href='#Page_239'>239</a>;
+    <ul>
+      <li>spectroscopic effects of sun’s rotation, <a href='#Page_249'>249</a>;</li>
+      <li>temperature of the moon, <a href='#Page_294'>294</a>;</li>
+      <li>fireball, <a href='#Page_386'>386</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Laplace, verified Newton’s law, <a href='#Page_11'>11</a>;
+    <ul>
+      <li>lunar acceleration, <a href='#Page_12'>12</a>;</li>
+      <li><i>Mécanique Céleste</i>, <a href='#Page_13'>13</a>, <a href='#Page_14'>14</a>;</li>
+      <li>nebular hypothesis, <a href='#Page_235'>235</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Lassell, large reflectors, <a href='#Page_24'>24</a>;
+    <ul>
+      <li>discoveries of Hyperion, Ariel, and Umbriel, <a href='#Page_24'>24</a>, <a href='#Page_341'>341</a>, <a href='#Page_347'>347</a>;</li>
+      <li>Saturn’s dark ring, <a href='#Page_336'>336</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Latitude, terrestrial, <a href='#Page_50'>50</a>, <a href='#Page_125'>125</a>;
+    <ul>
+      <li>celestial, <a href='#Page_68'>68</a>;</li>
+      <li>of sun, <a href='#Page_77'>77</a>;</li>
+      <li>geocentric, <a href='#Page_135'>135</a>;</li>
+      <li>geographical, <a href='#Page_135'>135</a>;</li>
+      <li>astronomical, <a href='#Page_136'>136</a>;</li>
+      <li>variation of, <a href='#Page_136'>136</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Leland, Miss, <a href='#Page_511'>511</a></li>
+  <li class='c033'>Leonid meteors, <a href='#Page_391'>391</a>–395</li>
+  <li class='c033'>Leonis, Gamma, <a href='#Page_413'>413</a></li>
+  <li class='c033'>— R, <a href='#Page_461'>461</a></li>
+  <li class='c033'>Lepaute, Madame, computation of Halley’s comet, <a href='#Page_16'>16</a></li>
+  <li class='c033'>Leverrier, discovery of Neptune, <a href='#Page_32'>32</a>;
+    <ul>
+      <li>intra-Mercurian planet, <a href='#Page_232'>232</a>;</li>
+      <li>mass of asteroids, <a href='#Page_315'>315</a>;</li>
+      <li>orbit of November meteors, <a href='#Page_395'>395</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Lewis, <a href='#Page_426'>426</a></li>
+  <li class='c033'>Libræ, Delta, <a href='#Page_473'>473</a></li>
+  <li class='c033'>Librations, of Mercury, <a href='#Page_277'>277</a>;
+    <ul>
+      <li>of Venus, <a href='#Page_281'>281</a>;</li>
+      <li>of the moon, <a href='#Page_93'>93</a>, <a href='#Page_289'>289</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Lick observatory, <a href='#Page_25'>25</a>, <a href='#Page_26'>26</a></li>
+  <li class='c033'>Light-equation, <a href='#Page_329'>329</a></li>
+  <li class='c033'>“Light journey,” <a href='#Page_420'>420</a></li>
+  <li class='c033'>Limited number of visible stars, <a href='#Page_538'>538</a>, <a href='#Page_545'>545</a></li>
+  <li class='c033'>Limiting apertures, <a href='#Page_212'>212</a></li>
+  <li class='c033'>Lippershey, inventor of the telescope, <a href='#Page_9'>9</a></li>
+  <li class='c033'>Lockyer, spectroscopic observations at the sun’s limb, <a href='#Page_254'>254</a>;
+    <ul>
+      <li>classification of prominences, <a href='#Page_250'>250</a>;</li>
+      <li>solar tornadoes, <a href='#Page_259'>259</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Loewy, Coudé telescope, <a href='#Page_27'>27</a>;
+    <ul>
+      <li>lunar photography, <a href='#Page_296'>296</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Longitude, terrestrial, <a href='#Page_50'>50</a>, <a href='#Page_125'>125</a>;
+    <ul>
+      <li>celestial, <a href='#Page_68'>68</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Lowell, rotation of Mercury, <a href='#Page_277'>277</a>;
+    <ul>
+      <li>observations of Venus, <a href='#Page_279'>279</a>, <a href='#Page_281'>281</a>;</li>
+      <li>lakes of Mars, <a href='#Page_301'>301</a>, <a href='#Page_302'>302</a>;</li>
+      <li>relation to canals, <a href='#Page_302'>302</a>–304</li>
+    </ul>
+  </li>
+  <li class='c033'>Luminous night-clouds, <a href='#Page_286'>286</a></li>
+  <li class='c033'>Lunar distances, <a href='#Page_129'>129</a></li>
+  <li class='c033'>— ecliptic limit, <a href='#Page_112'>112</a></li>
+  <li class='c033'>Lyncis (<i>12</i>), <a href='#Page_450'>450</a></li>
+  <li class='c033'>Lyra, annular nebula in, <a href='#Page_526'>526</a></li>
+  <li class='c033'>Lyræ, Beta, <a href='#Page_465'>465</a></li>
+  <li class='c033'>Lyraid meteors, <a href='#Page_393'>393</a>, <a href='#Page_395'>395</a></li>
+  <li class='center'>M</li>
+  <li class='c033'>Maclear, <a href='#Page_464'>464</a></li>
+  <li class='c033'>Mädler, search for Martian moons, <a href='#Page_309'>309</a>;
+    <ul>
+      <li>compression of Uranus, <a href='#Page_343'>343</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Madrid meteorite, <a href='#Page_385'>385</a></li>
+  <li class='c033'>Magellanic clouds, <a href='#Page_30'>30</a>, <a href='#Page_534'>534</a>–537</li>
+  <li class='c033'>Magnetism, terrestrial, <a href='#Page_287'>287</a>, <a href='#Page_288'>288</a></li>
+  <li class='c033'>Magnitude, of eclipses, <a href='#Page_113'>113</a>;
+    <ul>
+      <li>of stars, <a href='#Page_212'>212</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Magnitudes, star, <a href='#Page_403'>403</a>, <a href='#Page_404'>404</a></li>
+  <li class='c033'>Mann, <a href='#Page_433'>433</a></li>
+  <li class='c033'>Maps, <a href='#Page_133'>133</a></li>
+  <li class='c033'>Maraldi, <a href='#Page_462'>462</a>, <a href='#Page_470'>470</a>, <a href='#Page_511'>511</a></li>
+  <li class='c033'>Marchand, observations of the Zodiacal Light, <a href='#Page_273'>273</a></li>
+  <li class='c033'>Markwick, Col., <a href='#Page_547'>547</a></li>
+  <li class='c033'>Mars, phases of, <a href='#Page_104'>104</a>;
+    <ul>
+      <li>parallax of, <a href='#Page_147'>147</a>;</li>
+      <li>a superior planet, <a href='#Page_297'>297</a>;</li>
+      <li>seasons, <a href='#Page_298'>298</a>, <a href='#Page_301'>301</a>, <a href='#Page_302'>302</a>;</li>
+      <li>snow-caps, <a href='#Page_299'>299</a>, <a href='#Page_303'>303</a>, <a href='#Page_306'>306</a>;</li>
+      <li>land and water, <a href='#Page_299'>299</a>–301, <a href='#Page_305'>305</a>, <a href='#Page_306'>306</a>;</li>
+      <li>continents, <a href='#Page_300'>300</a>, <a href='#Page_301'>301</a>;</li>
+      <li>canals, <a href='#Page_301'>301</a>, <a href='#Page_304'>304</a>;</li>
+      <li>duplication, <a href='#Page_301'>301</a>, <a href='#Page_305'>305</a>;</li>
+      <li>spectrum, <a href='#Page_306'>306</a>;</li>
+      <li>atmosphere, <a href='#Page_307'>307</a>, <a href='#Page_313'>313</a>;</li>
+      <li>mountains, <a href='#Page_307'>307</a>;</li>
+      <li>climate, <a href='#Page_308'>308</a>;</li>
+      <li>moons, <a href='#Page_309'>309</a>, <a href='#Page_310'>310</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Marth, Neptune’s satellite, <a href='#Page_350'>350</a></li>
+  <li class='c033'>Mascari, rotation of Venus, <a href='#Page_280'>280</a></li>
+  <li class='c033'>Maskelyne, astronomer-royal, <a href='#Page_19'>19</a>;
+    <ul>
+      <li>founded <cite>Nautical Almanac</cite>, <a href='#Page_20'>20</a>;</li>
+      <li>star-motions, <a href='#Page_28'>28</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Mass, defined, <a href='#Page_151'>151</a>;
+    <ul>
+      <li>sun, <a href='#Page_156'>156</a>;</li>
+      <li>planets, <a href='#Page_157'>157</a>;</li>
+      <li>moon, <a href='#Page_158'>158</a>;</li>
+      <li>of asteroids, <a href='#Page_158'>158</a>;</li>
+      <li>earth, <a href='#Page_159'>159</a>;</li>
+      <li>satellites, <a href='#Page_159'>159</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Maunder, <a href='#Page_460'>460</a>, <a href='#Page_488'>488</a>, <a href='#Page_493'>493</a></li>
+  <li class='c033'>Maxwell, Clerk, constitution of Saturn’s rings, <a href='#Page_337'>337</a>, <a href='#Page_340'>340</a></li>
+  <li class='c033'>Mayer, Tobias, lunar tables, <a href='#Page_11'>11</a>;
+    <ul>
+      <li>star-motions, <a href='#Page_28'>28</a></li>
+    </ul>
+  </li>
+  <li class='c033'><span class='pageno' id='Page_572'>572</span>Mazapil meteorite, <a href='#Page_396'>396</a></li>
+  <li class='c033'>Measurement, of earth, <a href='#Page_42'>42</a>, <a href='#Page_129'>129</a>;
+    <ul>
+      <li>of sun’s distance, <a href='#Page_146'>146</a>;</li>
+      <li>of binary stars, <a href='#Page_208'>208</a>;</li>
+      <li>of planets, <a href='#Page_208'>208</a></li>
+    </ul>
+  </li>
+  <li class='c033'><i>Mécanique Céleste</i>, character, <a href='#Page_13'>13</a>, <a href='#Page_14'>14</a></li>
+  <li class='c033'>Megrez, <a href='#Page_402'>402</a></li>
+  <li class='c033'>Mercury, Copernican theory of movements, <a href='#Page_8'>8</a>;
+    <ul>
+      <li>transit of, <a href='#Page_101'>101</a>;</li>
+      <li>phases of, <a href='#Page_101'>101</a>;</li>
+      <li>orbit, <a href='#Page_273'>273</a>, <a href='#Page_274'>274</a>;</li>
+      <li>atmosphere, <a href='#Page_274'>274</a>, <a href='#Page_275'>275</a>;</li>
+      <li>rotation, <a href='#Page_275'>275</a>–277;</li>
+      <li>as an abode of life, <a href='#Page_277'>277</a>;</li>
+      <li>capture of Encke’s comet, <a href='#Page_372'>372</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Meridian, <a href='#Page_50'>50</a>;
+    <ul>
+      <li>line, <a href='#Page_51'>51</a>;</li>
+      <li>arc of, <a href='#Page_130'>130</a>;</li>
+      <li>circle, <a href='#Page_198'>198</a>;</li>
+      <li>photometer, <a href='#Page_214'>214</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Merope, <a href='#Page_498'>498</a>, <a href='#Page_499'>499</a></li>
+  <li class='c033'>Messier (<i>3</i>), <a href='#Page_509'>509</a></li>
+  <li class='c033'>— (<i>5</i>), <a href='#Page_510'>510</a></li>
+  <li class='c033'>— (<i>11</i>), <a href='#Page_506'>506</a></li>
+  <li class='c033'>— (<i>22</i>), <a href='#Page_514'>514</a></li>
+  <li class='c033'>— (<i>37</i>), <a href='#Page_505'>505</a></li>
+  <li class='c033'>— (<i>51</i>), <a href='#Page_533'>533</a></li>
+  <li class='c033'>— (<i>57</i>), <a href='#Page_526'>526</a></li>
+  <li class='c033'>— (<i>80</i>), new star in, <a href='#Page_485'>485</a></li>
+  <li class='c033'>— (<i>92</i>), <a href='#Page_509'>509</a></li>
+  <li class='c033'>— (<i>99</i>), <a href='#Page_534'>534</a></li>
+  <li class='c033'>— discoveries of nebulæ, <a href='#Page_30'>30</a></li>
+  <li class='c033'>Metonic cycle, <a href='#Page_92'>92</a></li>
+  <li class='c033'>Meteoric systems, <a href='#Page_231'>231</a>, <a href='#Page_390'>390</a>, <a href='#Page_391'>391</a>;
+    <ul>
+      <li>radiants, <a href='#Page_392'>392</a>, <a href='#Page_395'>395</a>, <a href='#Page_396'>396</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Meteorites, falls, <a href='#Page_385'>385</a>–387;
+    <ul>
+      <li>legal status, <a href='#Page_387'>387</a>;</li>
+      <li>velocities, <a href='#Page_387'>387</a>, <a href='#Page_388'>388</a>, <a href='#Page_390'>390</a>;</li>
+      <li>thumb-marks, <a href='#Page_388'>388</a>;</li>
+      <li>chemical composition, <a href='#Page_389'>389</a>;</li>
+      <li>enclosed diamonds, <a href='#Page_390'>390</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Meteors, Perseid, <a href='#Page_391'>391</a>, <a href='#Page_393'>393</a>;
+    <ul>
+      <li>Leonid, <a href='#Page_391'>391</a>–393;</li>
+      <li>Andromede, <a href='#Page_393'>393</a>–394, <a href='#Page_396'>396</a>;</li>
+      <li>relations to comets, <a href='#Page_393'>393</a>, <a href='#Page_395'>395</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Micrometer, wire, or pillar, <a href='#Page_205'>205</a>;
+    <ul>
+      <li>evolution of, <a href='#Page_207'>207</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Michell, prevision of binary stars, <a href='#Page_28'>28</a></li>
+  <li class='c033'>Midnight sun, <a href='#Page_63'>63</a></li>
+  <li class='c033'>Milky Way, <a href='#Page_402'>402</a>, <a href='#Page_430'>430</a>, <a href='#Page_555'>555</a>, <a href='#Page_557'>557</a></li>
+  <li class='c033'>— — star streams, <a href='#Page_9'>9</a>;
+    <ul>
+      <li>disc theory, <a href='#Page_29'>29</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Minimum deviation, <a href='#Page_215'>215</a></li>
+  <li class='c033'>Mira Ceti, <a href='#Page_458'>458</a>, <a href='#Page_459'>459</a></li>
+  <li class='c033'>Mitchell, <a href='#Page_421'>421</a>, <a href='#Page_431'>431</a></li>
+  <li class='c033'>Mizar, <a href='#Page_402'>402</a>, <a href='#Page_411'>411</a>, <a href='#Page_455'>455</a>, <a href='#Page_457'>457</a></li>
+  <li class='c033'>Molyneux, <a href='#Page_419'>419</a>, <a href='#Page_420'>420</a></li>
+  <li class='c033'>Montanari, <a href='#Page_471'>471</a></li>
+  <li class='c033'>Month, <a href='#Page_91'>91</a></li>
+  <li class='c033'>Moon, acceleration, <a href='#Page_12'>12</a>;
+    <ul>
+      <li><i>contumax sidus</i>, <a href='#Page_16'>16</a>;</li>
+      <li>observations, <a href='#Page_19'>19</a>;</li>
+      <li>apparent motion of, <a href='#Page_87'>87</a>;</li>
+      <li>orbit of, <a href='#Page_88'>88</a>, <a href='#Page_94'>94</a>;</li>
+      <li>phases of, <a href='#Page_89'>89</a>;</li>
+      <li>sidereal period of, <a href='#Page_89'>89</a>;</li>
+      <li>synodic period of, <a href='#Page_91'>91</a>;</li>
+      <li>rotation of, <a href='#Page_92'>92</a>;</li>
+      <li>librations of, <a href='#Page_93'>93</a>;</li>
+      <li>harvest, <a href='#Page_95'>95</a>;</li>
+      <li>high and low, <a href='#Page_97'>97</a>;</li>
+      <li>shadow of, <a href='#Page_115'>115</a>;</li>
+      <li>distance of, <a href='#Page_143'>143</a>;</li>
+      <li>size of, <a href='#Page_144'>144</a>;</li>
+      <li>mass of, <a href='#Page_158'>158</a>;</li>
+      <li>possible disintegration, <a href='#Page_233'>233</a>;</li>
+      <li>origin, <a href='#Page_236'>236</a>, <a href='#Page_237'>237</a>;</li>
+      <li>rotation, <a href='#Page_289'>289</a>;</li>
+      <li>cones and craters, <a href='#Page_290'>290</a>, <a href='#Page_292'>292</a>, <a href='#Page_293'>293</a>;</li>
+      <li>rays and rills, <a href='#Page_293'>293</a>;</li>
+      <li>absence of air and water, <a href='#Page_294'>294</a>, <a href='#Page_313'>313</a>;</li>
+      <li>temperature, <a href='#Page_294'>294</a>, <a href='#Page_295'>295</a>;</li>
+      <li>eclipses, <a href='#Page_295'>295</a>;</li>
+      <li>photography, <a href='#Page_295'>295</a>–297</li>
+    </ul>
+  </li>
+  <li class='c033'>Morning star, <a href='#Page_100'>100</a></li>
+  <li class='c033'>Müller, surface of Mercury, <a href='#Page_275'>275</a>;
+    <ul>
+      <li>photometry of asteroids, <a href='#Page_312'>312</a>;</li>
+      <li>albedo of Jupiter, <a href='#Page_320'>320</a>;</li>
+      <li>of Saturn, <a href='#Page_334'>334</a>;</li>
+      <li>of Neptune, <a href='#Page_349'>349</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Muscæ, R, <a href='#Page_468'>468</a></li>
+  <li class='center'>N</li>
+  <li class='c033'>Nadir, <a href='#Page_45'>45</a></li>
+  <li class='c033'>Nasir Eddin, planetary tables, <a href='#Page_5'>5</a></li>
+  <li class='c033'>Nasmyth, conjunction of Mercury and Venus, <a href='#Page_278'>278</a></li>
+  <li class='c033'>Nearest fixed stars, <a href='#Page_417'>417</a></li>
+  <li class='c033'>Nebula, Orion, <a href='#Page_23'>23</a>, <a href='#Page_25'>25</a>, <a href='#Page_30'>30</a></li>
+  <li class='c033'>Nebulæ, structure, <a href='#Page_23'>23</a>;
+    <ul>
+      <li>spiral, <a href='#Page_24'>24</a>;</li>
+      <li>photographs, <a href='#Page_23'>23</a>, <a href='#Page_25'>25</a>;</li>
+      <li>first discoveries, <a href='#Page_29'>29</a>, <a href='#Page_30'>30</a>;</li>
+      <li>status, <a href='#Page_30'>30</a>, <a href='#Page_31'>31</a>;</li>
+      <li>gaseous nature, <a href='#Page_30'>30</a>, <a href='#Page_35'>35</a>;</li>
+      <li>annular, <a href='#Page_526'>526</a>, <a href='#Page_527'>527</a>;</li>
+      <li>elliptical, <a href='#Page_529'>529</a>, <a href='#Page_533'>533</a>;</li>
+      <li>gaseous, <a href='#Page_517'>517</a>–524;</li>
+      <li>planetary, <a href='#Page_527'>527</a>–529;</li>
+      <li>spiral, <a href='#Page_533'>533</a>, <a href='#Page_534'>534</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Nebular hypothesis, <a href='#Page_30'>30</a>, <a href='#Page_35'>35</a>, <a href='#Page_235'>235</a>, <a href='#Page_530'>530</a></li>
+  <li class='c033'>Nebulous stars, <a href='#Page_529'>529</a></li>
+  <li class='c033'>Neptune, discovery, <a href='#Page_32'>32</a>, <a href='#Page_229'>229</a>;
+    <ul>
+      <li>distance from the sun, <a href='#Page_232'>232</a>;</li>
+      <li>dimensions, <a href='#Page_349'>349</a>;</li>
+      <li>compression, <a href='#Page_351'>351</a>;</li>
+      <li>retrograde rotation, <a href='#Page_351'>351</a>;</li>
+      <li>planets as viewed from, <a href='#Page_351'>351</a>, <a href='#Page_352'>352</a>;</li>
+      <li>family of comets, <a href='#Page_371'>371</a>, <a href='#Page_372'>372</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Neptune’s satellite, discovery, <a href='#Page_24'>24</a>;
+    <ul>
+      <li>plane of revolution, <a href='#Page_350'>350</a>;</li>
+      <li>precessional disturbance, <a href='#Page_351'>351</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Newall, <a href='#Page_25'>25</a>-inch refractor, <a href='#Page_26'>26</a></li>
+  <li class='c033'>Newcomb, Prof., past duration of sunlight, <a href='#Page_285'>285</a>;
+    <ul>
+      <li>light changes of Ariel, <a href='#Page_347'>347</a>;</li>
+      <li>satellite of Neptune, <a href='#Page_351'>351</a>;</li>
+      <li>the runaway star, <a href='#Page_424'>424</a>;</li>
+      <li>proper motion of Alcyone, <a href='#Page_501'>501</a></li>
+    </ul>
+  </li>
+  <li class='c033'>New stars, <a href='#Page_477'>477</a>–497</li>
+  <li class='c033'>Newton, H. A., capture of comets, <a href='#Page_372'>372</a>;
+    <ul>
+      <li>meteoric cult, <a href='#Page_387'>387</a>;</li>
+      <li>daily number of shooting stars, <a href='#Page_390'>390</a></li>
+    </ul>
+  </li>
+  <li class='c033'>— Sir Isaac, law of gravitation, <a href='#Page_10'>10</a>, <a href='#Page_11'>11</a>;
+    <ul>
+      <li>invention of reflecting telescope, <a href='#Page_21'>21</a>;</li>
+      <li>comet of 1680, <a href='#Page_355'>355</a>;</li>
+      <li>decay of comets, <a href='#Page_366'>366</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Newtonian telescope, <a href='#Page_179'>179</a></li>
+  <li class='c033'><span class='pageno' id='Page_573'>573</span>Nichol, Dr., <a href='#Page_508'>508</a></li>
+  <li class='c033'>Niesten, rotation of Venus, <a href='#Page_280'>280</a>;
+    <ul>
+      <li>mass of asteroids, <a href='#Page_315'>315</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Nodes, <a href='#Page_94'>94</a></li>
+  <li class='c033'>North polar distance, <a href='#Page_66'>66</a></li>
+  <li class='c033'>Nova Andromedæ, <a href='#Page_488'>488</a></li>
+  <li class='c033'>— Aurigæ, <a href='#Page_489'>489</a></li>
+  <li class='c033'>— Cassiopeiæ, <a href='#Page_479'>479</a></li>
+  <li class='c033'>— Cygni, <a href='#Page_486'>486</a></li>
+  <li class='c033'>— Ophiuchi, <a href='#Page_484'>484</a></li>
+  <li class='c033'>— Serpentarii, <a href='#Page_483'>483</a></li>
+  <li class='c033'>— Vulpeculæ, <a href='#Page_484'>484</a></li>
+  <li class='c033'>Nubecula Major, <a href='#Page_534'>534</a></li>
+  <li class='c033'>— Minor, <a href='#Page_535'>535</a></li>
+  <li class='c033'>Number of visible stars, <a href='#Page_538'>538</a>–544</li>
+  <li class='c033'>Nutation, <a href='#Page_169'>169</a></li>
+  <li class='center'>O</li>
+  <li class='c033'>Oases of Mars, <a href='#Page_302'>302</a>–305</li>
+  <li class='c033'>Object-glass, achromatic, <a href='#Page_177'>177</a>;
+    <ul>
+      <li>photographic, <a href='#Page_195'>195</a>;</li>
+      <li>photo-telescope, <a href='#Page_196'>196</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Objective prism, <a href='#Page_223'>223</a></li>
+  <li class='c033'>Obliquity of ecliptic, <a href='#Page_61'>61</a></li>
+  <li class='c033'>Observatories, <a href='#Page_191'>191</a></li>
+  <li class='c033'>— Lick, <a href='#Page_189'>189</a>, <a href='#Page_190'>190</a>, <a href='#Page_202'>202</a></li>
+  <li class='c033'>— Nice, <a href='#Page_192'>192</a></li>
+  <li class='c033'>— Yerkes, <a href='#Page_189'>189</a></li>
+  <li class='c033'>Occultations, <a href='#Page_121'>121</a></li>
+  <li class='c033'>— of stars, by the moon, <a href='#Page_294'>294</a>;
+    <ul>
+      <li>by comets, <a href='#Page_366'>366</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Olbers, discovery of Pallas and Vesta, <a href='#Page_311'>311</a>;
+    <ul>
+      <li>origin of asteroids, <a href='#Page_311'>311</a>, <a href='#Page_316'>316</a>;</li>
+      <li>electrical theory of comets, <a href='#Page_357'>357</a>;</li>
+      <li>classification, <a href='#Page_358'>358</a>, <a href='#Page_383'>383</a>;</li>
+      <li>comet discovered by, <a href='#Page_371'>371</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Ophiuchi, Nova, <a href='#Page_483'>483</a>–485</li>
+  <li class='c033'>— (<i>70</i>), <a href='#Page_441'>441</a></li>
+  <li class='c033'>— U, <a href='#Page_473'>473</a>, <a href='#Page_476'>476</a></li>
+  <li class='c033'>Opposition, <a href='#Page_103'>103</a>, <a href='#Page_105'>105</a></li>
+  <li class='c033'>Orbit, of earth, <a href='#Page_72'>72</a>, <a href='#Page_76'>76</a>;
+    <ul>
+      <li>of moon, <a href='#Page_88'>88</a>;</li>
+      <li>elements of a planetary orbit, <a href='#Page_106'>106</a>;</li>
+      <li>of binary stars, <a href='#Page_432'>432</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Orion, <a href='#Page_406'>406</a>, <a href='#Page_408'>408</a>, <a href='#Page_417'>417</a></li>
+  <li class='c033'>— great nebula in, <a href='#Page_517'>517</a>–521</li>
+  <li class='c002'>Orionis, Alpha (Betelgeuse), <a href='#Page_404'>404</a>, <a href='#Page_408'>408</a>, <a href='#Page_415'>415</a>, <a href='#Page_427'>427</a></li>
+  <li class='c033'>— Iota, <a href='#Page_414'>414</a></li>
+  <li class='c033'>— Sigma, <a href='#Page_414'>414</a></li>
+  <li class='c033'>— Theta, <a href='#Page_414'>414</a></li>
+  <li class='c033'>“Owl,” nebula, <a href='#Page_528'>528</a></li>
+  <li class='center'>P</li>
+  <li class='c033'>P (<i>34</i>) Cygni, <a href='#Page_482'>482</a></li>
+  <li class='c033'>Palisa, discoveries of asteroids, <a href='#Page_314'>314</a></li>
+  <li class='c033'>Palitzsch, <a href='#Page_470'>470</a></li>
+  <li class='c033'>Pallas, discovery, <a href='#Page_311'>311</a>;
+    <ul>
+      <li>diameter, <a href='#Page_312'>312</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Parallax of stars, <a href='#Page_419'>419</a>, <a href='#Page_420'>420</a></li>
+  <li class='c033'>— diurnal, <a href='#Page_140'>140</a>;
+    <ul>
+      <li>equatorial horizontal, <a href='#Page_140'>140</a>;</li>
+      <li>horizontal, <a href='#Page_140'>140</a>;</li>
+      <li>of sun, <a href='#Page_146'>146</a>;</li>
+      <li>of Mars, <a href='#Page_147'>147</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Parmentier, distribution of asteroids, <a href='#Page_316'>316</a></li>
+  <li class='c033'>Pegasus, Square of, <a href='#Page_409'>409</a></li>
+  <li class='c033'>Pegasi, Kappa, <a href='#Page_433'>433</a></li>
+  <li class='c033'>— (<i>85</i>), <a href='#Page_434'>434</a></li>
+  <li class='c033'>— U, <a href='#Page_469'>469</a></li>
+  <li class='c033'>Pendulum observations, <a href='#Page_135'>135</a>;
+    <ul>
+      <li>compensated, <a href='#Page_174'>174</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Penumbra, of earth’s shadow, <a href='#Page_111'>111</a></li>
+  <li class='c033'>Percentage of stars in Milky Way, <a href='#Page_547'>547</a>, <a href='#Page_548'>548</a></li>
+  <li class='c033'>Perigee, <a href='#Page_89'>89</a></li>
+  <li class='c033'>Perihelion, <a href='#Page_75'>75</a></li>
+  <li class='c033'>Perrotin, rotation of Venus, <a href='#Page_280'>280</a>;
+    <ul>
+      <li>of Uranus, <a href='#Page_343'>343</a>;</li>
+      <li>markings on Uranus, <a href='#Page_344'>344</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Persei, Beta (Algol). <i>See</i> Algol</li>
+  <li class='c033'>Perseid meteors, <a href='#Page_391'>391</a>;
+    <ul>
+      <li>associated with Tuttle’s comet, <a href='#Page_393'>393</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Perseus, <a href='#Page_407'>407</a></li>
+  <li class='c033'>— star clusters in, <a href='#Page_503'>503</a></li>
+  <li class='c033'>Perturbations, <a href='#Page_158'>158</a></li>
+  <li class='c033'>Peters, <a href='#Page_428'>428</a></li>
+  <li class='c033'>Phases of moon, <a href='#Page_89'>89</a>;
+    <ul>
+      <li>of Venus, <a href='#Page_101'>101</a>;</li>
+      <li>of Mars, <a href='#Page_104'>104</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Phocylides Holwarda, <a href='#Page_458'>458</a></li>
+  <li class='c033'>Photographic telescopes, <a href='#Page_194'>194</a></li>
+  <li class='c033'>Photography of nebulæ, <a href='#Page_23'>23</a>, <a href='#Page_25'>25</a>, <a href='#Page_38'>38</a>;
+    <ul>
+      <li>of sun-spots, <a href='#Page_36'>36</a>, <a href='#Page_243'>243</a>, <a href='#Page_244'>244</a>;</li>
+      <li>of the moon, <a href='#Page_36'>36</a>, <a href='#Page_295'>295</a>–297;</li>
+      <li>of stellar spectra, <a href='#Page_37'>37</a>;</li>
+      <li>of comets, <a href='#Page_38'>38</a>, <a href='#Page_354'>354</a>, <a href='#Page_377'>377</a>–383;</li>
+      <li>celestial, <a href='#Page_194'>194</a>;</li>
+      <li>of spectra, <a href='#Page_219'>219</a>, <a href='#Page_223'>223</a>;</li>
+      <li>of the eclipsed sun, <a href='#Page_254'>254</a>;</li>
+      <li>of the reversing layer, <a href='#Page_259'>259</a>;</li>
+      <li>of prominence-spectra, <a href='#Page_260'>260</a>;</li>
+      <li>of prominences and faculæ, <a href='#Page_261'>261</a>, <a href='#Page_262'>262</a>;</li>
+      <li>of the corona, <a href='#Page_267'>267</a>, <a href='#Page_269'>269</a>–271;</li>
+      <li>planetary, <a href='#Page_327'>327</a>; meteoric, <a href='#Page_396'>396</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Photoheliograph, <a href='#Page_197'>197</a></li>
+  <li class='c033'>Photometers, wedge, <a href='#Page_213'>213</a>;
+    <ul>
+      <li>meridian, <a href='#Page_214'>214</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Photosphere, visible structure, <a href='#Page_242'>242</a></li>
+  <li class='c033'>Piazzi, five-foot circle, <a href='#Page_20'>20</a>;
+    <ul>
+      <li>discovery of Ceres, <a href='#Page_311'>311</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Pickering, Prof. E. C., photometric measures of asteroids, <a href='#Page_312'>312</a>;
+    <ul>
+      <li>photograph of Jupiter, <a href='#Page_327'>327</a>;</li>
+      <li><span class='pageno' id='Page_574'>574</span>the spectrum of Alpha Centauri, <a href='#Page_441'>441</a>;</li>
+      <li>the spectrum of Pleione, <a href='#Page_498'>498</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Pickering, W. H., lunar photographs, <a href='#Page_296'>296</a>;
+    <ul>
+      <li>mounting of telescopes, <a href='#Page_297'>297</a>;</li>
+      <li>lakes and canals of Mars, <a href='#Page_301'>301</a>, <a href='#Page_304'>304</a>;</li>
+      <li>water area on Mars, <a href='#Page_305'>305</a>;</li>
+      <li>star collisions, <a href='#Page_495'>495</a>;</li>
+      <li>nebula surrounding Zeta Orionis, <a href='#Page_520'>520</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Pigott, <a href='#Page_467'>467</a></li>
+  <li class='c033'>“Pilgrim star,” <a href='#Page_479'>479</a>–482</li>
+  <li class='c033'>Planetary nebulæ, <a href='#Page_527'>527</a>–529</li>
+  <li class='c033'>Planets, apparent movements of, <a href='#Page_98'>98</a>;
+    <ul>
+      <li>interior and exterior, <a href='#Page_98'>98</a>;</li>
+      <li>conjunctions of, <a href='#Page_99'>99</a>, <a href='#Page_103'>103</a>;</li>
+      <li>phases of, <a href='#Page_101'>101</a>, <a href='#Page_104'>104</a>;</li>
+      <li>oppositions of, <a href='#Page_103'>103</a>;</li>
+      <li>synodic periods of, <a href='#Page_107'>107</a>;</li>
+      <li>times of revolution, <a href='#Page_107'>107</a>;</li>
+      <li>relative distances of, <a href='#Page_144'>144</a>;</li>
+      <li>distances of, <a href='#Page_150'>150</a>;</li>
+      <li>terrestrial, <a href='#Page_229'>229</a>;</li>
+      <li>giant, <a href='#Page_229'>229</a>, <a href='#Page_319'>319</a>, <a href='#Page_343'>343</a>;</li>
+      <li>trans-Neptunian, <a href='#Page_231'>231</a>;</li>
+      <li>intra-Mercurian, <a href='#Page_232'>232</a>;</li>
+      <li>decay, <a href='#Page_233'>233</a>;</li>
+      <li>comets captured by, <a href='#Page_371'>371</a>, <a href='#Page_372'>372</a></li>
+    </ul>
+  </li>
+  <li class='c033'>— minor. <i>See</i> Asteroids</li>
+  <li class='c033'>Pleiades, <a href='#Page_404'>404</a>, <a href='#Page_407'>407</a>, <a href='#Page_497'>497</a>–502, <a href='#Page_539'>539</a>, <a href='#Page_549'>549</a></li>
+  <li class='c033'>Pleione, <a href='#Page_498'>498</a></li>
+  <li class='c033'>“Plough,” <a href='#Page_400'>400</a>–402, <a href='#Page_405'>405</a></li>
+  <li class='c033'>Plummer, short-period comets, <a href='#Page_371'>371</a>;
+    <ul>
+      <li>Encke’s, <a href='#Page_372'>372</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Podmaniczky, Baroness, <a href='#Page_488'>488</a></li>
+  <li class='c033'>Pogson, <a href='#Page_485'>485</a></li>
+  <li class='c033'>Polar axis, <a href='#Page_185'>185</a></li>
+  <li class='c033'>Polaris. <i>See</i> Pole Star</li>
+  <li class='c033'>Pole, celestial, <a href='#Page_46'>46</a>; terrestrial, <a href='#Page_50'>50</a>;
+    <ul>
+      <li>movements of, <a href='#Page_138'>138</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Pole Star, <a href='#Page_46'>46</a>, <a href='#Page_405'>405</a>, <a href='#Page_412'>412</a>, <a href='#Page_421'>421</a>, <a href='#Page_427'>427</a></li>
+  <li class='c033'>Pollux, <a href='#Page_404'>404</a>, <a href='#Page_406'>406</a>, <a href='#Page_415'>415</a>, <a href='#Page_427'>427</a></li>
+  <li class='c033'>Pond, defects of Greenwich quadrant, <a href='#Page_19'>19</a>;
+    <ul>
+      <li>astronomer-royal, <a href='#Page_20'>20</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Position, angle, <a href='#Page_208'>208</a>;
+    <ul>
+      <li>circle, <a href='#Page_208'>208</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Poynting’s experiment, <a href='#Page_160'>160</a></li>
+  <li class='c033'>Præsepe, <a href='#Page_502'>502</a></li>
+  <li class='c033'>Precession, of equinoxes, <a href='#Page_69'>69</a>;
+    <ul>
+      <li>effects of, <a href='#Page_170'>170</a>;</li>
+      <li>luni-solar, <a href='#Page_169'>169</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Prime vertical, <a href='#Page_210'>210</a></li>
+  <li class='c033'>Principia, publication, <a href='#Page_10'>10</a>, <a href='#Page_13'>13</a>;
+    <ul>
+      <li>character, <a href='#Page_14'>14</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Prism, action of, <a href='#Page_215'>215</a>;
+    <ul>
+      <li>objective, <a href='#Page_223'>223</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Prismatic camera, <a href='#Page_223'>223</a></li>
+  <li class='c033'>— spectroscope, <a href='#Page_215'>215</a></li>
+  <li class='c033'>Pritchard, Prof., <a href='#Page_422'>422</a>, <a href='#Page_424'>424</a></li>
+  <li class='c033'>Proctor, Saturn’s rings, <a href='#Page_341'>341</a>;
+    <ul>
+      <li>distance of Uranus, <a href='#Page_344'>344</a>;</li>
+      <li>Proctor’s chart, <a href='#Page_548'>548</a>;</li>
+      <li>stars in streams, <a href='#Page_550'>550</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Procyon, supposed satellite, <a href='#Page_31'>31</a>, <a href='#Page_32'>32</a>;
+    <ul>
+      <li>order of magnitude, <a href='#Page_404'>404</a>;</li>
+      <li>parallax of Procyon, <a href='#Page_421'>421</a>;</li>
+      <li>Procyon approaching the Earth, <a href='#Page_427'>427</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Prominences, solar appendages, <a href='#Page_253'>253</a>, <a href='#Page_254'>254</a>;
+    <ul>
+      <li>spectrum, <a href='#Page_254'>254</a>, <a href='#Page_256'>256</a>, <a href='#Page_258'>258</a>, <a href='#Page_260'>260</a>;</li>
+      <li>daylight observations, <a href='#Page_254'>254</a>, <a href='#Page_255'>255</a>;</li>
+      <li>quiescent and eruptive, <a href='#Page_256'>256</a>;</li>
+      <li>periodicity, <a href='#Page_257'>257</a>;</li>
+      <li>rapid development, <a href='#Page_259'>259</a>;</li>
+      <li>spectral photography, <a href='#Page_260'>260</a>, <a href='#Page_261'>261</a></li>
+    </ul>
+  </li>
+  <li class='c033'>“Proper motions” of stars, <a href='#Page_423'>423</a>–431</li>
+  <li class='c033'>Ptolemaic system, <a href='#Page_3'>3</a>, <a href='#Page_4'>4</a>, <a href='#Page_6'>6</a></li>
+  <li class='center'>Q</li>
+  <li class='c033'>Quadrature, <a href='#Page_105'>105</a></li>
+  <li class='center'>R</li>
+  <li class='c033'>Rambaud, absorption in solar atmosphere, <a href='#Page_240'>240</a>;
+    <ul>
+      <li>fireball, <a href='#Page_380'>380</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Ramsay, terrestrial discovery of helium, <a href='#Page_255'>255</a></li>
+  <li class='c033'>Ramsden, astronomical circles, <a href='#Page_20'>20</a></li>
+  <li class='c033'>Raynard, the sun a nebulous body, <a href='#Page_253'>253</a>;
+    <ul>
+      <li>future of Saturn’s ring-system, <a href='#Page_340'>340</a>;</li>
+      <li>outflows from comets, <a href='#Page_380'>380</a>;</li>
+      <li>star streams, <a href='#Page_551'>551</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Ravené, gravitational disturbance by asteroids, <a href='#Page_315'>315</a></li>
+  <li class='c033'>R Centauri, <a href='#Page_478'>478</a></li>
+  <li class='c033'>Reading microscope, <a href='#Page_172'>172</a></li>
+  <li class='c033'>Recurrence of eclipses, <a href='#Page_119'>119</a></li>
+  <li class='c033'>“Red Bird,” <a href='#Page_415'>415</a></li>
+  <li class='c033'>Red spot on Jupiter, <a href='#Page_323'>323</a>, <a href='#Page_324'>324</a></li>
+  <li class='c033'>Red stars, <a href='#Page_416'>416</a></li>
+  <li class='c033'>Reduction of observations, <a href='#Page_18'>18</a>, <a href='#Page_19'>19</a></li>
+  <li class='c033'>Refracting telescope, <a href='#Page_176'>176</a></li>
+  <li class='c033'>Refraction, <a href='#Page_52'>52</a></li>
+  <li class='c033'>— in Venus, <a href='#Page_278'>278</a></li>
+  <li class='c033'>Reflecting telescope, <a href='#Page_178'>178</a></li>
+  <li class='c033'>Regression of moon’s nodes, <a href='#Page_94'>94</a></li>
+  <li class='c033'>Regulus, <a href='#Page_406'>406</a>, <a href='#Page_410'>410</a>, <a href='#Page_427'>427</a></li>
+  <li class='c033'>Retrogradation, <a href='#Page_89'>89</a>, <a href='#Page_103'>103</a></li>
+  <li class='c033'>Reversing layer, <a href='#Page_249'>249</a>, <a href='#Page_258'>258</a>, <a href='#Page_271'>271</a>;
+    <ul>
+      <li>photographed, <a href='#Page_259'>259</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Rich and poor regions, <a href='#Page_549'>549</a></li>
+  <li class='c033'>Richaud, <a href='#Page_440'>440</a></li>
+  <li class='c033'>Rigel, <a href='#Page_404'>404</a>, <a href='#Page_414'>414</a>, <a href='#Page_427'>427</a></li>
+  <li class='c033'>Right ascension, <a href='#Page_66'>66</a></li>
+  <li class='c033'>Roberts, Dr., <a href='#Page_23'>23</a>, <a href='#Page_107'>107</a>, <a href='#Page_502'>502</a>, <a href='#Page_503'>503</a>, <a href='#Page_508'>508</a>, <a href='#Page_511'>511</a>, <a href='#Page_520'>520</a>, <a href='#Page_525'>525</a>–528, <a href='#Page_530'>530</a>, <a href='#Page_533'>533</a>, <a href='#Page_534'>534</a>, <a href='#Page_539'>539</a>, <a href='#Page_540'>540</a></li>
+  <li class='c033'><span class='pageno' id='Page_575'>575</span>Roberts, A. W., <a href='#Page_440'>440</a>, <a href='#Page_441'>441</a>, <a href='#Page_469'>469</a></li>
+  <li class='c033'>Roche, minimum distance of satellites, <a href='#Page_340'>340</a></li>
+  <li class='c033'>Römer, velocity of light, <a href='#Page_329'>329</a></li>
+  <li class='c033'>Rosse, Earl of, giant reflector, <a href='#Page_24'>24</a></li>
+  <li class='c033'>Roszl, mass of 311 asteroids, <a href='#Page_315'>315</a></li>
+  <li class='c033'>Rotation of earth, <a href='#Page_47'>47</a>, <a href='#Page_48'>48</a>;
+    <ul>
+      <li>of moon, <a href='#Page_92'>92</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Rowland grating, <a href='#Page_217'>217</a></li>
+  <li class='c033'>— solar elements, <a href='#Page_250'>250</a></li>
+  <li class='c033'>Russell, photograph of Swift’s comet, <a href='#Page_377'>377</a>;
+    <ul>
+      <li>Kappa Crucis, <a href='#Page_506'>506</a>;</li>
+      <li>the “key-hole” nebula, <a href='#Page_522'>522</a>;</li>
+      <li>the Magellanic clouds, <a href='#Page_536'>536</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Rutherfurd, photographs of the moon, <a href='#Page_295'>295</a></li>
+  <li class='center'>S</li>
+  <li class='c033'>Sacrobosco, treatise on the sphere, <a href='#Page_6'>6</a></li>
+  <li class='c033'>Sagittarii, Zeta, <a href='#Page_434'>434</a></li>
+  <li class='c033'>Saros, <a href='#Page_120'>120</a></li>
+  <li class='c033'>Satellites, movements of Satellites, <a href='#Page_108'>108</a>;
+    <ul>
+      <li>masses of Satellites, <a href='#Page_159'>159</a></li>
+    </ul>
+  </li>
+  <li class='c033'>— discoveries, <a href='#Page_9'>9</a>, <a href='#Page_23'>23</a>, <a href='#Page_25'>25</a>, <a href='#Page_26'>26</a>, <a href='#Page_309'>309</a>, <a href='#Page_347'>347</a>;
+    <ul>
+      <li>apportionment, <a href='#Page_230'>230</a>;</li>
+      <li>formation checked by tidal friction, <a href='#Page_277'>277</a>, <a href='#Page_282'>282</a>;</li>
+      <li>planes of revolution, <a href='#Page_328'>328</a>, <a href='#Page_347'>347</a>, <a href='#Page_348'>348</a>, <a href='#Page_350'>350</a>;</li>
+      <li>transits, <a href='#Page_329'>329</a>, <a href='#Page_330'>330</a>, <a href='#Page_342'>342</a>;</li>
+      <li>eclipses, <a href='#Page_329'>329</a>, <a href='#Page_342'>342</a>;</li>
+      <li>variability, <a href='#Page_330'>330</a>, <a href='#Page_341'>341</a>, <a href='#Page_347'>347</a>;</li>
+      <li>rotation, <a href='#Page_331'>331</a>, <a href='#Page_341'>341</a>, <a href='#Page_342'>342</a>, <a href='#Page_347'>347</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Saturn, density, <a href='#Page_333'>333</a>;
+    <ul>
+      <li>spectrum, <a href='#Page_334'>334</a>;</li>
+      <li>rotation, <a href='#Page_334'>334</a>, <a href='#Page_339'>339</a>;</li>
+      <li>dimensions, <a href='#Page_535'>535</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Saturn’s ring-system, dusky member, <a href='#Page_25'>25</a>, <a href='#Page_336'>336</a>, <a href='#Page_338'>338</a>;
+    <ul>
+      <li>dimensions, <a href='#Page_336'>336</a>;</li>
+      <li>constitution, <a href='#Page_337'>337</a>, <a href='#Page_339'>339</a>, <a href='#Page_340'>340</a>;</li>
+      <li>albedo, <a href='#Page_338'>338</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Sawyer, U Ophiuchi discovered, <a href='#Page_473'>473</a>;
+    <ul>
+      <li>variability of R Canis Majoris detected, <a href='#Page_473'>473</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Schaeberle, photographs of corona of 1893, <a href='#Page_270'>270</a>;
+    <ul>
+      <li>land and water on Mars, <a href='#Page_306'>306</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Scheiner, spectra of sun-spots, <a href='#Page_251'>251</a></li>
+  <li class='c033'>Schiaparelli, rotation of Mercury, <a href='#Page_275'>275</a>;
+    <ul>
+      <li>map of Mercury, <a href='#Page_277'>277</a>;</li>
+      <li>rotation of Venus, <a href='#Page_280'>280</a>, <a href='#Page_281'>281</a>;</li>
+      <li>canals of Mars, <a href='#Page_301'>301</a>;</li>
+      <li>duplication, <a href='#Page_305'>305</a>;</li>
+      <li>climate of Mars, <a href='#Page_308'>308</a>;</li>
+      <li>compression of Uranus, <a href='#Page_343'>343</a>;</li>
+      <li>comets and meteors, <a href='#Page_393'>393</a>;</li>
+      <li>theory of extinction of light, <a href='#Page_544'>544</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Schiehallion experiment, <a href='#Page_161'>161</a></li>
+  <li class='c033'>Schmidt, map of the moon, <a href='#Page_290'>290</a></li>
+  <li class='c033'>Schönfeld, <a href='#Page_461'>461</a>, <a href='#Page_464'>464</a>, <a href='#Page_466'>466</a>, <a href='#Page_467'>467</a>, <a href='#Page_473'>473</a>, <a href='#Page_483'>483</a></li>
+  <li class='c033'>Schorr, <a href='#Page_442'>442</a></li>
+  <li class='c033'>Schur, <a href='#Page_433'>433</a>, <a href='#Page_442'>442</a></li>
+  <li class='c033'>Schuster, photograph of eclipsed sun, <a href='#Page_268'>268</a></li>
+  <li class='c033'>Schwabe, discovery of sun-spot periodicity, <a href='#Page_245'>245</a></li>
+  <li class='c033'>Seasons, <a href='#Page_61'>61</a></li>
+  <li class='c033'>Secchi, observations of prominences, <a href='#Page_256'>256</a>;
+    <ul>
+      <li>spectrum of Uranus, <a href='#Page_345'>345</a></li>
+    </ul>
+  </li>
+  <li class='c033'>See, Dr., <a href='#Page_413'>413</a>, <a href='#Page_433'>433</a>–435, <a href='#Page_440'>440</a>, <a href='#Page_442'>442</a>, <a href='#Page_447'>447</a>, <a href='#Page_448'>448</a></li>
+  <li class='c033'>Seeliger, photometric measures of Saturn’s rings, <a href='#Page_339'>339</a></li>
+  <li class='c033'>Serpentarii, Nova, <a href='#Page_483'>483</a></li>
+  <li class='c033'>Sextant, <a href='#Page_211'>211</a></li>
+  <li class='c033'>Shackleton, photograph of the reversing layer, <a href='#Page_259'>259</a></li>
+  <li class='c033'>Ship, position of, <a href='#Page_128'>128</a></li>
+  <li class='c033'>“Sickle” in Leo, <a href='#Page_406'>406</a></li>
+  <li class='c033'>Siderostat, <a href='#Page_194'>194</a></li>
+  <li class='c033'>Sidgreaves, elevations of chromosphere, <a href='#Page_258'>258</a></li>
+  <li class='c033'>Sirius, proper motion, <a href='#Page_17'>17</a>, <a href='#Page_31'>31</a>;
+    <ul>
+      <li>companion, <a href='#Page_32'>32</a>;</li>
+      <li>spectrum, <a href='#Page_37'>37</a>;</li>
+      <li>size, <a href='#Page_403'>403</a>;</li>
+      <li>position, <a href='#Page_409'>409</a>;</li>
+      <li>colour, <a href='#Page_414'>414</a>;</li>
+      <li>distance, <a href='#Page_418'>418</a>, <a href='#Page_421'>421</a>;</li>
+      <li>discovery of proper motion, <a href='#Page_423'>423</a>;</li>
+      <li>a binary star, <a href='#Page_437'>437</a>;</li>
+      <li>comparative magnitude, <a href='#Page_438'>438</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Smyth, <a href='#Page_447'>447</a>, <a href='#Page_448'>448</a>, <a href='#Page_502'>502</a>, <a href='#Page_505'>505</a>, <a href='#Page_508'>508</a></li>
+  <li class='c033'>Solar, constant, <a href='#Page_239'>239</a></li>
+  <li class='c033'>— diagonal, <a href='#Page_183'>183</a></li>
+  <li class='c033'>— eclipses, <a href='#Page_113'>113</a></li>
+  <li class='c033'>— ecliptic limit, <a href='#Page_118'>118</a></li>
+  <li class='c033'>— System, dominated by gravity, <a href='#Page_29'>29</a>;
+    <ul>
+      <li>constitution, <a href='#Page_229'>229</a>, <a href='#Page_232'>232</a>;</li>
+      <li>dimensions, <a href='#Page_231'>231</a>;</li>
+      <li>stability, <a href='#Page_232'>232</a>;</li>
+      <li>origin, <a href='#Page_235'>235</a>, <a href='#Page_236'>236</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Southern Cross, <a href='#Page_410'>410</a>, <a href='#Page_416'>416</a>, <a href='#Page_549'>549</a></li>
+  <li class='c033'>Shouting, <a href='#Page_68'>68</a>, <a href='#Page_198'>198</a></li>
+  <li class='c033'>Spectroheliograph, <a href='#Page_225'>225</a></li>
+  <li class='c033'>Spectroscope, prismatic, <a href='#Page_215'>215</a>;
+    <ul>
+      <li>direct vision, <a href='#Page_216'>216</a>;</li>
+      <li>grating, <a href='#Page_216'>216</a>;</li>
+      <li>Lick star-, <a href='#Page_219'>219</a>;</li>
+      <li>Rowland, <a href='#Page_217'>217</a>;</li>
+      <li>tele-, <a href='#Page_219'>219</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Spectroscopic measurements of rotation;
+    <ul>
+      <li>the sun, <a href='#Page_248'>248</a>;</li>
+      <li>Venus, <a href='#Page_281'>281</a>;</li>
+      <li>Saturn, <a href='#Page_339'>339</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Spectrum, solar, <a href='#Page_34'>34</a>, <a href='#Page_250'>250</a>;
+    <ul>
+      <li>of stars and nebulæ, <a href='#Page_35'>35</a>, <a href='#Page_37'>37</a>;</li>
+      <li>measurement of, <a href='#Page_218'>218</a>;</li>
+      <li>sun-spot, <a href='#Page_250'>250</a>, <a href='#Page_251'>251</a>;</li>
+      <li>prominence, <a href='#Page_254'>254</a>, <a href='#Page_256'>256</a>, <a href='#Page_261'>261</a>;</li>
+      <li>chromospheric, <a href='#Page_258'>258</a>;</li>
+      <li>of Mercury, <a href='#Page_275'>275</a>;</li>
+      <li>of Venus, <a href='#Page_279'>279</a>;</li>
+      <li>auroral, <a href='#Page_288'>288</a>;</li>
+      <li>of Jupiter, <a href='#Page_326'>326</a>;</li>
+      <li>of Saturn’s rings, <a href='#Page_338'>338</a>;</li>
+      <li><span class='pageno' id='Page_576'>576</span>of Uranus, <a href='#Page_345'>345</a>, <a href='#Page_346'>346</a>;</li>
+      <li>of Neptune, <a href='#Page_351'>351</a>;</li>
+      <li>of comets, <a href='#Page_368'>368</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Spherical excess, <a href='#Page_133'>133</a></li>
+  <li class='c033'>Spica, <a href='#Page_404'>404</a>, <a href='#Page_410'>410</a></li>
+  <li class='c033'>Spiral nebulæ, <a href='#Page_533'>533</a>, <a href='#Page_534'>534</a></li>
+  <li class='c033'>Spoerer, solar rotation, <a href='#Page_249'>249</a></li>
+  <li class='c033'>Star of Bethlehem, <a href='#Page_101'>101</a></li>
+  <li class='c033'>Star-charting, photographic, <a href='#Page_38'>38</a></li>
+  <li class='c033'>— cluster, <a href='#Page_17'>17</a></li>
+  <li class='c033'>— spectroscope, <a href='#Page_219'>219</a></li>
+  <li class='c033'>— time, <a href='#Page_68'>68</a></li>
+  <li class='c033'>Stars, temporary, <a href='#Page_3'>3</a>, <a href='#Page_8'>8</a>, <a href='#Page_477'>477</a>;
+    <ul>
+      <li>proper motions of, <a href='#Page_17'>17</a>, <a href='#Page_19'>19</a>, <a href='#Page_28'>28</a>, <a href='#Page_425'>425</a>, <a href='#Page_427'>427</a>;</li>
+      <li>fixed, <a href='#Page_45'>45</a>;</li>
+      <li>circumpolar, <a href='#Page_46'>46</a>;</li>
+      <li>diurnal motion of, <a href='#Page_46'>46</a>;</li>
+      <li>aberration of, <a href='#Page_58'>58</a>;</li>
+      <li>catalogues of, <a href='#Page_71'>71</a>;</li>
+      <li>clock, <a href='#Page_82'>82</a>;</li>
+      <li>morning and evening, <a href='#Page_100'>100</a>;</li>
+      <li>magnitudes, <a href='#Page_403'>403</a>, <a href='#Page_404'>404</a>;</li>
+      <li>Pole, <a href='#Page_405'>405</a>, <a href='#Page_412'>412</a>;</li>
+      <li>double, <a href='#Page_410'>410</a>;</li>
+      <li>coloured, <a href='#Page_416'>416</a>;</li>
+      <li>red, <a href='#Page_416'>416</a>;</li>
+      <li>nearest, <a href='#Page_417'>417</a>;</li>
+      <li>binary, <a href='#Page_431'>431</a>;</li>
+      <li>variable, <a href='#Page_458'>458</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Stationary points, <a href='#Page_103'>103</a></li>
+  <li class='c033'>Stone, mass of Titan, <a href='#Page_342'>342</a></li>
+  <li class='c033'>Stoney, G. Johnstone, atmospheres of planets, <a href='#Page_313'>313</a></li>
+  <li class='c033'>Stratonoff, sun’s rotation from faculæ, <a href='#Page_249'>249</a></li>
+  <li class='c033'>Suess, theory of lunar formations, <a href='#Page_292'>292</a></li>
+  <li class='c033'>Sun, translation, <a href='#Page_28'>28</a>, <a href='#Page_229'>229</a>;
+    <ul>
+      <li>apparent movements of, <a href='#Page_55'>55</a>, <a href='#Page_77'>77</a>;</li>
+      <li>midnight, <a href='#Page_63'>63</a>;</li>
+      <li>apparent diameter of, <a href='#Page_72'>72</a>;</li>
+      <li>mean, <a href='#Page_79'>79</a>;</li>
+      <li>eclipses of, <a href='#Page_113'>113</a>;</li>
+      <li>distance of, <a href='#Page_146'>146</a>;</li>
+      <li>mass of, <a href='#Page_156'>156</a>;</li>
+      <li>maintenance of heat, <a href='#Page_234'>234</a>;</li>
+      <li>radiative power, <a href='#Page_237'>237</a>–239, <a href='#Page_241'>241</a>, <a href='#Page_242'>242</a>;</li>
+      <li>temperature, <a href='#Page_239'>239</a>, <a href='#Page_240'>240</a>;</li>
+      <li>magnitude, <a href='#Page_240'>240</a>, <a href='#Page_241'>241</a>;</li>
+      <li>luminous surface, <a href='#Page_242'>242</a>;</li>
+      <li>spots, <a href='#Page_243'>243</a>–249, <a href='#Page_251'>251</a>, <a href='#Page_252'>252</a>;</li>
+      <li>periodicity, <a href='#Page_246'>246</a>;</li>
+      <li>rotation, <a href='#Page_247'>247</a>–249;</li>
+      <li>chemistry, <a href='#Page_250'>250</a>;</li>
+      <li>theories, <a href='#Page_252'>252</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Sun-dial, <a href='#Page_78'>78</a></li>
+  <li class='c033'>Sun’s motion in space, <a href='#Page_428'>428</a></li>
+  <li class='c033'>Sun-spots, observed by Galileo, <a href='#Page_9'>9</a>;
+    <ul>
+      <li>construction, <a href='#Page_243'>243</a>, <a href='#Page_251'>251</a>;</li>
+      <li>zones, <a href='#Page_245'>245</a>, <a href='#Page_247'>247</a>;</li>
+      <li>periodicity, <a href='#Page_245'>245</a>, <a href='#Page_247'>247</a>;</li>
+      <li>irregular movements, <a href='#Page_247'>247</a>–249;</li>
+      <li>spectra, <a href='#Page_250'>250</a>–252</li>
+    </ul>
+  </li>
+  <li class='c033'>Sutton, <a href='#Page_553'>553</a></li>
+  <li class='c033'>Swift, Lewis, comet discovered by, <a href='#Page_377'>377</a>, <a href='#Page_378'>378</a>, <a href='#Page_383'>383</a></li>
+  <li class='c033'>Sykora, elevation of spotted areas on the sun, <a href='#Page_252'>252</a></li>
+  <li class='c033'>Synodic period, of moon, <a href='#Page_91'>91</a>;
+    <ul>
+      <li>of planets, <a href='#Page_107'>107</a></li>
+    </ul>
+  </li>
+  <li class='center'>T</li>
+  <li class='c033'>Tacchini, spectrum of Venus, <a href='#Page_279'>279</a>;
+    <ul>
+      <li>rotation, <a href='#Page_280'>280</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Talcott’s latitude method, <a href='#Page_124'>124</a></li>
+  <li class='c033'>Tauri, Alpha. <i>See</i> Aldebaran</li>
+  <li class='c033'>— Lambda, <a href='#Page_473'>473</a></li>
+  <li class='c033'>Tebbutt’s comet, <a href='#Page_362'>362</a>, <a href='#Page_368'>368</a></li>
+  <li class='c033'>Telescope, invention of, <a href='#Page_9'>9</a>;
+    <ul>
+      <li>achromatic, <a href='#Page_20'>20</a>, <a href='#Page_21'>21</a>;</li>
+      <li>reflecting, <a href='#Page_21'>21</a>, <a href='#Page_24'>24</a>, <a href='#Page_25'>25</a>, <a href='#Page_178'>178</a>;</li>
+      <li>refracting, <a href='#Page_20'>20</a>, <a href='#Page_25'>25</a>–27, <a href='#Page_176'>176</a>;</li>
+      <li>future improvement, <a href='#Page_26'>26</a>, <a href='#Page_27'>27</a>, <a href='#Page_297'>297</a>;</li>
+      <li>Newtonian, <a href='#Page_179'>179</a>;</li>
+      <li>Cassegrain, <a href='#Page_180'>180</a>, <a href='#Page_181'>181</a>;</li>
+      <li>Herschellian, <a href='#Page_180'>180</a>;</li>
+      <li>Skew Cassegrain, <a href='#Page_181'>181</a>;</li>
+      <li>magnifying power of, <a href='#Page_184'>184</a>;</li>
+      <li>illuminating power of, <a href='#Page_184'>184</a>;</li>
+      <li>altazimuth, <a href='#Page_184'>184</a>;</li>
+      <li>equatorial, <a href='#Page_185'>185</a>;</li>
+      <li>Rosse, <a href='#Page_187'>187</a>;</li>
+      <li>Common, <a href='#Page_5'>5</a>-foot, <a href='#Page_188'>188</a>;</li>
+      <li>Lick, <a href='#Page_190'>190</a>;</li>
+      <li>fixed, <a href='#Page_194'>194</a>;</li>
+      <li>photographic, <a href='#Page_194'>194</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Telespectroscope, <a href='#Page_219'>219</a></li>
+  <li class='c033'>Tempel, <a href='#Page_501'>501</a></li>
+  <li class='c033'>“Temporary stars,” <a href='#Page_477'>477</a>–497</li>
+  <li class='c033'>Theodolite, <a href='#Page_205'>205</a></li>
+  <li class='c033'>Thiele, <a href='#Page_435'>435</a>, <a href='#Page_447'>447</a>, <a href='#Page_451'>451</a></li>
+  <li class='c033'>Thome, comet of 1887, <a href='#Page_360'>360</a></li>
+  <li class='c033'>Tidal evolution, <a href='#Page_167'>167</a></li>
+  <li class='c033'>Tidal friction, <a href='#Page_166'>166</a>;
+    <ul>
+      <li>in earth-moon system, <a href='#Page_236'>236</a>, <a href='#Page_283'>283</a>, <a href='#Page_284'>284</a>;</li>
+      <li>on Mercury, <a href='#Page_277'>277</a>;</li>
+      <li>effect on satellite-formation, <a href='#Page_278'>278</a>, <a href='#Page_282'>282</a>;</li>
+      <li>on Venus, <a href='#Page_282'>282</a>;</li>
+      <li>on Phobos, <a href='#Page_310'>310</a>;</li>
+      <li>on Saturnian satellites, <a href='#Page_342'>342</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Tides, <a href='#Page_162'>162</a>;
+    <ul>
+      <li>spring and neap, <a href='#Page_164'>164</a>;</li>
+      <li>priming and lagging, <a href='#Page_164'>164</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Time, apparent, <a href='#Page_78'>78</a>;
+    <ul>
+      <li>equation of, <a href='#Page_79'>79</a>;</li>
+      <li>mean solar, <a href='#Page_79'>79</a>;</li>
+      <li>determination of, <a href='#Page_82'>82</a>;</li>
+      <li>at different places, <a href='#Page_83'>83</a>;</li>
+      <li>Greenwich mean, <a href='#Page_83'>83</a>;</li>
+      <li>local, <a href='#Page_83'>83</a>;</li>
+      <li>telegraphy, <a href='#Page_84'>84</a>;</li>
+      <li>zone, <a href='#Page_84'>84</a>;</li>
+      <li>balls, <a href='#Page_85'>85</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Tisserand, revolutions of Jupiter’s fifth satellite, <a href='#Page_331'>331</a>;
+    <ul>
+      <li>disturbance of Neptune’s satellite, <a href='#Page_351'>351</a>;</li>
+      <li>capture of comets, <a href='#Page_372'>372</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Todd, Miss M. L., drawing of corona, <a href='#Page_268'>268</a></li>
+  <li class='c033'>— Prof., trans-Neptunian planet, <a href='#Page_231'>231</a></li>
+  <li class='c033'>Toucani (<i>41</i>), <a href='#Page_513'>513</a></li>
+  <li class='c033'>Transit circle, <a href='#Page_198'>198</a>–202</li>
+  <li class='c033'>— instrument, <a href='#Page_202'>202</a></li>
+  <li class='c033'>— of Venus, <a href='#Page_101'>101</a>, <a href='#Page_148'>148</a></li>
+  <li class='c033'>Triangulation, <a href='#Page_53'>53</a></li>
+  <li class='c033'>Troughton, instrumental improvements, <a href='#Page_19'>19</a>, <a href='#Page_20'>20</a></li>
+  <li class='c033'>Trouvelot, mountains of Venus, <a href='#Page_279'>279</a>;
+    <ul>
+      <li>rotation, <a href='#Page_280'>280</a></li>
+    </ul>
+  </li>
+  <li class='c033'><span class='pageno' id='Page_577'>577</span>Twilight, <a href='#Page_53'>53</a></li>
+  <li class='c033'>Tycho Brahé, <a href='#Page_5'>5</a>, <a href='#Page_8'>8</a>, <a href='#Page_9'>9</a>, <a href='#Page_405'>405</a>, <a href='#Page_418'>418</a>, <a href='#Page_479'>479</a>, <a href='#Page_481'>481</a>, <a href='#Page_529'>529</a></li>
+  <li class='center'>U</li>
+  <li class='c033'>Ulugh Beigh, observations at Samarcand, <a href='#Page_5'>5</a></li>
+  <li class='c033'>Umbra of earth’s shadow, <a href='#Page_111'>111</a></li>
+  <li class='c033'>Uranus, discovery, <a href='#Page_22'>22</a>, <a href='#Page_229'>229</a>;
+    <ul>
+      <li>perturbations, <a href='#Page_32'>32</a>, <a href='#Page_231'>231</a>;</li>
+      <li>dimensions and markings, <a href='#Page_343'>343</a>–345;</li>
+      <li>analogy with Neptune, <a href='#Page_343'>343</a>, <a href='#Page_351'>351</a>;</li>
+      <li>rotation, <a href='#Page_344'>344</a>, <a href='#Page_348'>348</a>;</li>
+      <li>spectrum, <a href='#Page_345'>345</a>, <a href='#Page_346'>346</a>;</li>
+      <li>satellites, <a href='#Page_347'>347</a>, <a href='#Page_348'>348</a>;</li>
+      <li>comets captured by, <a href='#Page_371'>371</a>, <a href='#Page_395'>395</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Ursa Major, stars in, <a href='#Page_400'>400</a>, <a href='#Page_401'>401</a></li>
+  <li class='c033'>Ursæ Majoris, Xi, <a href='#Page_440'>440</a></li>
+  <li class='center'>V</li>
+  <li class='c033'>Variation of latitude, <a href='#Page_136'>136</a></li>
+  <li class='c033'>Variable stars, <a href='#Page_458'>458</a></li>
+  <li class='c033'>Vega, <a href='#Page_403'>403</a>, <a href='#Page_406'>406</a>, <a href='#Page_414'>414</a>, <a href='#Page_422'>422</a>, <a href='#Page_427'>427</a></li>
+  <li class='c033'>Venus, phases observed by Galileo, <a href='#Page_9'>9</a>;
+    <ul>
+      <li>transits, <a href='#Page_17'>17</a>;</li>
+      <li>phases of, <a href='#Page_101'>101</a>;</li>
+      <li>transit of, <a href='#Page_101'>101</a>, <a href='#Page_148'>148</a>;</li>
+      <li>atmosphere, <a href='#Page_278'>278</a>, <a href='#Page_281'>281</a>, <a href='#Page_282'>282</a>;</li>
+      <li>ashen light, <a href='#Page_279'>279</a>;</li>
+      <li>spectrum, <a href='#Page_279'>279</a>;</li>
+      <li>rotation, <a href='#Page_280'>280</a>, <a href='#Page_281'>281</a>;</li>
+      <li>imaginary satellite, <a href='#Page_282'>282</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Vernier, <a href='#Page_172'>172</a></li>
+  <li class='c033'>Very, distribution of lunar heat, <a href='#Page_295'>295</a></li>
+  <li class='c033'>Vesta, discovery, <a href='#Page_311'>311</a>;
+    <ul>
+      <li>diameter and brightness, <a href='#Page_312'>312</a>;</li>
+      <li>mass, <a href='#Page_313'>313</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Villarceau, <a href='#Page_433'>433</a></li>
+  <li class='c033'>Virginis, Alpha (Spica), <a href='#Page_404'>404</a>, <a href='#Page_410'>410</a></li>
+  <li class='c033'>— Gamma, <a href='#Page_413'>413</a>, <a href='#Page_444'>444</a>–450</li>
+  <li class='c033'>— Tau, <a href='#Page_456'>456</a></li>
+  <li class='c033'>— W, <a href='#Page_469'>469</a></li>
+  <li class='c033'>Visible stars, number of, <a href='#Page_538'>538</a>–546</li>
+  <li class='c033'>Vogel, spectrum of Jupiter, <a href='#Page_326'>326</a>;
+    <ul>
+      <li>of Uranus, <a href='#Page_345'>345</a>;</li>
+      <li>binary or multiple system of Beta Lyræ, <a href='#Page_466'>466</a>;</li>
+      <li>diameter of Algol, <a href='#Page_472'>472</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Volcanic action, terrestrial, <a href='#Page_284'>284</a>;
+    <ul>
+      <li>lunar, <a href='#Page_290'>290</a>, <a href='#Page_292'>292</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Von Gothard, <a href='#Page_465'>465</a></li>
+  <li class='c033'>Vulpeculæ, Nova, <a href='#Page_484'>484</a></li>
+  <li class='c033'>— S, <a href='#Page_484'>484</a></li>
+  <li class='center'>W</li>
+  <li class='c033'>Ward, <a href='#Page_488'>488</a></li>
+  <li class='c033'>Way, Milky, <a href='#Page_402'>402</a>, <a href='#Page_430'>430</a>, <a href='#Page_549'>549</a>, <a href='#Page_557'>557</a></li>
+  <li class='c033'>Webb, <a href='#Page_528'>528</a></li>
+  <li class='c033'>Wedge, photometer, <a href='#Page_213'>213</a></li>
+  <li class='c033'>Weight, defined, <a href='#Page_151'>151</a>;
+    <ul>
+      <li>of the earth, <a href='#Page_160'>160</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Wells’ comet, <a href='#Page_368'>368</a></li>
+  <li class='c033'>Williams, A. Stanley, rotation of Venus, <a href='#Page_280'>280</a>;
+    <ul>
+      <li>of Jupiter, <a href='#Page_325'>325</a>;</li>
+      <li>photographs of Jupiter, <a href='#Page_327'>327</a>;</li>
+      <li>spots on Saturn, <a href='#Page_334'>334</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Wilson, Alexander, depression of sun-spots, <a href='#Page_251'>251</a></li>
+  <li class='c033'>— W. E., temperature of the sun, <a href='#Page_240'>240</a></li>
+  <li class='c033'>Winnecke, <a href='#Page_422'>422</a></li>
+  <li class='c033'>Winnecke’s comet, <a href='#Page_368'>368</a>, <a href='#Page_371'>371</a>, <a href='#Page_372'>372</a></li>
+  <li class='c033'>Wire micrometer, <a href='#Page_205'>205</a></li>
+  <li class='c033'>Wolf, Max, photographic discovery of asteroids, <a href='#Page_314'>314</a>;
+    <ul>
+      <li>comet discovered by, <a href='#Page_377'>377</a>;</li>
+      <li>chart of the Pleiades, <a href='#Page_499'>499</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Wrublewsky, <a href='#Page_434'>434</a></li>
+  <li class='center'>Y</li>
+  <li class='c033'>Year, <a href='#Page_85'>85</a>;
+    <ul>
+      <li>sidereal, <a href='#Page_85'>85</a>;</li>
+      <li>tropical, <a href='#Page_85'>85</a>;</li>
+      <li>leap, <a href='#Page_86'>86</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Yendell, <a href='#Page_474'>474</a></li>
+  <li class='c033'>Yerkes, <a href='#Page_40'>40</a>-inch refractor, <a href='#Page_26'>26</a>, <a href='#Page_27'>27</a></li>
+  <li class='c033'>Young, solar eruption, <a href='#Page_256'>256</a>;
+    <ul>
+      <li>spectrum of chromosphere, <a href='#Page_258'>258</a>;</li>
+      <li>reversing layer, <a href='#Page_258'>258</a>;</li>
+      <li>spectrum of Venus, <a href='#Page_279'>279</a>;</li>
+      <li>brightness of Phobos, <a href='#Page_309'>309</a>;</li>
+      <li>belts of Uranus, <a href='#Page_343'>343</a>;</li>
+      <li>size of Uranus, <a href='#Page_345'>345</a>;</li>
+      <li>the sun and planets, seen from Neptune, <a href='#Page_349'>349</a>, <a href='#Page_352'>352</a>;</li>
+      <li><span class='pageno' id='Page_578'>578</span>Andromede meteors, <a href='#Page_394'>394</a></li>
+    </ul>
+  </li>
+  <li class='center'>Z</li>
+  <li class='c033'>Zenith, <a href='#Page_45'>45</a></li>
+  <li class='c033'>— telescope, <a href='#Page_210'>210</a></li>
+  <li class='c033'>Zodiac, <a href='#Page_60'>60</a></li>
+  <li class='c033'>Zodiacal Light, <a href='#Page_272'>272</a>, <a href='#Page_273'>273</a></li>
+  <li class='c033'>Zöllner, albedo of Mars, <a href='#Page_298'>298</a>, <a href='#Page_334'>334</a>;
+    <ul>
+      <li>of Jupiter, <a href='#Page_320'>320</a>;</li>
+      <li>of Neptune, <a href='#Page_349'>349</a>;</li>
+      <li>estimate of sunlight, <a href='#Page_543'>543</a></li>
+    </ul>
+  </li>
+  <li class='c033'>Zone time, <a href='#Page_84'>84</a></li>
+</ul>
+
+<div class='nf-center-c0'>
+<div class='nf-center c002'>
+    <div>THE END.</div>
+  </div>
+</div>
+
+<hr class='c034'>
+<div class='footnote' id='f1'>
+<p class='c007'><a href='#r1'>1</a>. There is a very complete paper on “How to find Easter,” by Dr. Downing,
+in the <cite>Journal</cite> of the British Astronomical Association, vol. ii., p. 264.</p>
+</div>
+<div class='footnote' id='f2'>
+<p class='c007'><a href='#r2'>2</a>. The application of Kepler’s third law gives us P = <i>a</i><sup>³⁄₂</sup> years, but as this is not
+strictly true, both P and <i>a</i> must be given where the greatest possible accuracy is
+desired.</p>
+</div>
+<div class='footnote' id='f3'>
+<p class='c007'><a href='#r3'>3</a>. The diagram is based upon one given by Prof. Albrech in the <cite>Astronomische
+Nachrichten</cite>, No. 3333. The dotted part of the curve could not be directly
+derived on account of insufficient observations.</p>
+</div>
+<div class='footnote' id='f4'>
+<p class='c007'><a href='#r4'>4</a>. The focal length of a lens is the distance from its centre at which an image of
+a very distant object, such as the sun, is formed.</p>
+</div>
+<div class='footnote' id='f5'>
+<p class='c007'><a href='#r5'>5</a>. In a British inch there are 25·4 millimetres.</p>
+</div>
+<div class='footnote' id='f6'>
+<p class='c007'><a href='#r6'>6</a>. Proctor: “Old and New Astronomy,” p. 327.</p>
+</div>
+<div class='footnote' id='f7'>
+<p class='c007'><a href='#r7'>7</a>. Langley: “The New Astronomy,” p. 108.</p>
+</div>
+<div class='footnote' id='f8'>
+<p class='c007'><a href='#r8'>8</a>. The “bolometer,” invented by Langley, measures heat with exquisite refinement
+by means of its electrical effects.</p>
+</div>
+<div class='footnote' id='f9'>
+<p class='c007'><a href='#r9'>9</a>. W. E. Wilson: <cite>Monthly Notices</cite>, vol. lv., p. 457.</p>
+</div>
+<div class='footnote' id='f10'>
+<p class='c007'><a href='#r10'>10</a>. <cite>Observatory</cite>, vol. xviii., p. 344.</p>
+</div>
+<div class='footnote' id='f11'>
+<p class='c007'><a href='#r11'>11</a>. Frost-Scheiner: “Astronomical Spectroscopy,” p. 177.</p>
+</div>
+<div class='footnote' id='f12'>
+<p class='c007'><a href='#r12'>12</a>. <cite>Astronomische Nachrichten</cite>, No. 3330.</p>
+</div>
+<div class='footnote' id='f13'>
+<p class='c007'><a href='#r13'>13</a>. <cite>Knowledge</cite>, vol. vi., p. 13.</p>
+</div>
+<div class='footnote' id='f14'>
+<p class='c007'><a href='#r14'>14</a>. “The Sun,” p. 206, first edition.</p>
+</div>
+<div class='footnote' id='f15'>
+<p class='c007'><a href='#r15'>15</a>. “Memoirs of the Royal Astronomical Society,” vol. xli., p. 435.</p>
+</div>
+<div class='footnote' id='f16'>
+<p class='c007'><a href='#r16'>16</a>. <cite>Astronomy and Astro-Physics</cite>, vol. xiii., p. 122.</p>
+</div>
+<div class='footnote' id='f17'>
+<p class='c007'><a href='#r17'>17</a>. <cite>Comptes Rendus</cite>, December 26, 1893.</p>
+</div>
+<div class='footnote' id='f18'>
+<p class='c007'><a href='#r18'>18</a>. <cite>Knowledge</cite>, vol. iv., p. 105.</p>
+</div>
+<div class='footnote' id='f19'>
+<p class='c007'><a href='#r19'>19</a>. “Rapport de la Mission envoyée an Sénégal,” p. 31.</p>
+</div>
+<div class='footnote' id='f20'>
+<p class='c007'><a href='#r20'>20</a>. “Harvard Annals,” vol. xix., part ii.; 1893.</p>
+</div>
+<div class='footnote' id='f21'>
+<p class='c007'><a href='#r21'>21</a>. “The Solar Corona discussed by Spherical Harmonics;” Washington,
+1889.</p>
+</div>
+<div class='footnote' id='f22'>
+<p class='c007'><a href='#r22'>22</a>. <cite>Bulletin Astronomique</cite>, April, 1896.</p>
+</div>
+<div class='footnote' id='f23'>
+<p class='c007'><a href='#r23'>23</a>. According to G. Müller, <cite>Potsdam Publicationen</cite>, No. 30, p. 369, Zöllner
+fixed the albedo of Mercury at 0·13.</p>
+</div>
+<div class='footnote' id='f24'>
+<p class='c007'><a href='#r24'>24</a>. <cite>Astr. Nach.</cite>, No. 3171.</p>
+</div>
+<div class='footnote' id='f25'>
+<p class='c007'><a href='#r25'>25</a>. <cite>Astr. Nach.</cite>, No. 3406.</p>
+</div>
+<div class='footnote' id='f26'>
+<p class='c007'><a href='#r26'>26</a>. <cite>Ibid.</cite>, No. 2944.</p>
+</div>
+<div class='footnote' id='f27'>
+<p class='c007'><a href='#r27'>27</a>. <cite>Astr. Nach.</cite>, No. 3332.</p>
+</div>
+<div class='footnote' id='f28'>
+<p class='c007'><a href='#r28'>28</a>. This was in principle suggested by Proctor in “The Old and New
+Astronomy.”</p>
+</div>
+<div class='footnote' id='f29'>
+<p class='c007'><a href='#r29'>29</a>. <cite>Nature</cite>, vol. li., p. 227.</p>
+</div>
+<div class='footnote' id='f30'>
+<p class='c007'><a href='#r30'>30</a>. Kelvin, <cite>Nature</cite>, p. 440; Clarence King, <cite>American Journal of Science</cite>,
+January, 1893.</p>
+</div>
+<div class='footnote' id='f31'>
+<p class='c007'><a href='#r31'>31</a>. <cite>Ciel et Terre</cite>, 16th March, 1895.</p>
+</div>
+<div class='footnote' id='f32'>
+<p class='c007'><a href='#r32'>32</a>. <cite>Himmel und Erde</cite>, Feb., 1889; <cite>Astr. Nach.</cite>, No. 3347; A. Battandier,
+<cite>L’Astronomie</cite>, 1894.</p>
+</div>
+<div class='footnote' id='f33'>
+<p class='c007'><a href='#r33'>33</a>. Balfour Stewart: “Ency. Brit.,” vol. xvi. pp. 164, 165.</p>
+</div>
+<div class='footnote' id='f34'>
+<p class='c007'><a href='#r34'>34</a>. A. Paulsen: <cite>Ciel et Terre</cite>, 1 Juillet, 1895, p. 202.</p>
+</div>
+<div class='footnote' id='f35'>
+<p class='c007'><a href='#r35'>35</a>. Elger: “The Moon,” p. 73.</p>
+</div>
+<div class='footnote' id='f36'>
+<p class='c007'><a href='#r36'>36</a>. “Publications, Astronomical Society of the Pacific,” vol. vii., p. 144.</p>
+</div>
+<div class='footnote' id='f37'>
+<p class='c007'><a href='#r37'>37</a>. “Harvard Annals,” vol. xxxii., part i., p. 109.</p>
+</div>
+<div class='footnote' id='f38'>
+<p class='c007'><a href='#r38'>38</a>. <cite>Astronomy and Astro-Physics</cite>, Nov., 1894, p. 718.</p>
+</div>
+<div class='footnote' id='f39'>
+<p class='c007'><a href='#r39'>39</a>. “Popular Astronomy,” 1895, p. 347.</p>
+</div>
+<div class='footnote' id='f40'>
+<p class='c007'><a href='#r40'>40</a>. <cite>Astr. Nach.</cite>, No. 3271 (Schiaparelli).</p>
+</div>
+<div class='footnote' id='f41'>
+<p class='c007'><a href='#r41'>41</a>. “Popular Astronomy,” vol. i., p. 348.</p>
+</div>
+<div class='footnote' id='f42'>
+<p class='c007'><a href='#r42'>42</a>. <cite>Scientific American</cite>, Feb. 29, 1896.</p>
+</div>
+<div class='footnote' id='f43'>
+<p class='c007'><a href='#r43'>43</a>. Schiaparelli: <cite>Astronomy and Astro-Physics</cite>, Nov., 1894, p. 720.</p>
+</div>
+<div class='footnote' id='f44'>
+<p class='c007'><a href='#r44'>44</a>. <cite>Astronomy and Astro-Physics</cite>, August, 1894, p. 554.</p>
+</div>
+<div class='footnote' id='f45'>
+<p class='c007'><a href='#r45'>45</a>. “Publ. Astro. Soc. of the Pacific,” vol. iv., p. 196.</p>
+</div>
+<div class='footnote' id='f46'>
+<p class='c007'><a href='#r46'>46</a>. <cite>Monthly Notices</cite>, vol. lvi., p. 166.</p>
+</div>
+<div class='footnote' id='f47'>
+<p class='c007'><a href='#r47'>47</a>. Campbell: “Publ. A. S. P.,” vol. vi., p. 273.</p>
+</div>
+<div class='footnote' id='f48'>
+<p class='c007'><a href='#r48'>48</a>. <i>Ibid.</i>, vol. ii., p. 248.</p>
+</div>
+<div class='footnote' id='f49'>
+<p class='c007'><a href='#r49'>49</a>. <i>Ibid.</i>, vol. vi., p. 110.</p>
+</div>
+<div class='footnote' id='f50'>
+<p class='c007'><a href='#r50'>50</a>. <cite>Astronomy and Astro-Physics</cite>, October, 1894, p. 640.</p>
+</div>
+<div class='footnote' id='f51'>
+<p class='c007'><a href='#r51'>51</a>. <cite>Potsdam Publicationen</cite>, No. 30, 1893.</p>
+</div>
+<div class='footnote' id='f52'>
+<p class='c007'><a href='#r52'>52</a>. Barnard: <cite>Monthly Notices</cite>, vol. lvi., p. 55.</p>
+</div>
+<div class='footnote' id='f53'>
+<p class='c007'><a href='#r53'>53</a>. John Hopkins’ <cite>University Circular</cite>, Jan., 1895.</p>
+</div>
+<div class='footnote' id='f54'>
+<p class='c007'><a href='#r54'>54</a>. <cite>Astr. Nach.</cite>, No. 3359.</p>
+</div>
+<div class='footnote' id='f55'>
+<p class='c007'><a href='#r55'>55</a>. <cite>Monthly Notices</cite>, vol. lvi., p. 250.</p>
+</div>
+<div class='footnote' id='f56'>
+<p class='c007'><a href='#r56'>56</a>. Barnard: <cite>Astr. Journal</cite>, No. 325, 1894.</p>
+</div>
+<div class='footnote' id='f57'>
+<p class='c007'><a href='#r57'>57</a>. “Publ. A. S. P.,” vol. ii., p. 286.</p>
+</div>
+<div class='footnote' id='f58'>
+<p class='c007'><a href='#r58'>58</a>. Maunder: <cite>Knowledge</cite>, vol. xix., p. 5.</p>
+</div>
+<div class='footnote' id='f59'>
+<p class='c007'><a href='#r59'>59</a>. <cite>Monthly Notices</cite>, vol. lvi., p. 143.</p>
+</div>
+<div class='footnote' id='f60'>
+<p class='c007'><a href='#r60'>60</a>. “Scientific Proceedings, R. Dublin Society,” vol. viii., p. 398.</p>
+</div>
+<div class='footnote' id='f61'>
+<p class='c007'><a href='#r61'>61</a>. <cite>Astro.-Phys. Journal</cite>, May, 1896, p. 394; “Rapport de l’Observatoire de
+Paris,” 1895, p. 22.</p>
+</div>
+<div class='footnote' id='f62'>
+<p class='c007'><a href='#r62'>62</a>. Proctor: “Old and New Astronomy,” p. 584.</p>
+</div>
+<div class='footnote' id='f63'>
+<p class='c007'><a href='#r63'>63</a>. “The subject of slant-markings,” Mr. Stanley Williams remarks (<i>loc. cit.</i>),
+“has only just begun to be investigated.”</p>
+</div>
+<div class='footnote' id='f64'>
+<p class='c007'><a href='#r64'>64</a>. “Jupiter and his System,” by Ellen M. Clerke, p. 43.</p>
+</div>
+<div class='footnote' id='f65'>
+<p class='c007'><a href='#r65'>65</a>. <cite>Comptes Rendus</cite>, t. cxix., p. 581.</p>
+</div>
+<div class='footnote' id='f66'>
+<p class='c007'><a href='#r66'>66</a>. G. H. Darwin: <cite>Harper’s Magazine</cite>, June, 1889.</p>
+</div>
+<div class='footnote' id='f67'>
+<p class='c007'><a href='#r67'>67</a>. Barnard, <cite>Monthly Notices</cite>, vol. lvi., p. 163.</p>
+</div>
+<div class='footnote' id='f68'>
+<p class='c007'><a href='#r68'>68</a>. Lewis: <cite>Observatory</cite>, vol. xviii., p. 379.</p>
+</div>
+<div class='footnote' id='f69'>
+<p class='c007'><a href='#r69'>69</a>. <cite>Monthly Notices</cite>, vol. lii., p. 419.</p>
+</div>
+<div class='footnote' id='f70'>
+<p class='c007'><a href='#r70'>70</a>. “Abhandlungen Akad. der Wissensch.” München, Bl. xvi., p. 403.</p>
+</div>
+<div class='footnote' id='f71'>
+<p class='c007'><a href='#r71'>71</a>. <cite>Astro-Physical Journal</cite>, May, June, 1895.</p>
+</div>
+<div class='footnote' id='f72'>
+<p class='c007'><a href='#r72'>72</a>. “Old and New Astronomy,” p. 640.</p>
+</div>
+<div class='footnote' id='f73'>
+<p class='c007'><a href='#r73'>73</a>. “Phil. Trans.,” vol. lxxxii., p. 17.</p>
+</div>
+<div class='footnote' id='f74'>
+<p class='c007'><a href='#r74'>74</a>. “Publications Astr. Soc. of the Pacific,” vol. iii., p. 284.</p>
+</div>
+<div class='footnote' id='f75'>
+<p class='c007'><a href='#r75'>75</a>. <cite>Astr. Journal</cite>, No. 370.</p>
+</div>
+<div class='footnote' id='f76'>
+<p class='c007'><a href='#r76'>76</a>. Perrotin: “Vierteljahrsschrift Astr. Ges.,” Jahrg. xxiv., p. 267.</p>
+</div>
+<div class='footnote' id='f77'>
+<p class='c007'><a href='#r77'>77</a>. “Annales de l’Observatoire de Nice,” t. ii., 1887.</p>
+</div>
+<div class='footnote' id='f78'>
+<p class='c007'><a href='#r78'>78</a>. Keeler: <cite>Astr. Nach.</cite>, No. 2927.</p>
+</div>
+<div class='footnote' id='f79'>
+<p class='c007'><a href='#r79'>79</a>. Gregory: <cite>Nature</cite>, vol. xl., p. 236.</p>
+</div>
+<div class='footnote' id='f80'>
+<p class='c007'><a href='#r80'>80</a>. “General Astronomy,” p. 372.</p>
+</div>
+<div class='footnote' id='f81'>
+<p class='c007'><a href='#r81'>81</a>. <cite>Astronomical Journal</cite>, No. 342.</p>
+</div>
+<div class='footnote' id='f82'>
+<p class='c007'><a href='#r82'>82</a>. Tisserand: <cite>Astronomy and Astro-Physics</cite>, vol. xiii., p. 291 (1894).</p>
+</div>
+<div class='footnote' id='f83'>
+<p class='c007'><a href='#r83'>83</a>. <cite>Comptes Rendus</cite>, t. cvii., p. 804.</p>
+</div>
+<div class='footnote' id='f84'>
+<p class='c007'><a href='#r84'>84</a>. <cite>Astronomical Journal</cite>, No. 186.</p>
+</div>
+<div class='footnote' id='f85'>
+<p class='c007'><a href='#r85'>85</a>. “General Astronomy,” p. 372.</p>
+</div>
+<div class='footnote' id='f86'>
+<p class='c007'><a href='#r86'>86</a>. “Observations at the Cape of Good Hope,” p. 396.</p>
+</div>
+<div class='footnote' id='f87'>
+<p class='c007'><a href='#r87'>87</a>. “Monat. Correspondenz,” Bd. xxv., pp. 3–22, 1812.</p>
+</div>
+<div class='footnote' id='f88'>
+<p class='c007'><a href='#r88'>88</a>. Fessenden: <cite>Astro-Physical Journal</cite>, vol. iii., p. 40.</p>
+</div>
+<div class='footnote' id='f89'>
+<p class='c007'><a href='#r89'>89</a>. <cite>Astr. Nach.</cite>, No. 2837.</p>
+</div>
+<div class='footnote' id='f90'>
+<p class='c007'><a href='#r90'>90</a>. Guillemin: “The World of Comets,” p. 282.</p>
+</div>
+<div class='footnote' id='f91'>
+<p class='c007'><a href='#r91'>91</a>. <cite>Astr. Nach.</cite>, No. 2437.</p>
+</div>
+<div class='footnote' id='f92'>
+<p class='c007'><a href='#r92'>92</a>. <cite>Knowledge</cite>, Feb., 1896, p. 41.</p>
+</div>
+<div class='footnote' id='f93'>
+<p class='c007'><a href='#r93'>93</a>. For an account of its spectral changes, see Campbell in <cite>Astr. and Astr.-Physics</cite>,
+vol. xi., p. 698.</p>
+</div>
+<div class='footnote' id='f94'>
+<p class='c007'><a href='#r94'>94</a>. Barnard, <cite>Knowledge</cite>, vol. viii., p. 229.</p>
+</div>
+<div class='footnote' id='f95'>
+<p class='c007'><a href='#r95'>95</a>. Denning: <cite>Astronomy and Astro-Physics</cite>, vol. xii., p. 371.</p>
+</div>
+<div class='footnote' id='f96'>
+<p class='c007'><a href='#r96'>96</a>. <cite>Astroph. Journal</cite>, Jan., 1896, p. 42.</p>
+</div>
+<div class='footnote' id='f97'>
+<p class='c007'><a href='#r97'>97</a>. Ranyard: <cite>Knowledge</cite>, vol. ix., p. 159.</p>
+</div>
+<div class='footnote' id='f98'>
+<p class='c007'><a href='#r98'>98</a>. “Publications Astr. Pac. Society,” vol. vii., p. 166.</p>
+</div>
+<div class='footnote' id='f99'>
+<p class='c007'><a href='#r99'>99</a>. Hussey: <i>loc. cit.</i>, p. 171.</p>
+</div>
+<div class='footnote' id='f100'>
+<p class='c007'><a href='#r100'>100</a>. Holden: “Publ. Astr. Pac. Society,” vol. ii., p. 19. H. A. Newton: <i>Ibid.</i>,
+vol. iii., p. 91.</p>
+</div>
+<div class='footnote' id='f101'>
+<p class='c007'><a href='#r101'>101</a>. “Report Bri. Ass.,” 1891, p. 805.</p>
+</div>
+<div class='footnote' id='f102'>
+<p class='c007'><a href='#r102'>102</a>. S. Meunier: “Encycl. Chimique,” t. ii., p. 461.</p>
+</div>
+<div class='footnote' id='f103'>
+<p class='c007'><a href='#r103'>103</a>. Young: “Gen. Astr.,” p. 435.</p>
+</div>
+<div class='footnote' id='f104'>
+<p class='c007'><a href='#r104'>104</a>. Cornish: <cite>Knowledge</cite>, vol. vi., p. 163.</p>
+</div>
+<div class='footnote' id='f105'>
+<p class='c007'><a href='#r105'>105</a>. <cite>Journal Brit. Astr. Ass.</cite>, vol. vi., p. 432.</p>
+</div>
+<div class='footnote' id='f106'>
+<p class='c007'><a href='#r106'>106</a>. H. A. Newton: “Proc. Amer. Phil. Society,” vol. xxxii.</p>
+</div>
+<div class='footnote' id='f107'>
+<p class='c007'><a href='#r107'>107</a>. Quoted by Sir F. Palgrave: “Phil. Trans.,” vol. cxxx., p. 175.</p>
+</div>
+<div class='footnote' id='f108'>
+<p class='c007'><a href='#r108'>108</a>. <cite>Observatory</cite>, April, 1895.</p>
+</div>
+<div class='footnote' id='f109'>
+<p class='c007'><a href='#r109'>109</a>. <cite>Observatory</cite>, Jan., 1896.</p>
+</div>
+<div class='footnote' id='f110'>
+<p class='c007'><a href='#r110'>110</a>. It has been recently seen again in America.</p>
+</div>
+<div class='footnote' id='f111'>
+<p class='c007'><a href='#r111'>111</a>. <cite>Journal of the British Astronomical Association</cite>, March, 1891.</p>
+</div>
+<div class='footnote' id='f112'>
+<p class='c007'><a href='#r112'>112</a>. <cite>Nature</cite>, Feb. 13, 1896.</p>
+</div>
+<div class='footnote' id='f113'>
+<p class='c007'><a href='#r113'>113</a>. “Planetary and Stellar Studies,” p. 257.</p>
+</div>
+<div class='footnote' id='f114'>
+<p class='c007'><a href='#r114'>114</a>. See Chapter V.</p>
+</div>
+<div class='footnote' id='f115'>
+<p class='c007'><a href='#r115'>115</a>. <cite>Comptes Rendus</cite>, March 30, 1896.</p>
+</div>
+<div class='footnote' id='f116'>
+<p class='c007'><a href='#r116'>116</a>. <cite>Nature</cite>, April 30, 1896.</p>
+</div>
+<div class='footnote' id='f117'>
+<p class='c007'><a href='#r117'>117</a>. “Cape Observations,” p. 34.</p>
+</div>
+<div class='footnote' id='f118'>
+<p class='c007'><a href='#r118'>118</a>. <cite>Journal of the British Astronomical Association</cite>, vol. iv., No. 11, p. 21.</p>
+</div>
+<div class='footnote' id='f119'>
+<p class='c007'><a href='#r119'>119</a>. <cite>Journal of the British Astronomical Association</cite>, vol. vi., No. 6, p. 312.</p>
+</div>
+<div class='footnote' id='f120'>
+<p class='c007'><a href='#r120'>120</a>. Recent observations show that the total variation is 2·71 magnitudes—the
+largest variation known for an Algol star.</p>
+</div>
+<div class='footnote' id='f121'>
+<p class='c007'><a href='#r121'>121</a>. “Cosmos,” Bohn’s edition, vol. iii., p. 205.</p>
+</div>
+<div class='footnote' id='f122'>
+<p class='c007'><a href='#r122'>122</a>. It was, however, asserted by Herlicius that he had seen it on Sept. 27.</p>
+</div>
+<div class='footnote' id='f123'>
+<p class='c007'><a href='#r123'>123</a>. The spectrum, however, seems to have since become continuous.</p>
+</div>
+<div class='footnote' id='f124'>
+<p class='c007'><a href='#r124'>124</a>. <cite>Astronomical Journal</cite>, No. 100.</p>
+</div>
+<div class='footnote' id='f125'>
+<p class='c007'><a href='#r125'>125</a>. <cite>Journal of the British Astronomical Association</cite>, March, 1892.</p>
+</div>
+<div class='footnote' id='f126'>
+<p class='c007'><a href='#r126'>126</a>. <cite>Journal of the British Astronomical Association</cite>, February, 1895, vol. v. No. 4.</p>
+</div>
+<div class='footnote' id='f127'>
+<p class='c007'><a href='#r127'>127</a>. <i>Ibid.</i>, April, 1895, p. 328.</p>
+</div>
+<div class='footnote' id='f128'>
+<p class='c007'><a href='#r128'>128</a>. <cite>Journal of the British Astronomical Association</cite>, February, 1892.</p>
+</div>
+<div class='footnote' id='f129'>
+<p class='c007'><a href='#r129'>129</a>. <cite>The Observatory</cite>, December, 1895.</p>
+</div>
+<div class='footnote' id='f130'>
+<p class='c007'><a href='#r130'>130</a>. “Planetary and Stellar Studies,” p. 188.</p>
+</div>
+<div class='footnote' id='f131'>
+<p class='c007'><a href='#r131'>131</a>. <cite>Nature</cite>, September 6, 1894.</p>
+</div>
+<div class='footnote' id='f132'>
+<p class='c007'><a href='#r132'>132</a>. <cite>Nature</cite>, June 4, 1896.</p>
+</div>
+<div class='footnote' id='f133'>
+<p class='c007'><a href='#r133'>133</a>. “Cosmos,” vol. iii., Bohn’s edition, p. 192.</p>
+</div>
+<div class='footnote' id='f134'>
+<p class='c007'><a href='#r134'>134</a>. Humboldt’s “Cosmos,” Bohn’s edition, vol. iv., pp. 327, 328.</p>
+</div>
+<div class='footnote' id='f135'>
+<p class='c007'><a href='#r135'>135</a>. <cite>Monthly Notices</cite>, Royal Astronomical Society, June, 1888.</p>
+</div>
+<div class='footnote' id='f136'>
+<p class='c007'><a href='#r136'>136</a>. “Old and New Astronomy,” p. 794.</p>
+</div>
+<div class='footnote' id='f137'>
+<p class='c007'><a href='#r137'>137</a>. <cite>Nature</cite>, June 4, 1896.</p>
+</div>
+<div class='footnote' id='f138'>
+<p class='c007'><a href='#r138'>138</a>. <cite>Nature</cite>, September, 1894.</p>
+</div>
+<div class='footnote' id='f139'>
+<p class='c007'><a href='#r139'>139</a>. <i>Ibid.</i>, October 4, 1894.</p>
+</div>
+<div class='footnote' id='f140'>
+<p class='c007'><a href='#r140'>140</a>. “Outlines of Astronomy,” tenth edition, p. 657.</p>
+</div>
+<div class='footnote' id='f141'>
+<p class='c007'><a href='#r141'>141</a>. <cite>Nature</cite>, November, 21, 1895.</p>
+</div>
+<div class='footnote' id='f142'>
+<p class='c007'><a href='#r142'>142</a>. <cite>Nature</cite>, January 16, 1896.</p>
+</div>
+<div class='footnote' id='f143'>
+<p class='c007'><a href='#r143'>143</a>. <cite>Nature</cite>, August 9, 1888.</p>
+</div>
+<div class='footnote' id='f144'>
+<p class='c007'><a href='#r144'>144</a>. Humboldt’s “Cosmos,” Bohn’s edition, vol, iii., p. 143.</p>
+</div>
+<div class='footnote' id='f145'>
+<p class='c007'><a href='#r145'>145</a>. See <cite>Knowledge</cite>, June, 1895.</p>
+</div>
+<div class='footnote' id='f146'>
+<p class='c007'><a href='#r146'>146</a>. “The Universe and the Coming Transits,” p. 200.</p>
+</div>
+<div class='footnote' id='f147'>
+<p class='c007'><a href='#r147'>147</a>. <cite>Journal of the British Astronomical Association</cite>, May, 1895, p. 383.</p>
+</div>
+<div class='footnote' id='f148'>
+<p class='c007'><a href='#r148'>148</a>. <cite>Knowledge</cite>, May, 1896.</p>
+</div>
+<div class='footnote' id='f149'>
+<p class='c007'><a href='#r149'>149</a>. <cite>Knowledge</cite>, July, 1891.</p>
+</div>
+<div class='footnote' id='f150'>
+<p class='c007'><a href='#r150'>150</a>. <cite>Knowledge</cite>, January, 1894, p. 17.</p>
+</div>
+<div class='footnote' id='f151'>
+<p class='c007'><a href='#r151'>151</a>. <cite>Journal of the British Astronomical Association</cite>, April, 1895, p. 304.</p>
+</div>
+<div class='footnote' id='f152'>
+<p class='c007'><a href='#r152'>152</a>. The Italics are Herschel’s.</p>
+</div>
+<div class='footnote' id='f153'>
+<p class='c007'><a href='#r153'>153</a>. A full discussion of Struve’s views will be found in Chapter XVI. of “The
+Visible Universe,” by the present writer.</p>
+</div>
+<div class='footnote' id='f154'>
+<p class='c007'><a href='#r154'>154</a>. “The Meteoritic Theory,” pp. 380, 381.</p>
+</div>
+
+<div class='pbb'>
+ <hr class='pb c005'>
+</div>
+
+<div class='chapter ph2'>
+
+<div class='nf-center-c0'>
+<div class='nf-center c001'>
+    <div>D. APPLETON AND COMPANY’S PUBLICATIONS.</div>
+  </div>
+</div>
+
+</div>
+
+<p class='c035'><cite>THE SUN.</cite> By <span class='sc'>C. A. Young</span>, Ph. D., LL. D., Professor
+of Astronomy in Princeton University. New and revised edition,
+with numerous Illustrations. 12mo. Cloth, $2.00.</p>
+
+<p class='c036'>“In this book we see a master’s hand. Professor Young has no superiors, if he
+has rivals, among astronomers in this country.... ‘The Sun’ is a book of facts and
+achievements, and not a discussion of theories, and it will be read and appreciated by
+all scientific students, and not by them alone. Being written in untechnical language,
+it is equally adapted to a large class of educated readers not engaged in scientific pursuits.”—<cite>Journal
+of Education, Boston.</cite></p>
+
+<p class='c036'>“Professor Young’s work is essentially a record of facts and achievements, rather
+than of theories and attempts at the interpretation of mysteries; yet the great questions
+still remaining to be answered are of course discussed, and in a masterly manner.”—<cite>Philadelphia
+Evening Bulletin.</cite></p>
+
+<p class='c036'>“It is one of the best books of popular science ever written, and deserves a host of
+readers.”—<cite>The Dial, Chicago.</cite></p>
+
+<p class='c036'>“You feel throughout that a master is leading you amid the intricacies and mazes
+of one of the most absorbing of studies.... Many a one whose views are hazy and
+dim will find here just that enlightenment, without an overburdened technicality, that
+will prove most useful.”—<cite>The Interior.</cite></p>
+
+<p class='c035'><cite>THE STORY OF THE SUN.</cite> By Sir <span class='sc'>Robert S. Ball</span>,
+F. R. S., author of “An Atlas of Astronomy,” “The Cause of an
+Ice Age,” etc. 8vo. Cloth, $5.00.</p>
+
+<p class='c036'>“Sir Robert Ball has the happy gift of making abstruse problems intelligible to the
+‘wayfaring man’ by the aid of simple language and a few diagrams. Science moves
+so fast that there was room for a volume which should enlighten the general reader on
+the present state of knowledge about solar phenomena, and that place the present
+treatise admirably fills.”—<cite>London Chronicle.</cite></p>
+
+<p class='c036'>“As a specimen of the publisher’s art it is superb. It is printed on paper which
+entices the reader to make marginal notes of reference to other books in his library,
+the type is large, the binding is excellent, and the volume is neither too large nor
+too small to handle without fatigue.”—<cite>New York Herald.</cite></p>
+
+<p class='c035'><cite>AN ATLAS OF ASTRONOMY.</cite> By Sir <span class='sc'>Robert S.
+Ball</span>, F. R. S., Professor of Astronomy and Geometry at the
+University of Cambridge; Author of “Starland,” “The Cause
+of an Ice Age,” etc. With 72 Plates, Explanatory Text, and
+Complete Index. Small 4to. Cloth, $4.00.</p>
+
+<p class='c036'>“The high reputation of Sir Robert Ball as a writer on astronomy at once popular
+and scientific is in itself more than sufficient recommendation of his newly published
+‘Atlas of Astronomy.’ The plates are clear and well arranged, and those of
+them which represent the more striking aspects of the more important heavenly
+bodies are very beautifully executed. The introduction is written with Sir Robert
+Ball’s well-known lucidity and simplicity of exposition, and altogether the Atlas is
+admirably adapted to meet the needs and smooth the difficulties of young and inexperienced
+students of astronomy, as well as materially to assist the researches of those
+are more advanced.”—<cite>London Times.</cite></p>
+
+<p class='c035'><cite>OUTLINES OF ASTRONOMY.</cite> By Sir <span class='sc'>J. J. W. Herschel</span>.
+With Plates and Woodcuts. Eleventh edition. 8vo.
+Cloth, $4.00.</p>
+
+<p class='c037'><cite>ORIGIN OF THE STARS, AND THE CAUSES OF
+THEIR MOTIONS AND THEIR LIGHT.</cite> By <span class='sc'>Jacob
+Ennis</span>. 12mo. Cloth, $2.00.</p>
+
+<p class='c037'><cite>SPECTRUM ANALYSIS, IN ITS APPLICATION TO
+TERRESTRIAL SUBSTANCES, and the Physical Constitution
+of the Heavenly Bodies.</cite> Familiarly explained by Dr. H.
+Schellen, Director der Realschule I. O. Cologne. Translated
+from the second enlarged and revised German edition by Jane
+and Caroline Lassell. Edited, with Notes, by William Huggins,
+LL. D. With numerous Woodcuts, Colored Plates, and
+Portraits; also, Angström’s and Kirchhoff’s Maps. 8vo.
+Cloth, $6.00.</p>
+
+<p class='c036'>“Certainly, as regards mere knowledge, the ‘Spectrum Analysis’ has let us into
+many secrets of the physical universe which Newton and Laplace would have declared
+impossible for man’s intellect to attain. The science is still in its infancy, but it is
+prosecuted by some of the ablest, most patient, and most enthusiastic observers, and
+some of the keenest thinkers at present existing on our little, insignificant physical
+globe.”—<cite>Boston Globe.</cite></p>
+
+<p class='c035'><cite>STUDIES IN SPECTRUM ANALYSIS.</cite> By <span class='sc'>J. Norman
+Lockyer</span>, F.R.S., Correspondent of the Institute of France,
+etc. With 60 Illustrations. 12mo. Cloth, $2.50.</p>
+
+<p class='c036'>“The study of spectrum analysis is one fraught with a peculiar fascination, and
+some of the author’s experiments are exceedingly picturesque in their results. They
+are so lucidly described, too, that the reader keeps on, from page to page, never flagging
+in interest in the matter before him, nor putting down the book until the last
+page is reached.”—<cite>New York Evening Express.</cite></p>
+
+<p class='c035'><cite>ASTRONOMY AND GEOLOGY COMPARED.</cite> By
+Lord <span class='sc'>Ormathwaite</span>. 18mo. Tinted paper. Cloth, $1.00.</p>
+
+<p class='c037'><cite>THE STORY OF THE STARS.</cite> By <span class='sc'>G. F. Chambers</span>,
+F. R. A. S., author of “Handbook of Descriptive and Practical
+Astronomy,” etc. With 24 Illustrations. Library of Useful
+Stories. 16mo. Cloth, 40 cents.</p>
+
+<p class='c036'>“One can here get a clear conception of the relative condition of the stars and
+constellations, and of the existent universe so far as it is disclosed to view. The
+author presents his wonderful and at times bewildering facts in a bright and cheery
+spirit that makes the book doubly attractive.”—<cite>Boston Home Journal.</cite></p>
+
+<div class='nf-center-c0'>
+<div class='nf-center c002'>
+    <div>RICHARD A. PROCTOR’S WORKS.</div>
+  </div>
+</div>
+
+<p class='c035'><cite>OTHER WORLDS THAN OURS. The Plurality of
+Worlds, Studied under the Light of Recent Scientific Researches.</cite>
+By <span class='sc'>Richard Anthony Proctor</span>. With Illustrations, some
+colored. 12mo. Cloth, $1.75.</p>
+
+<p class='c036'><span class='sc'>Contents.</span>—Introduction. What the Earth teaches us. What we learn from the
+Sun. The Inferior Planets. Mars, the Miniature of our Earth. Jupiter, the Giant
+of the Solar System. Saturn, the Ringed World. Uranus and Neptune, the Arctic
+Planets. The Moon and other Satellites. Meteors and Comets: Their Office in the
+Solar System. Other Suns than Ours. Of Minor Stars, and of the Distribution of
+Stars in Space. The Nebulæ: Are they External Galaxies? Supervision and Control.</p>
+
+<p class='c035'><cite>OUR PLACE AMONG INFINITIES.</cite> A Series of
+Essays contrasting our Little Abode in Space and Time with the
+Infinities around us. To which are added Essays on the Jewish
+Sabbath and Astrology. 12mo. Cloth, $1.75.</p>
+
+<p class='c036'><span class='sc'>Contents.</span>—Past and Future of the Earth. Seeming Wastes in Nature. New
+Theory of Life in other Worlds. A Missing Comet. The Lost Comet and its Meteor
+Train. Jupiter. Saturn and its System. A Giant Sun. The Star Depths. Star
+Gauging. Saturn and the Sabbath of the Jews. Thoughts on Astrology.</p>
+
+<p class='c035'><cite>THE EXPANSE OF HEAVEN.</cite> A Series of Essays on
+the Wonders of the Firmament. 12mo. Cloth, $2.00.</p>
+
+<p class='c036'><span class='sc'>Contents.</span>—A Dream that was not all a Dream. The Sun. The Queen of Night.
+The Evening Star. The Ruddy Planet. Life in the Ruddy Planet. The Prince of
+Planets. Jupiter’s Family of Moons. The Ring-Girdled Planet. Newton and the
+Law of the Universe. The Discovery of Two Giant Planets. The Lost Comet.
+Visitants from the Star Depths. Whence come the Comets? The Comet Families of
+the Giant Planets. The Earth’s Journey through Showers. How the Planets Grew.
+Our Daily Light. The Flight of Light. A Cluster of Suns. Worlds ruled by Colored
+Suns. The King of Suns. Four Orders of Suns. The Depths of Space. Charting
+the Star Depths. The Star Depths Astir with Life. The Drifting Stars. The Milky
+Way.</p>
+
+<p class='c035'><cite>THE MOON: Her Motions, Aspect, Scenery, and Physical
+Conditions.</cite> With Three Lunar Photographs, Map, and many
+Plates, Charts, etc. 12mo. Cloth, $2.00.</p>
+
+<p class='c036'><span class='sc'>Contents.</span>—The Moon’s Distance, Size, and Mass. The Moon’s Motions. The
+Moon’s Changes of Aspect, Rotation, Libration, etc. Study of the Moon’s Surface.
+Lunar Celestial Phenomena. Condition of the Moon’s Surface. Index to the Map
+of the Moon.</p>
+
+<p class='c035'><cite>LIGHT SCIENCE FOR LEISURE HOURS.</cite> A Series
+of Familiar Essays on Scientific Subjects, Natural Phenomena,
+etc. 12mo. Cloth, $1.75.</p>
+
+<p class='c037'><cite>POPULAR ASTRONOMY.</cite> A General Description of
+the Heavens. By <span class='sc'>Camille Flammarion</span>. Translated from
+the French by J. Ellard Gore. With 3 Plates and 288 Illustrations.
+8vo. Cloth, $4.50.</p>
+
+<p class='c036'>“The fullest and most elaborate compendium of popular knowledge of astronomy....
+The book might reasonably be pronounced the most desirable of its kind.”—<cite>New
+York Sun.</cite></p>
+
+<p class='c036'>“M. Flammarion has produced a work that charms while it interests. He has
+classified astronomy so perfectly that any person of ordinary intelligence may learn
+from his book practically all the men in the observatories know.”—<cite>New York Times.</cite></p>
+
+<p class='c036'>“Flammarion talks, and his conversation is free from those technical expressions
+which make the obscure style more obscure. He treats the most abstruse problems
+in such a fashion that you see through them more clearly than you ever thought it
+possible to do without years of study.”—<cite>New York Herald.</cite></p>
+
+<p class='c036'>“While the translator has done excellent work, he has also added largely to the
+value of the book by his carefully prepared notes, in which he brings every astronomical
+theme down to date.”—<cite>Chicago Inter-Ocean.</cite></p>
+
+<p class='c036'>“The book is one of extreme interest, and to our mind far surpasses in fascination
+any novel that was ever written.”—<cite>London Literary World.</cite></p>
+
+<p class='c035'><cite>ASTRONOMY WITH AN OPERA-GLASS.</cite> A Popular
+Introduction to the Study of the Starry Heavens with the
+Simplest of Optical Instruments. By <span class='sc'>Garrett P. Serviss</span>.
+8vo. Cloth, $1.50.</p>
+
+<p class='c036'>“The glimpses he allows to be seen of far-stretching vistas opening out on every
+side of his modest course of observation help to fix the attention of the negligent, and
+lighten the toil of the painstaking student.... Mr. Serviss writes with freshness
+and vivacity.”—<cite>London Saturday Review.</cite></p>
+
+<p class='c036'>“By its aid thousands of people who have resigned themselves to the ignorance
+in which they were left at school, by our wretched system of teaching by the book
+only, will thank Mr. Serviss for the suggestions he has so well carried out”—<cite>New
+York Times.</cite></p>
+
+<p class='c036'>“We are glad to welcome this popular introduction to the study of the heavens....
+There could hardly be a more pleasant road to astronomical knowledge than it
+affords.... A child may understand the text, which reads more like a collection of
+anecdotes than anything else, but this does not mar its scientific value.”—<cite>Nature.</cite></p>
+
+<p class='c036'>“Mr. Garrett P. Serviss’s book, ‘Astronomy with an Opera-Glass,’ offers us an
+admirable handbook and guide in the cultivation of this noble æsthetic discipline (the
+study of the stars).”—<cite>New York Home Journal.</cite></p>
+
+<p class='c036'>“The book should belong to every family library.”—<cite>Boston Home Journal.</cite></p>
+
+<div class='nf-center-c0'>
+<div class='nf-center c002'>
+    <div>WORKS BY ARABELLA B. BUCKLEY (MRS. FISHER).</div>
+  </div>
+</div>
+
+<p class='c038'>“Mrs. Fisher has a positive genius for presenting the science of living things in shapes that
+boys and girls can understand.”—<cite>Philadelphia Ledger.</cite></p>
+
+<p class='c036'>“Mrs. Fisher’s apprenticeship as secretary to Sir Charles Lyell has resulted in a life-long
+devotion to the natural sciences, the fruits of which have been a series of charming books valuable
+to the young, and indeed to grown-ups.”—<cite>The Critic.</cite></p>
+
+<p class='c035'><cite>THE FAIRY-LAND OF SCIENCE.</cite> With 74 Illustrations.
+Cloth, $1.50.</p>
+
+<p class='c036'><span class='sc'>Contents.</span>—The Fairy-Land of Science: How to Enter it; How to Use it; How to Enjoy
+it.—Sunbeams, and the Work they do.—The Aërial Ocean in which we Live.—A Drop of Water
+on its Travels.—The Two Great Sculptors, Water and Ice.—The Voices of Nature, and How we
+Hear them.—The Life of a Primrose.—The History of a Piece of Coal.—Bees in the Hive.—Bees
+and Flowers.</p>
+
+<p class='c035'><cite>THROUGH MAGIC GLASSES, and other Lectures.</cite> A Sequel
+to “The Fairy-Land of Science.” Cloth, $1.50.</p>
+
+<p class='c036'><span class='sc'>Contents.</span>—The Magician’s Chamber by Moonlight.—Magic Glasses and How to Use
+Them.—Fairy Rings and How They are Made.—The Life History of Lichens and Mosses.—The
+History of a Lava-Stream.—An Hour with the Sun.—An Evening with the Stars.—Little
+Beings from a Miniature Ocean.—The Dartmoor Ponies.—The Magician’s Dream of Ancient Days.</p>
+
+<p class='c035'><cite>LIFE AND HER CHILDREN: Glimpses of Animal Life,
+from the Amœba to the Insects.</cite> With over 100 Illustrations. Cloth, $1.50.</p>
+
+<p class='c036'><span class='sc'>Contents.</span>—Life’s Simplest Children; How they Live, and Move, and Build.—How Sponges
+Live.—The Lasso-Throwers of the Ponds and Oceans.—How Star-Fish Walk and Sea-Urchins
+Grow.—The Mantle-covered Animals, and How they Live with Heads and without them.—The
+Outcasts of Animal Life, and the Elastic-ringed Animals by Sea and by Land.—The Mailed
+Warriors of the Sea, with Ringed Bodies and Jointed Feet.—The Snare-Weavers and their
+Hunting Relations.—Insect Suckers and Biters which Change their Coats but not their Bodies.—Insect
+Sippers and Gnawers which Remodel their Bodies within their Coats.—Intelligent Insects
+with Helpless Children, as illustrated by the Ants.</p>
+
+<p class='c035'><cite>WINNERS IN LIFE’S RACE; or, The Great Backboned
+Family.</cite> With numerous Illustrations. Cloth, $1.50.</p>
+
+<p class='c036'><span class='sc'>Contents.</span>—The Threshold of Backboned Life.—How the Quaint Old Fishes of Ancient
+Times have Lived on into Our Day.—The Bony Fish, and How they have Spread over Sea, and
+Lake, and River.—How the Backboned Animals pass from Water-breathing to Air-breathing, and
+find their Way out upon the Land.—The Cold-blooded Air-breathers of the Globe in Times both
+Past and Present.—The Feathered Conquerors of the Air.—Their Wanderings over Sea and
+Marsh, Desert and Plain.—From Running to Flying, from Mound-laying to Nest-building, from
+Cry to Song.—The Mammalia or Milk-Givers, the simplest Suckling Mother, the active Pouchbearers,
+and the Imperfect-toothed Animals.—From the Lower and Small Milk-Givers which find
+Safety in Concealment, to the Intelligent Apes and Monkeys.—The Large Milk-givers which have
+conquered the World by Strength and Intelligence.—How the Backboned Animals have returned
+to the Water, and Large Milk-givers Imitate the Fish.—A Bird’s-eye View of the Rise and
+Progress of Backboned Life.</p>
+
+<p class='c035'><cite>A SHORT HISTORY OF NATURAL SCIENCE;
+and of the Progress of Discovery from the Time of the Greeks to the
+Present Time.</cite> New edition, revised and rearranged. With 77 Illustrations.
+Cloth, $2.00.</p>
+
+<p class='c036'>“The work, though mainly intended for children and young persons, may be most
+advantageously read by many persons of riper age, and may serve to implant in their
+minds a fuller and clearer conception of ‘the promises, the achievements, and claims
+of science.’”—<cite>Journal of Science.</cite></p>
+
+<p class='c035'><cite>PRINCIPLES OF GEOLOGY; or, The Modern Changes
+of the Earth and its Inhabitants, considered as illustrative of
+Geology.</cite> By Sir <span class='sc'>Charles Lyell</span>, Bart. Illustrated with Maps,
+Plates, and Woodcuts. Two vols., royal 8vo. Cloth, $8.00.</p>
+
+<p class='c036'>The “Principles of Geology” may be looked upon with pride not only as a representative
+of English science, but as without a rival of its kind anywhere. Growing in
+fullness and accuracy with the growth of experience and observation in every region of
+the world, the work has incorporated with itself each established discovery, and has
+been modified by every hypothesis of value which has been brought to bear upon, or
+been evolved from, the most recent body of facts.</p>
+
+<p class='c035'><cite>THE AUTOBIOGRAPHY OF THE EARTH.</cite> A
+Popular Account of Geological History. By Rev. <span class='sc'>H. N.
+Hutchinson</span>, B. A., F. G. S. With 27 Illustrations. Crown
+8vo. Cloth, $1.50.</p>
+
+<p class='c036'>“A scientific work, divested of technicalities, and put into a bright, narrative form
+which can not but attract even the most general reader.”—<cite>Boston Transcript.</cite></p>
+
+<p class='c036'>“It is hardly conceivable that the rich results of the science of geology can be so
+treated as to prove uninteresting to thinking people, be they young or old. When,
+therefore, we say that Mr. Hutchinson’s book is extremely interesting, no more might
+be implied than that the author has skillfully used the vast materials at his hand. But
+Mr. Hutchinson has successfully carried out a difficult design on an admirable plan,
+and has adhered to that plan throughout. His sketch of historical geology has a
+genuine continuity.”—<cite>Saturday Review.</cite></p>
+
+<p class='c035'><cite>A REPRINT OF ANNUAL REPORTS AND OTHER
+PAPERS OF THE GEOLOGY OF THE VIRGINIAS.</cite>
+By the late <span class='sc'>William Barton Rogers</span>, LL. D., etc., Director
+of the Geological Survey of Virginia from 1835 to 1841. With
+numerous Maps, Colored Charts, etc. 12mo. Cloth, $5.00.</p>
+
+<p class='c035'><cite>NATURAL RESOURCES OF THE UNITED
+STATES.</cite> By <span class='sc'>Jacob Harris Patton</span>, M. A., Ph. D. 8vo,
+523 pages. Cloth, $3.00.</p>
+
+<p class='c036'>“This portly octavo of over 500 pages is an encyclopedic directory to its subject,
+and a work of comprehensive scope, exhaustive research, scientific spirit, and good
+literary form.... Capitalists, investors, railroad projectors, land speculators, and all
+who need to know the distribution of land values, will find a vast amount of information
+in this well-arranged work, the contents of which, it is safe to say, could not be
+found assembled in similar compass elsewhere.”—<cite>Boston Literary World.</cite></p>
+
+<p class='c036'>“As interesting to read as it is valuable to consult. By the employment of fine
+white paper and large, clear type, the publishers have made it an elegant specimen of
+the printer’s art.”—<cite>New York Sun.</cite></p>
+
+<div class='nf-center-c0'>
+<div class='nf-center c002'>
+    <div>BOOKS BY PROF. G. FREDERICK WRIGHT.</div>
+  </div>
+</div>
+
+<p class='c035'><cite>GREENLAND ICEFIELDS, AND LIFE IN THE
+NORTH ATLANTIC</cite>. With a New Discussion of the Causes
+of the Ice Age. By <span class='sc'>G. Frederick Wright</span>, D. D., LL. D.,
+F. G. S. A., author of “The Ice Age in North America,” “Man
+and the Glacial Period,” etc., and <span class='sc'>Warren Upham</span>, A. M.,
+F. G. S. A., late of the Geological Surveys of New Hampshire,
+Minnesota, and the United States. With numerous Maps and
+Illustrations. 12mo. Cloth, $2.00.</p>
+
+<p class='c036'>The immediate impulse to the preparation of this volume arose in connection with
+a trip to Greenland by Professor Wright in the summer of 1894 on the steamer
+Miranda. The work aims to give within moderate limits a comprehensive view of the
+scenery, the glacial phenomena, the natural history, the people, and the explorations
+of Greenland. The photographs are all original, and the maps have been prepared
+to show the latest state of knowledge concerning the region. The volume treats of
+the ice of the Labrador current, the coast of Labrador, Spitzbergen ice in Davis Strait,
+the Greenland Eskimos, Europeans in Greenland, explorations of the inland ice, the
+plants and animals of Greenland, changes of level since the advent of the Glacial
+period, and includes a summary of the bearing of the facts upon glacial theories.
+The work is of both popular and scientific interest.</p>
+
+<p class='c035'><cite>THE ICE AGE IN NORTH AMERICA, and its
+Bearings upon the Antiquity of Man.</cite> With an Appendix on
+“The Probable Cause of Glaciation,” by <span class='sc'>Warren Upham</span>,
+F. G. S. A., Assistant on the Geological Surveys of New Hampshire,
+Minnesota, and the United States. New and enlarged
+edition. With 150 Maps and Illustrations. 8vo, 625 pages, and
+Index. Cloth, $5.00.</p>
+
+<p class='c036'>“The author has seen with his own eyes the most important phenomena of the
+Ice age on this continent from Maine to Alaska. In the work itself, elementary
+description is combined with a broad, scientific, and philosophic method, without
+abandoning for a moment the purely scientific character. Professor Wright has contrived
+to give the whole a philosophical direction which lends interest and inspiration
+to it, and which in the chapters on Man and the Glacial Period rises to something like
+dramatic intensity.”—<cite>The Independent.</cite></p>
+
+<p class='c035'><cite>MAN AND THE GLACIAL PERIOD.</cite> International
+Scientific Series. With numerous Illustrations. 12mo. Cloth,
+$1.75.</p>
+
+<p class='c036'>“The earlier chapters describing glacial action, and the traces of it in North
+America—especially the defining of its limits, such as the terminal moraine of the
+great movement itself—are of great interest and value. The maps and diagrams
+are of much assistance in enabling the reader to grasp the vast extent of the movement.”—<cite>London
+Spectator.</cite></p>
+
+<p class='c035'><cite>PIONEERS OF SCIENCE IN AMERICA.</cite> Sketches
+of their Lives and Scientific Work. Edited and revised by
+<span class='sc'>William Jay Youmans</span>, M. D. With Portraits. 8vo. Cloth,
+$4.00.</p>
+
+<p class='c036'>Impelled solely by an enthusiastic love of Nature, and neither asking nor receiving
+outside aid, these early workers opened the way and initiated the movement through
+which American science has reached its present commanding position. This book
+gives some account of these men, their early struggles, their scientific labors, and,
+whenever possible, something of their personal characteristics. This information,
+often very difficult to obtain, has been collected from a great variety of sources, with
+the utmost care to secure accuracy. It is presented in a series of sketches, some fifty
+in all, each with a single exception accompanied with a well-authenticated portrait.</p>
+
+<p class='c036'>“Fills a place that needed filling, and is likely to be widely read.”—<cite>New York Sun.</cite></p>
+
+<p class='c036'>“It is certainly a useful and convenient volume, and readable too, if we judge correctly
+of the degree of accuracy of the whole by critical examination of those cases in
+which our own knowledge enables us to form an opinion.... In general, it seems to
+us that the handy volume is specially to be commended for setting in just historical
+perspective many of the earlier scientists who are neither very generally nor very well
+known.”—<cite>New York Evening Post.</cite></p>
+
+<p class='c036'>“A wonderfully interesting volume. Many a young man will find it fascinating.
+The compilation of the book is a work well done, well worth the doing.”—<cite>Philadelphia
+Press.</cite></p>
+
+<p class='c036'>“One of the most valuable books which we have received.”—<cite>Boston Advertiser.</cite></p>
+
+<p class='c036'>“A book of no little educational value.... An extremely valuable work of reference.”—<cite>Boston
+Beacon.</cite></p>
+
+<p class='c036'>“A valuable handbook for those whose work runs on these same lines, and is likely
+to prove of lasting interest to those for whom ‘<i>les documents humain</i>’ are second only
+to history in importance—nay, are a vital part of history.”—<cite>Boston Transcript.</cite></p>
+
+<p class='c036'>“A biographical history of science in America, noteworthy for its completeness and
+scope.... All of the sketches are excellently prepared and unusually interesting.”—<cite>Chicago
+Record.</cite></p>
+
+<p class='c036'>“One of the most valuable contributions to American literature recently made....
+The pleasing style in which these sketches are written, the plans taken to secure accuracy,
+and the information conveyed, combine to give them great value and interest.
+No better or more inspiring reading could be placed in the hands of an intelligent and
+aspiring young man.”—<cite>New York Christian Work.</cite></p>
+
+<p class='c036'>“A book whose interest and value are not for to-day or to-morrow, but for indefinite
+time.”—<cite>Rochester Herald.</cite></p>
+
+<p class='c036'>“It is difficult to imagine a reader of ordinary intelligence who would not be entertained
+by the book.... Conciseness, exactness, urbanity of tone, and interestingness
+are the four qualities which chiefly impress the reader of these sketches.”—<cite>Buffalo
+Express.</cite></p>
+
+<p class='c036'>“Full of interesting and valuable matter.”—<cite>The Churchman.</cite></p>
+
+<div class='nf-center-c0'>
+<div class='nf-center c002'>
+    <div>THE ANTHROPOLOGICAL SERIES.</div>
+    <div class='c002'>NOW READY.</div>
+  </div>
+</div>
+
+<p class='c035'><cite>THE BEGINNINGS OF ART.</cite> By <span class='sc'>Ernst Grosse</span>,
+Professor of Philosophy in the University of Freiburg. A new
+volume in the Anthropological Series, edited by Professor Frederick
+Starr. Illustrated. 12mo. Cloth, $1.75.</p>
+
+<p class='c036'>“This book can not fail to interest students of every branch of art, while the general
+reader who will dare to take hold of it will have his mind broadened and enriched
+beyond what he would conceive a work of many times its dimensions might effect.”—<cite>Brooklyn
+Eagle.</cite></p>
+
+<p class='c036'>“The volume is clearly written, and should prove a popular exposition of a deeply
+interesting theme.”—<cite>Philadelphia Public Ledger.</cite></p>
+
+<p class='c035'><cite>WOMAN’S SHARE IN PRIMITIVE CULTURE.</cite>
+By <span class='sc'>Otis Tufton Mason</span>, A. M., Curator of the Department of
+Ethnology in the United States National Museum. With numerous
+Illustrations. 12mo. Cloth, $1.75.</p>
+
+<p class='c036'>“A most interesting <i>résumé</i> of the revelations which science has made concerning
+the habits of human beings in primitive times, and especially as to the place, the duties,
+and the customs of women.”—<cite>Philadelphia Inquirer.</cite></p>
+
+<p class='c035'><cite>THE PYGMIES.</cite> By <span class='sc'>A. de Quatrefages</span>, late Professor
+of Anthropology at the Museum of Natural History, Paris.
+With numerous Illustrations. 12mo. Cloth, $1.75.</p>
+
+<p class='c036'>“Probably no one was better equipped to illustrate the general subject than Quatrefages.
+While constantly occupied upon the anatomical and osseous phases of his subject,
+he was none the less well acquainted with what literature and history had to say
+concerning the pygmies.... This book ought to be in every divinity school in which
+man as well as God is studied, and from which missionaries go out to convert the
+human being of reality and not the man of rhetoric and text-books.”—<cite>Boston Literary
+World.</cite></p>
+
+<p class='c035'><cite>THE BEGINNINGS OF WRITING.</cite> By <span class='sc'>W. J. Hoffman</span>,
+M. D. With numerous Illustrations. 12mo. Cloth,
+$1.75.</p>
+
+<p class='c036'>This interesting book gives a most attractive account of the rude methods employed
+by primitive man for recording his deeds. The earliest writing consists of pictographs
+which were traced on stone, wood, bone, skins, and various paperlike substances. Dr.
+Hoffman shows how the several classes of symbols used in these records are to be interpreted,
+and traces the growth of conventional signs up to syllabaries and alphabets—the
+two classes of signs employed by modern peoples.</p>
+
+<div class='nf-center-c0'>
+<div class='nf-center c002'>
+    <div>IN PREPARATION.</div>
+  </div>
+</div>
+
+<div class='lg-container-b'>
+  <div class='linegroup'>
+    <div class='group'>
+      <div class='line'><cite>THE SOUTH SEA ISLANDERS.</cite> By Dr. <span class='sc'>Schmeltz</span>.</div>
+      <div class='line'><cite>THE ZUÑI.</cite> By <span class='sc'>Frank Hamilton Cushing</span>.</div>
+      <div class='line'><cite>THE AZTECS.</cite> By Mrs. <span class='sc'>Zelia Nuttall</span>.</div>
+    </div>
+  </div>
+</div>
+
+<div class='nf-center-c0'>
+  <div class='nf-center'>
+    <div><span class='sc'>Recent Volumes of the International Scientific Series.</span></div>
+  </div>
+</div>
+
+<p class='c035'><cite>THE AURORA BOREALIS.</cite> By <span class='sc'>Alfred Angot</span>, Honorary
+Meteorologist to the Central Meteorological Office of
+France. With 18 Illustrations. $1.75.</p>
+
+<p class='c036'>While there have been many monographs in different languages upon various
+phases of this subject, there has been a want of a convenient and comprehensive survey
+of the whole field. Professor Angot has cited a few illustrations of each class of
+phenomena, and, without encumbering his book with a mass of minor details, he presents
+a picture of the actual state of present knowledge, with a summary both of definite
+results and of the points demanding additional investigation.</p>
+
+<p class='c035'><cite>THE EVOLUTION OF THE ART OF MUSIC.</cite> By
+<span class='sc'>C. Hubert H. Parry</span>, D. C. L., M. A., etc. $1.75.</p>
+
+<p class='c036'>Dr. Parry’s high rank among modern writers upon music assures to this book a
+cordial welcome. It was first published as “The Art of Music,” in octavo form. The
+title of this revised edition has been slightly amplified, with a view of suggesting the
+intention of the work more effectually.</p>
+
+<p class='c035'><cite>WHAT IS ELECTRICITY?</cite> By <span class='sc'>John Trowbridge</span>,
+S. D., Rumford Professor and Lecturer on the Applications of
+Science to the Useful Arts, Harvard University. Illustrated.
+$1.50.</p>
+
+<p class='c036'>Professor Trowbridge’s long experience both as an original investigator and as a
+teacher imparts a peculiar value to this important work. Finding that no treatise
+could be recommended which answers the question, What is Electricity? satisfactorily,
+he has explained in a popular way the electro-magnetic theory of light and heat, and
+the subject of periodic currents and electric waves, seeking an answer for his titular
+question in the study of the transformation of energy and a consideration of the hypotheses
+of movements in the ether.</p>
+
+<p class='c035'><cite>ICE-WORK, PRESENT AND PAST.</cite> By <span class='sc'>T. G. Bonney</span>,
+D. Sc., F. R. S., F. S. A., etc., Professor of Geology at
+University College, London. $1.50.</p>
+
+<p class='c036'>In his work Professor Bonney has endeavored to give greater prominence to those
+facts of glacial geology on which all inferences must be founded. After setting forth
+the facts shown in various regions, he has given the various interpretations which
+have been proposed, adding his comments and criticisms. He also explains a method
+by which he believes we can approximate to the temperature at various places during
+the Glacial epoch, and the different explanations of this general refrigeration are stated
+and briefly discussed.</p>
+
+<p class='c035'><cite>MOVEMENT.</cite> By <span class='sc'>E. J. Marey</span>, Member of the Institute
+and of the Academy of Medicine; Professor at the College of
+France; Author of “Animal Mechanism.” Translated by Eric
+Pritchard, M. A. With 200 Illustrations. $1.75.</p>
+
+<p class='c036'>The present work describes the methods employed in the extended development of
+photography of moving objects attained in the last few years, and shows the importance
+of such researches in mechanics and other departments of physics, the fine arts,
+physiology, and zoölogy, and in regulating the walking or marching of men and the
+gait of horses.</p>
+
+<div class='nf-center-c0'>
+<div class='nf-center c002'>
+    <div>D. APPLETON AND COMPANY, NEW YORK.</div>
+  </div>
+</div>
+
+<div class='pbb'>
+ <hr class='pb c005'>
+</div>
+<div class='tnotes x-ebookmaker'>
+
+<div class='chapter ph2'>
+
+<div class='nf-center-c0'>
+<div class='nf-center c001'>
+    <div>TRANSCRIBER’S NOTES</div>
+  </div>
+</div>
+
+</div>
+
+<table class='table0'>
+  <tr>
+    <th class='c019'>Page</th>
+    <th class='c019'>Changed from</th>
+    <th class='c021'>Changed to</th>
+  </tr>
+  <tr>
+    <td class='c009'><a href='#t475'>475</a></td>
+    <td class='c022'>the diameter of our sun, I find, from the above date, that the</td>
+    <td class='c029'>the diameter of our sun, I find, from the above data, that the</td>
+  </tr>
+</table>
+
+  <ul  class='ul_1'>
+    <li>Typos fixed; non-standard spelling and dialect retained.
+
+    </li>
+    <li>Used numbers for footnotes, placing them all at the end of the last chapter.
+    </li>
+  </ul>
+
+</div>
+
+<div style='text-align:center'>*** END OF THE PROJECT GUTENBERG EBOOK 77516 ***</div>
+  </body>
+  <!-- created with ppgen.py 3.57e (with regex) on 2025-11-29 23:01:54 GMT -->
+</html>
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diff --git a/LICENSE.txt b/LICENSE.txt
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+This book, including all associated images, markup, improvements,
+metadata, and any other content or labor, has been confirmed to be
+in the PUBLIC DOMAIN IN THE UNITED STATES.
+
+Procedures for determining public domain status are described in
+the "Copyright How-To" at https://www.gutenberg.org.
+
+No investigation has been made concerning possible copyrights in
+jurisdictions other than the United States. Anyone seeking to utilize
+this eBook outside of the United States should confirm copyright
+status under the laws that apply to them.
diff --git a/README.md b/README.md
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@@ -0,0 +1,2 @@
+Project Gutenberg (https://www.gutenberg.org) public repository for eBook #77516
+(https://www.gutenberg.org/ebooks/77516)